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Mon. Not. R. Astron. Soc. 344, 325–335 (2003) Suspected cool R Coronae Borealis stars in the Magellanic Clouds D. H. Morgan, 1 D. Hatzidimitriou, 2 R. D. Cannon 3 and B. F. W. Croke 4 1 Institute for Astronomy, University of Edinburgh, Royal Observatory, Blackford Hill, Edinburgh EH9 3HJ 2 Department of Physics, University of Crete, Heraklion, Greece 3 Anglo-Australian Observatory, PO Box 296, Epping, NSW 2121, Australia 4 Integrated Catchment Assessment and Management Centre, Centre for Resource and Environment Studies, Australian National University, Canberra, ACT 0200, Australia Accepted 2003 May 19. Received 2003 May 7; in original form 2003 March 24 ABSTRACT Six stars out of a sample of 2300 carbon stars in the Magellanic Clouds have been identified as having strong C 2 bands but CN bands that are very weak or absent. It is argued that five of these are likely to be R Coronae Borealis (RCB) stars on the basis of their spectral characteristics and peculiar colours. Most are variables and the Large Magellanic Cloud (LMC) members have extreme radial velocities that are more like the planetary nebula population than the carbon stars. This sample consists of four LMC members (only one of them previously recognized as an RCB star), one Small Magellanic Cloud (SMC) member (the first RCB star reported in the SMC) and one foreground Galactic star. Key words: stars: carbon – Magellanic Clouds. 1 INTRODUCTION R Coronae Borealis (RCB) stars are very interesting objects, as they constitute important sites of dust formation and evolution. They are hydrogen-deficient, carbon-rich supergiants that fade dramatically at irregular intervals due to circumstellar-dust formation. RCB stars are very rare objects, representing either an unusual or a short-lived stage of stellar evolution. Theoretically, two major scenarios have been proposed to account for the main characteristics of RCB-type stars, one involving the merger of two white dwarfs, and the other involving a final helium shell flash in an evolved planetary nebula central star (Iben, Tutukov & Yungelson 1996). Dust apparently forms in the non-equilibrium conditions caused by pulsations in the atmospheres of these stars (Clayton 1996). Infrared observations led Feast (Feast et al. 1997; Feast 1997) to suggest that the dust forms at about 1500 K, relatively close to the stellar surface, and is ejected in puffs in random directions from the star, rather than in spherical shells or in discs of fixed plane. RCB stars in the Magellanic Clouds, being at known distances, play an important role in furthering our understanding of this rare class of object, as no direct measurement of the distance to any Galactic RCB star exists to date (see Alcock et al. 2001). There are 13 known RCB stars in the Large Magellanic Cloud (LMC) and none yet in the Small Magellanic Cloud (SMC). Three of the 13 LMC stars have been known for over three decades (Feast 1972), while the other 10 were discovered very recently during the MACHO survey (Alcock et al. 2001). The RCB stars fall into three spectral groups: hot, warm and cool; C 2 bands can be seen in the two cooler groups, but are no more than E-mail: [email protected] weakly visible in the warm stars. The red CN bands, on the other hand, are usually very weak, even in cool RCB stars with strong C 2 bands (Alcock et al. 2001). A small number of stars with these spectral characteristics have been identified from a large sample of 2230 carbon stars observed as part of an extensive spectroscopic study of the carbon stars in the Magellanic Clouds (Cannon et al. 1999), carried out using the 2dF multi-object spectroscopic facility on the Anglo-Australian Telescope (Lewis et al. 2002). These stars could therefore be RCB variables and are the subject of this paper. The observations and the identification of the sample of CN-weak stars are described in Sections 2 and 3, the remainder of the paper being concerned with the nature of the stars in the sample, looking in turn at their spectra, photometric colours, brightness variations, radial velocities and proper motions. 2 OBSERVATIONS Most of the LMC carbon stars observed with 2dF were selected from the newly completed catalogue of 7760 carbon stars by (Kontizas et al. 2001 hereafter KDMK01). The SMC stars were cho- sen from the catalogues of Rebeirot, Azzopardi & Westerlund (1993) and Morgan & Hatzidimitriou (1995), which give complete spa- tial coverage of the inner and outer parts, respectively. The SIMBAD identifiers for these catalogues are [KDM2001], RAW and [MH95] which will be abbreviated here to KDM, RAW and MH. These cata- logues were constructed from low-dispersion spectral surveys taken in the blue-green, by identifying carbon stars through their strong (1, 0) C 2 λ4737 and (0, 0) C 2 λ5165 Swan bands. The 2dF instrument on the Anglo-Australian Telescope (AAT) enables up to 400 spectra to be recorded simultaneously, in a pair of identical fibre-fed spectrographs fitted with TEK C 2003 RAS

Suspected cool R Coronae Borealis stars in the Magellanic Clouds

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Page 1: Suspected cool R Coronae Borealis stars in the Magellanic Clouds

Mon. Not. R. Astron. Soc. 344, 325–335 (2003)

Suspected cool R Coronae Borealis stars in the Magellanic Clouds

D. H. Morgan,1� D. Hatzidimitriou,2 R. D. Cannon3 and B. F. W. Croke4

1Institute for Astronomy, University of Edinburgh, Royal Observatory, Blackford Hill, Edinburgh EH9 3HJ2Department of Physics, University of Crete, Heraklion, Greece3Anglo-Australian Observatory, PO Box 296, Epping, NSW 2121, Australia4Integrated Catchment Assessment and Management Centre, Centre for Resource and Environment Studies, Australian National University,Canberra, ACT 0200, Australia

Accepted 2003 May 19. Received 2003 May 7; in original form 2003 March 24

ABSTRACTSix stars out of a sample of ∼2300 carbon stars in the Magellanic Clouds have been identified ashaving strong C2 bands but CN bands that are very weak or absent. It is argued that five of theseare likely to be R Coronae Borealis (RCB) stars on the basis of their spectral characteristics andpeculiar colours. Most are variables and the Large Magellanic Cloud (LMC) members haveextreme radial velocities that are more like the planetary nebula population than the carbonstars. This sample consists of four LMC members (only one of them previously recognized asan RCB star), one Small Magellanic Cloud (SMC) member (the first RCB star reported in theSMC) and one foreground Galactic star.

Key words: stars: carbon – Magellanic Clouds.

1 I N T RO D U C T I O N

R Coronae Borealis (RCB) stars are very interesting objects, as theyconstitute important sites of dust formation and evolution. They arehydrogen-deficient, carbon-rich supergiants that fade dramaticallyat irregular intervals due to circumstellar-dust formation.

RCB stars are very rare objects, representing either an unusualor a short-lived stage of stellar evolution. Theoretically, two majorscenarios have been proposed to account for the main characteristicsof RCB-type stars, one involving the merger of two white dwarfs,and the other involving a final helium shell flash in an evolvedplanetary nebula central star (Iben, Tutukov & Yungelson 1996).Dust apparently forms in the non-equilibrium conditions caused bypulsations in the atmospheres of these stars (Clayton 1996). Infraredobservations led Feast (Feast et al. 1997; Feast 1997) to suggest thatthe dust forms at about 1500 K, relatively close to the stellar surface,and is ejected in puffs in random directions from the star, rather thanin spherical shells or in discs of fixed plane.

RCB stars in the Magellanic Clouds, being at known distances,play an important role in furthering our understanding of this rareclass of object, as no direct measurement of the distance to anyGalactic RCB star exists to date (see Alcock et al. 2001). There are13 known RCB stars in the Large Magellanic Cloud (LMC) and noneyet in the Small Magellanic Cloud (SMC). Three of the 13 LMCstars have been known for over three decades (Feast 1972), while theother 10 were discovered very recently during the MACHO survey(Alcock et al. 2001).

The RCB stars fall into three spectral groups: hot, warm and cool;C2 bands can be seen in the two cooler groups, but are no more than

�E-mail: [email protected]

weakly visible in the warm stars. The red CN bands, on the otherhand, are usually very weak, even in cool RCB stars with strongC2 bands (Alcock et al. 2001). A small number of stars with thesespectral characteristics have been identified from a large sample of∼2230 carbon stars observed as part of an extensive spectroscopicstudy of the carbon stars in the Magellanic Clouds (Cannon et al.1999), carried out using the 2dF multi-object spectroscopic facilityon the Anglo-Australian Telescope (Lewis et al. 2002). These starscould therefore be RCB variables and are the subject of this paper.The observations and the identification of the sample of CN-weakstars are described in Sections 2 and 3, the remainder of the paperbeing concerned with the nature of the stars in the sample, lookingin turn at their spectra, photometric colours, brightness variations,radial velocities and proper motions.

2 O B S E RVAT I O N S

Most of the LMC carbon stars observed with 2dF were selectedfrom the newly completed catalogue of 7760 carbon stars by(Kontizas et al. 2001 hereafter KDMK01). The SMC stars were cho-sen from the catalogues of Rebeirot, Azzopardi & Westerlund (1993)and Morgan & Hatzidimitriou (1995), which give complete spa-tial coverage of the inner and outer parts, respectively. The SIMBAD

identifiers for these catalogues are [KDM2001], RAW and [MH95]which will be abbreviated here to KDM, RAW and MH. These cata-logues were constructed from low-dispersion spectral surveys takenin the blue-green, by identifying carbon stars through their strong(1, 0) C2 λ4737 and (0, 0) C2 λ5165 Swan bands.

The 2dF instrument on the Anglo-Australian Telescope (AAT)enables up to 400 spectra to be recorded simultaneously, ina pair of identical fibre-fed spectrographs fitted with TEK

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1024 charge-coupled devices (CCDs) (Lewis et al. 2002, seehttp://www.aao.gov.au/2df/). The spectra for the LMC part of thisprogramme were obtained during 1998 January and November.These spectra were taken in the red, centred on the �ν = +2 Swanbands near 6200 A. The first set of observations covered four 2◦ di-ameter fields in the wavelength range 5675–6785 A, and the secondset gave five fields in the wavelength range 5585–6700 A. Thus,the strong (0, 1) Swan band at 5635 A was included in the secondset of observations but not in the first set, whereas the Li I λ6708line, which is the subject of a separate paper (Hatzidimitriou et al.2003), was included only in the first set. One field was made com-mon to both sets of observations to provide checks on the system.Most of the stars observed fall in the brightness range R ∼ 14–16.Altogether, ∼1500 carbon star spectra were obtained on eight fieldswith centres arranged approximately on a 10◦-long north–south stripacross the central parts of the LMC.

Some of the SMC spectra were obtained during the second of thetwo observing runs mentioned above. The remainder were obtainedon several nights during 2000 as part of the ATAC Service ObservingScheme. One field was observed twice because the sky conditionscaused a truncation of the exposures and consequently much poorerquality spectra. Altogether, spectra were obtained in the 5585–6700 A wavelength range for ∼730 SMC stars.

The 1200R gratings were used throughout, yielding spectra with1.1 A pixel−1 and an effective resolution of ∼2.5 A. Several ex-posures were taken for each field, typically 3 × 900 or 2 ×1200 s, together with offset sky, arc and flat-field exposures. Thedata were reduced with the AAO’s 2DFDR data reduction package(Bailey et al. 2001) and the IRAF package, and yielded spectra witha signal-to-noise ratio usually around 30 pixel−1. Full details of theobservations are given by Cannon et al. (in preparation).

One central LMC field was re-observed on 2002 January 9 usingthe 1200B gratings to give blue spectra in the wavelength range4190–5290 A at 1.1 A pixel−1. This too was done as part of theATAC Service Observing Scheme.

Another set of carbon star spectra had been obtained beforehandwith the FLAIR fibre system (Parker & Watson 1995) on the UK1.2-m Schmidt Telescope (UKST) as a test for the 2dF observations.Two 3000-s exposures were obtained on 1996 December 12 with

Table 1. Carbon stars with weak CN bands.

Name Source RA (J2000) Dec. Ra I a J b Hb Kb I c J c Kc vrad

(km s−1)

RAW 21 1 0 37 47.40 −73 39 02.0 15.62 14.62 17.07 15.25 13.45 139KDM 2373 2 5 10 28.50 −69 47 04.3 13.79 13.26 14.54 13.78 12.97 13.88 13.14 12.05 205KDM 5651 2 5 41 23.49 −70 58 01.8 14.43 13.90 13.48 13.20 12.78 13.65 13.19 12.46 206KDM 2492 3 5 11 31.03 −67 55 49.7 14.98 14.27 12.84 12.57 12.19 13.49 12.84 255KDM 7101 4 6 04 05.53 −72 51 23.1 14.18 16.41 13.43 12.88 12.21 13.20 12.65 11.84 220KDM 6546 4 5 51 46.57 −75 42 21.0 11.92 11.37 9.02 8.41 8.16 10.08 8.92 8.00 331

Name: KDM – Kontizas et al. (2001), RAW – Rebeirot et al. (1993).The ‘Source’ column identifies the source of the spectral identification: 1, 2dF SMC; 2, 2dF LMC; 3, 2dF LMC and known RCB star; 4, FLAIR LMC.KDM 2492 is the known RCB star HV 5637 and MACHO∗J051131.4-675552.No star is in the Westerlund et al. (1978) or Kunkel et al. (1997) catalogues.Cross-identification with Sanduleak & Philip (1977) (SP) and Hartwick & Cowley (1988) (HC) are: KDM 2373 ≡ SP 39 12 ≡HC 182, KDM 5651 ≡SP 55 17≡HC 119, KDM 2492 ≡ SP 37 16 ≡HC 179 and KDM 7101 ≡ SP 65 2KDM 6546 is also CGCS 1138 (Stephenson 1989) and possibly HC 193 (see the text in Section 9.2).Hartwick & Cowley (1988) quote vrad = 258 km s−1 for KDM 2492 but give no velocities for KDM 2373 or KDM 5651.Source of photometry: aSuperCOSMOS; b2-MASS; cDENIS (see the text for details).The R and I observations were made 4 months apart for KDM 7101, but within 2 d for the other stars.From GSC II: V = 14.60 for KDM 2373, V = 15.13 for KDM 5651, V = 14.70 for KDM 2492.Photometry by Walker (1979) of KDM 6546 is B = 13.29, V = 11.58, R = 10.76, I = 10.03.

a wavelength coverage of 5730–6452 A at 1.34 A pixel−1. Therewere 50 fibres, including four positioned on the sky. The target starswere taken from KDMK01 and Kunkel, Irwin & Demers (1997).The field was centred on ESO/SERC Field 33 (5.5h , −75◦).

3 S A M P L E S E L E C T I O N

As has been reported elsewhere (Morgan et al. 2003), the 2dF spectrawere given a preliminary classification, identifying N stars, J starsand C2-weak stars. During this process, four stars were found tohave strong C2 bands but very weak CN bands. Subsequent analysisof the data set based on bandstrengths, such as that carried out byMorgan et al. (2003), did not reveal any other stars with a similarlevel of CN weakness. The FLAIR data were also examined by eye;two candidates with weak CN bands were identified.

The six selected stars are listed in Table 1. The coordinates areJ2000; the photometric and kinematic data in the other columnswill be described later. The second column notes the source of theidentification. One of the 2dF identifications is a member of theSMC and another is a previously identified RCB star. Various cross-identifications are given in the footnotes.

It is worth mentioning the star RAW 76, which was reported byWesterlund et al. (1995) to have very weak CN bands for its C2

bandstrength. Inspection of the published spectrum shows that theCN weakness is not as extreme as in the stars of Table 1.

Two of the 2dF stars were observed twice: KDM 5651 and RAW21. The LMC star gave two similar spectra, but the second spectrumof the SMC star contained no counts, even though it was obtainedunder better sky conditions. This was presumably due to it beingextremely variable (see later).

4 T H E S P E C T R A

The six selected spectra are shown in Fig. 1 accompanied by thespectrum of the typical N star KDM 3181 shown for comparisonpurposes. Residual sky lines are generally negligible; they are justnoticeable in RAW 21 and KDM 7101 because the former was fromthe truncated observation noted earlier and the latter was from the

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Figure 1. Spectra of the six stars in Table 1 plus that of the typical N star KDM 3181. KDM 7101 and KDM 6546 were observed with FLAIR. The emissionlines at 6300 and 6363 A in RAW 21 are from the night sky.

FLAIR system for which sky subtraction is more difficult due to themuch larger fibres.

The upper six spectra are dominated by the blue-degraded�ν = −2 series of Swan C2 bands, yet there is little, if any, sign ofthe red-degraded (6, 1), (7, 2), (4, 0) and (5, 1) CN bands that areusually just as prominent as the C2 bands, as can be seen in KDM3181 around 5750, 5890, 6210 and 6350 A, respectively. The (6,2) CN band near 6500 A appears to be present, but is probably en-hanced by the overlapping (4, 7) C2 band at 6533 A and the group of

metal lines near 6500 A, including Ba II λ6496. The most extremestar is the SMC star RAW 21; its C2 bands are stronger than thoseof the LMC stars, as can be seen clearly in Fig. 1 through the (3, 6)band at 6599 A. The least extreme star is KDM 6546; the (4, 0) CNλ6206 band can be distinguished quite easily. KDM 6546 is muchbrighter than the other stars and will be shown later to be a Galacticstar.

These spectral characteristics are also found in cool RCB stars.Alcock et al. (2001) show (their fig. 7) the spectra of seven confirmed

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cool RCB stars in the LMC – three in the wavelength range 3600–7400 A and four in the wavelength range 3600–5400 A. Alcock et al.(2001) also show (their fig. 8) the spectra of stars like the peculiarRCB star DY Per, which appear, as far as the strongest molecularbands are concerned, like typical LMC N stars and are thought tobe cooler than normal RCB stars (Keenan & Barnbaum 1997). Bycomparing figs 7 and 8 of Alcock et al. (2001), it is clear that the�ν = −2 series of C2 Swan bands is strong in both types of star butthe usually strong (4, 0) CN λ6206, (5, 1) CN λ5730 and (7, 1) CNλ5239 bands are very weak in or absent from the normal RCB stars.Thus, the absence of these CN bands is a feature common to thestars of Fig. 1 and the RCB stars. (See also Bessell & Wood 1983,for the spectrum of another LMC RCB star with a similar spectrum.)Note that the spectrum of HV 5637 (KDM 2492) is shown in Fig.1 of this paper and in fig. 7 of Alcock et al. (2001). Similar spectralfeatures can be seen in Galactic RCB stars, e.g. S Aps (Kilkennyet al. 1992) and Z Umi (Benson et al. 1994) and, to a lesser extent,in the Galactic CHd1 star HD182040 (Barnbaum, Stone & Keenan1996). The spectra of Fig. 1 are also very like those of three carbon-rich variable stars put forward by Lloyd Evans, Kilkenny & vanWyk (1991) as possible new RCB stars. The RCB stars are, however,reported to show the strong blue-degraded (0, 1) CN λ4216 memberof the violet band system, but this band is rarely seen in our limitedsample of 2dF blue spectra because the flux there is usually verylow and the band falls close to the end of the spectral coverage.

The red isotopic bands involving 13C are not normally present inRCB stars (Lloyd Evans et al. 1991). Nor can they be seen in thestars of Fig. 1. There is a feature that at first sight might appearto be (0, 2) 13C12C λ6168, but, on closer inspection, is seen tobe at a slightly bluer wavelength and in most cases is simply Ca I

λ6162. There are no features near where the (1, 3) 13C12C λ6100 and(0, 1) 13C12C λ5635 bands would be.

As mentioned in Section 1, RCB stars are hydrogen-deficientstars that usually show a complete lack of Balmer lines and the CHband. None of the 2dF spectra in Fig. 1 shows Hα in absorption,which is consistent with the hydrogen-deficient nature of an RCBstar. Although Hα is not usually seen in carbon stars, it is morecommon in stars as blue as these (see later).

There are some other lines to note. First, Ca I λ6162 is clearly seen.Secondly, there is a weak but obvious line or bandhead at 6420 A(corrected for radial velocity) in the five upper spectra of Fig. 1,which is not present in any of the 2dF spectra of N stars. This canalso be seen in HD 182040 (Barnbaum et al. 1996), which is a non-varying hydrogen-deficient carbon star with weak CN bands. Otherspectral similarities between HD 182040 and the RCB stars can beseen in the wavelength region redward of 6200 A, though the Swanbands are much weaker in HD 182040. The Na D lines are seen in allthe stars, though not always with the same profile; in RAW 21 andKDM 2373 the pair are well separated, whereas in KDM 2492 andKDM 5651 (both spectra) they are almost completely blended. Thisblending is not uncommon in RCB stars, which often have Na D lineprofiles that are complex and variable (e.g. in DY Per, Keenan &Barnbaum 1997 and R CrB, Rao et al. 1999). The spectral resolutionis not good enough to show the detailed profiles accurately. Thecombined equivalent width of the Na D lines is slightly larger in theSMC star (� 3 A) than in the LMC stars (� 2 A). It also appearsslightly larger in KDM 7101 (� 3 A) observed with FLAIR, but thatmay be due to the greater difficulty of determining the continuumin the lower-resolution FLAIR spectrum.

An alternative proposition to the RCB-star hypothesis is that oneor more of the stars are Galactic dwarf carbon (dC) stars in a line-of-sight coincidence with the LMC or SMC. Like RCB stars, dC

stars can be characterized by relatively strong C2 bandheads of the�ν = −2 series. Degrading their spectral resolution could make the2dF spectra look like that of the dC star CLS 96 (Green et al. 1992).However, the absence of the isotopic bands from the stars of Fig. 1argues against the dC-star option because these bands are seen indC stars (Green et al. 1992).

Another possible Galactic candidate is the faint high-latitude car-bon (FHLC) star. These stars have weak C2 and CN bands, as ex-pected for stars of their colour (see Green & Margon 1990), and aresaid to have relatively weak CN Green et al. (1992). However, noneof the stars discovered by Totten (1998) from a survey of much ofthe sky at high and intermediate Galactic latitudes (though exclud-ing the Magellanic Cloud area) has CN bands that are as weak asthose of the five uppermost stars of Fig. 1. Therefore, it is highlyunlikely that these five stars are Galactic FHLC stars.

5 P H OTO M E T R I C DATA

Near-infrared photometry of sources in much of the MagellanicClouds can now be extracted from the Second Incremental Data Re-lease of the Two-Micron All-sky Survey (2-MASS) (Skrutskie et al.1997) which is available on-line at http://irsa.ipac.caltech.edu/. Itis also available from the Deep Near-Infrared Survey of the south-ern sky (DENIS) (Cioni et al. 2000) via the Strasbourg Data Centre.2-MASS JHK measurements are listed in Table 1, as are DENIS IJKmeasurements, adjusted to bring them into line with the 2-MASSdata (see Morgan et al. 2003).

None of the stars in Table 1 has an IRAS source within 1 arcmin.The infrared photometry of the candidate RCB stars is shown in

Figs 2 and 3, superimposed on 2-MASS data for the rest of the LMCcarbon stars observed by 2dF. The SMC star was brightened by 0.46mag to bring it to the same distance as the LMC stars, as defined bythe mean distance moduli calculated by Westerlund (1997). Each ofthe stars in Table 1 is identified by a unique symbol. The symbolsplotted in heavy type are from the 2-MASS survey and those inlighter type are from the DENIS survey.

RI photometry of the LMC stars is available from KDMK01. Thiswas derived from measurements of UKST I/SR Sky Survey plates

Figure 2. K versus J − K colour–magnitude diagram. The stars of Table1 are the large symbols: RAW 21, five-pointed star; KDM 2373, open plus;KDM 5651, six-pointed star; KDM 2492, circle + dot; KDM 7101, circle+ plus; KDM 6546, square. The symbols are shown in heavy type for 2-MASS data and in light type for DENIS data. The data for RAW 21 havebeen brightened by 0.46 mag (see the text). The small open circles are theRCB stars from Alcock et al. (2001). The dots are the other carbon stars inthe 2dF data set.

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Figure 3. J − H versus H − K colour–magnitude diagram. The symbolsare as in Fig. 2. The dotted line marks the location of the dC stars and thedashed line that of the FHLC stars (see the text).

Figure 4. I versus R − I colour–magnitude diagram as constructed solelyfrom the SuperCOSMOS data. The symbols are as in Fig. 2.

(Morgan 1995) made with the SuperCOSMOS measuring machine.Calibration was made relative to carbon star standards, using a boot-strap procedure to apply it to the whole LMC. Full details are givenby KDMK01. Equivalent photometry has been obtained for SMCstars using similar techniques, but is as yet unpublished. The blend-ing of the carbon star images with close line-of-sight companions isworse in the RI photometry than in the infrared photometry becausethe neighbouring stars are usually bluer than the carbon stars. Nev-ertheless, the extraction software used to obtain the RI photometrycopes with blended images in all but the severest situations. The RIplate pairs were, with one exception, taken close together in time,so colours should not be affected by stellar variability. Fig. 4 is thecolour–magnitude diagram for these RI measurements. The stars ofTable 1 are identified by the same symbols used in Figs 2 and 3.

RCB stars have infrared-emitting dust shells that affect their near-infrared JHK colours to varying degrees depending very much ontime (Kilkenny & Whittet 1984; Feast 1997). The colours of theRCB stars listed by Alcock et al. (2001) are also plotted in Figs 2and 3. (In Fig. 2, the abscissa values of the two bluest are shiftedby 0.07 mag to aid clarity.) Apart from KDM 6546, all the stars ofTable 1 lie with the RCB stars in these diagrams covering a widerange of colour and being fainter than the typical N star. In contrast,the DY-Per-like stars of Alcock et al. (2001) (not plotted) lie with

the N stars in Figs 2 and 3. Similarly, in Fig. 4, the candidate RCBstars are consistent with a type that is usually blue but undergoesoccasional periods of obscuration by dust. The exception in all threediagrams is KDM 6546 which is too bright to be an LMC member.

According to Green et al. (1992), the dC stars are found withblue (J − H ) colours, displaced from normal carbon stars in thenear-infrared two-colour diagram. The region they occupy is markedon Fig. 3 and overlaps the bluest part of the region occupied by theRCB stars. Two of the stars (KDM 2492 ≡ HV 5637 and KDM5651) could be dC stars on this criterion, but the other three couldnot. On the basis of its colours, the Galactic star KDM 6546 couldbe a dC star, but it could also be an FHLC star because the near-infrared colours of these are also displaced from the sequence fornormal carbon stars (Green et al. 1992) as marked on Fig. 3.

Alcock et al. (2001) also show the RCB stars (at maximum light)in the V versus (V − R) colour–magnitude diagram. The HubbleSpace Telescope (HST) Guide Star Catalogue GSC II (http://www-gsss.stsci.edu) provides V magnitudes for stars in the central partsof the LMC including three of the stars of Table 1 – the known RCBstar HV 5637 (KDM 2492) and two of the new candidate RCBstars: KDM 2373 and KDM 5651. The GSC II V magnitude of HV5637 is 14.7 (which is the same as Alcock et al.’s value) and theother two RCB candidates have similar values of 15.1 and 14.6 mag,respectively. In contrast, GSC II V magnitudes of a set of 52 carbonstars located just south of the LMC Bar are V = 16.2 ± 0.5. Most ofthe cool RCB stars are brighter than this at V ∼ 15.0 (see fig. 10 ofAlcock et al. 2001). Hence, this too supports the RCB candidatureof KDM 2373 and KDM 5651.

6 VA R I A B I L I T Y

Carbon stars are known to vary: typically N and R stars show opticalvariations with amplitudes �1 mag and periods ranging from 100to 600 d, though a few N stars have large amplitudes within the 1–3.5 mag range occupied by the classical carbon-rich Miras (Grenonet al. 2000). The periods of the Miras are 250–500 d. There arealso semiregular variables that can have much larger magnitudevariations over periods of up to 6000 d and the irregular RCB starswhich can suddenly fall in brightness by ∼4 mag, sometimes by asmuch as ∼8 mag. RCB stars are not constant at maximum brightnessbut vary with amplitudes in V of a few tenths of a magnitude overperiods of 40–100 d (Clayton 1996). The other hydrogen-deficientstars – the CHd stars – remain relatively constant, varying like RCBstars at maximum though by generally smaller amounts (Lawsonet al. 1990).

With the possibility that the selected CN-weak stars could beRCB variables, all available sources of photometry were searchedfor any information that would help build-up useful light curves.

Measurements of other UKST plates have been taken from fivesources.

(i) The overlap zones between adjacent UKST survey fields.Since these are large at LMC declinations, many stars lie in morethan one survey field. The photometric measurements made in sec-ond or even third fields, though not published by KDMK01, havebeen used.

(ii) A second pair of UKST I/SR plates on Field 56, which hadbeen measured during the construction of the source catalogue (seeKDMK01 for details). The magnitudes of the stars seen on thisI/SR pair have also been obtained. Calibration was achieved bycomparison with the calibrated magnitudes from KDMK01.

(iii) A series of short exposures taken on TechPan 4415 filmthrough the standard OG590 filter as contemporaneous partners to

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the films of the recent AAO/UKST Hα Survey of the MagellanicClouds (Parker & Phillipps 1998). These had been scanned by Su-perCOSMOS and used as image checks in the preparation of theKDMK01 catalogue. Since then, the KDMK01 carbon stars havebeen paired with the OR film measurements to ±1 arcsec. A goodlinear relationship was seen to link the uncalibrated magnitudes fromthe film data with the calibrated magnitudes from the catalogue de-spite the slightly different wavebands involved (5950–6900 A, cf.6300–6900 A). These linear relationships were used to calibrate themeasurements of the films and thereby provide R measurements atsignificantly later epochs than the older Sky Survey plates.

(iv) The SuperCOSMOS Sky Survey (SSS) data base. This con-tains calibrated SuperCOSMOS measurements of the deep all-skyphotographic sky surveys (Morgan 1995) and has recently becomeavailable on-line (Hambly et al. 2001). Searches of the SSS haveprovided BJ , R and I magnitudes from the SERC-J, AAO-R andSERC-I/SR Surveys, respectively, plus an R-band measurementfrom the ESO-R Survey. However, caution must be applied to theinterpretation of these measurements because crowding is a greaterproblem in the deep plates than in the shallower R plates and filmsmentioned above and because the SSS calibrations were not de-signed for work with very red carbon stars.

(v) The GSC II Guide Star Catalogue. Most of the measurementsin the GSC II Catalogue were made from the same survey platesscanned for the SSS and consequently do not help in building up lightcurves of the RCB-star candidates. However, the GSC II Catalogueincludes measurements of some very short-exposure UKST R platesand one V plate of the central parts of the LMC. These too have beenextracted from the on-line data base.

The R and I measurements taken from these photometric sourcesare shown in Fig. 5 for the period 1975 to date, supplementedwith data from a variety of other sources. The additional datainclude sample R magnitudes for the three stars that appear inthe publically available MACHO data base (http://wwwmacho.mcmaster.ca/Data/MachoData.html). The detailed light curves forKDM 2373 and KDM 5651 will be described later, and that forKDM 2492 has been plotted by Alcock et al. (2001). The sample

Figure 5. Magnitude variation in the R (circles), I (squares) and J (triangles) wavebands.

points relate to the start, middle and end of the MACHO coverage.I magnitudes from the DENIS Survey are shown, though that forRAW 21 (the most recent I measurement plotted) is actually an up-per limit based on the faintest stars seen in the vicinity of RAW 21which was not itself detected. This non-detection is certainly dueto variability rather than error because the star is not visible on anUKST V-plate taken 1 month later. Also included are measurementsin R and I from Walker (1979) for KDM 6546 and in I from Udalskiet al. (1998) for RAW 21.

Fig. 5 also shows J magnitudes from the 2-MASS and DENISSurveys (see Table 1) plus data for KDM 2492 from Glass, Lawson& Laney (1994). Errors due to slightly differing wavebands, mea-surement techniques or calibration are likely to be small on the scaleof these plots.

None of the LMC stars listed in Table 1 was identified as anLPV star by Hughes (1989) or Reid, Hughes & Glass (1995). Thesestudies used UKST I-plates taken in the periods 1976–1984 and1976–82, respectively, but did not cover the entire LMC. The mostvariable LMC star in Fig. 5 (KDM 7101) is not in the area coveredby the LPV surveys.

Three stars (RAW 21, KDM 2373 and KDM 7101) show evi-dence of extreme variability, with brightness falls of at least 3 mag,which are compatible with the stars being of the RCB type. Another(KDM 5651) has undergone changes in magnitude of ∼2 mag, butthe known RCB star HV 5637 ≡ KDM 2492 shows no evidence ofvariability greater than ∼1 mag. However, the time coverage avail-able is not good enough to rule out magnitude changes greater thanthose seen. On the other hand, according to Alcock et al. (2001),variability with poor time coverage has in the past generated manywrong RCB candidates, which were later rejected when spectrabecame available. Examples are symbiotic stars, cataclysmic vari-ables or semiregular variables (Lawson & Cottrell 1990). However,the new RCB candidates are not symbiotic because they have noHα emission; cataclysmic variables are too faint. It is worth notingthat the JHK colours of spectroscopically confirmed carbon-richMira variables (Wood, Bessell & Paltoglou 1985; Feast et al. 1989)lie in a sequence that is essentially the same as that of the N stars(Fig. 3), though with most lying at or beyond the red end of the N-star

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sequence. Hence, the colours of the Mira variables are not like thoseof the RCB stars. Nevertheless, well-defined light curves that showthe characteristics of the RCB phenomenon are still needed to con-firm beyond doubt the RCB status of the stars of Table 1.

7 R A D I A L V E L O C I T I E S

Radial velocities of the LMC stars were obtained using a cross-correlation in FIGARO. The high signal-to-noise ratio attained formost stars and the large number of spectral features resulted in aninternal precision of about 1/20 pixel though with external accura-cies closer to 1/10 pixel. Radial velocities of the SMC stars weremeasured using the IRAF package. Zero-points were set to matchthe 2dF velocities with those published by Kunkel et al. (1997).This was done separately for the two 2dF CCDs to account for theknown ∼10–15 km s−1 velocity offset between the two wavelengthcalibrations systems (Cannon 2002).

The LMC radial velocities were measured relative to a mean tem-plate using the full wavelength range. Since the cross-correlationroutines use the numerous small-scale fluctuations (that are duemainly to CN) as much as the large-scale ones (that are mainly dueto C2), it is better not to use stars strong in CN as templates forthose with little or no CN. For the stars with almost no discernibleCN, the best match with the template lies in the central wavelengthrange 5915–6205 A. The effect of this spectral mismatch was de-termined by measuring the change in velocity produced when thewavelength range used for the cross-correlation was reduced fromthe whole spectrum to just the central part. This was done for each ofthe four 2dF spectra (KDM 5651 was observed twice), each againstsix templates chosen to be N stars with high-quality spectra in thesame CCD-field combination. It turned out that limiting the cross-correlation to this central portion of the spectrum caused the radialvelocities of the CN-weak stars to increase by ∼11 ± 3 km s−1

accompanied by improvements in the cross-correlation peaks from∼0.65 to ∼0.85. In contrast, equivalent cross-correlations of these24 templates with one another gave a negligible velocity shift of ∼1± 1 km s−1. The radial velocities quoted in Table 1 are corrected forthis effect. The 2dF radial velocity of 255 km s−1 for KDM 2492is close to the published value of 258 km s−1 (Hartwick & Cow-ley 1988). The two velocity measurements of KDM 5651 agree to±2 km s−1.

Details of the radial velocities of the entire 2dF sample, along withtheir measurement method and their accuracies will be presented ina following paper. However, this will not be so for the stars observedwith FLAIR. For these stars, the chosen radial velocity template wasKDM 2784. The zero-point was again fixed using stars from Kunkelet al. (1997). 11 of these gave cross-correlation peaks greater than0.75 and provided a zero-point with an accuracy of ±12 km s−1. Acheck was provided by four stars that were observed with both 2dFand FLAIR. The final radial velocities differed by 8 ± 12 km s−1,which is within the errors of the zero-point settings.

Fig. 6 shows the radial velocities of the LMC stars of Table 1 asa function of declination, superimposed on the 2dF velocities forthe LMC carbon stars as a whole. Also shown are velocities fromKunkel et al. (1997) for stars that lie within the same RA range asthe 2dF observations (i.e. 4.8–5.8 h). With just one exception, thepeaks of the cross-correlation function are greater than 0.55, whichcorresponds to an rms of the differences between repeat measure-ments of about 5 km s−1. It is clear from Fig. 6 that the four RCBstar candidates have radial velocities that are consistent with LMCmembership, but are always at the low end of the range of radialvelocities seen for carbon stars at the same declination. As with the

Figure 6. Radial velocity (km s−1) against declination (deg) for LMCcarbon stars. Most points are from the 2dF data set; the small number ofslightly heavier points are from Kunkel et al. (1997). The large symbolsmark the stars of Table 1 as in Fig. 2.

photometry, KDM 6546 is different; its radial velocity is very high,probably indicating that it is a Galactic star.

Converting these velocities to Galactocentric velocities reducesthem all by �200 km s−1. FHLC stars more than 10 kpc above theGalactic plane have a mean rotation velocity of ∼0 km s−1 and avelocity dispersion of ∼100 km s−1 (Totten 1998). This means thatLMC carbon stars cannot necessarily be distinguished from FHLCstars by their radial velocities.

As mentioned in Section 1, one of the models describing the for-mation of RCB stars is the final helium shell flash (Iben et al. 1996)in which the aforementioned flash causes a planetary nebula centralstar to grow rapidly to become a supergiant star. In this way, theRCB stars could be related to the planetary nebula population ratherthan to the carbon stars. Fig. 7 shows the carbon stars of Fig. 6with the radial velocities of the planetary nebulae superimposed.The latter were taken from Morgan & Parker (1998), the trianglesbeing the velocities derived in that paper from FLAIR (excludingthe few objects for which only one or two lines were used in thevelocity derivation and any where the rms from the individual lines

Figure 7. Radial velocity (km s−1) against declination (◦) for LMC carbonstars and planetary nebulaer. The carbon star data are from 2dF as in Fig. 6and the planetary nebulaer velocities are from the compilation by Morgan &Parker (1998) with circles marking velocities from slit spectra and trianglesmarking velocities from the FLAIR fibre-optic system.

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exceeded 20 km s−1), and the circles being the generally more ac-curate velocities published by other authors from slit spectroscopy(see references given by Morgan & Parker 1998). Only those plan-etary nebulae located in the same RA range as the 2dF observationswere included (i.e. RA 4.8–5.8 h). At many declinations, especiallythose around −71◦ where two of the four LMC candidate RCB starsare located, the planetary nebulae have velocities that are generallylower than those of the carbon stars. Since the RCB star candidatesalso have very low radial velocities, it would appear that the RCBstars are more likely to be associated kinematically with the plane-tary nebulae than with the N stars, providing further support for thefinal helium shell flash model (Iben et al. 1996). Unfortunately, thenumber of RCB stars is too small to give a firm result on this point.

8 P RO P E R M OT I O N S

The SSS data base provides stellar proper motions derived from theSERC-J (BJ) and AAO-R surveys, which were taken about 10–15yr apart. Unfortunately, they are not reliable for the stars in Table 1because these stars are located in regions so crowded that imagedeblending by SuperCOSMOS is very difficult, particularly as thecarbon stars are so red that the BJ and R images are quite different.

Three of the stars in Table 1 have proper motions listed in theUCAC1 Catalogue (Zacharias et al. 2000). KDM 6546, KDM 5651,and KDM 2492 have (�α, �δ) = (4.9, 15.1), (3.1, 18.8) and (21.3,34.8) mas yr−1, respectively. The errors quoted in the catalogue aremean errors in the range 9–15 mas yr−1. The second epoch positionswere based on highly dispersed images but the first were basedon UKST plates and therefore suffer from the crowding problemsmentioned above. This is likely to be a problem for the last two ofthese stars which do have elliptical images on the Sky Survey plates.

The proper motions of eight of the nine known Galactic dC starsare ∼40 mas yr−1 or greater, with two ∼700 mas yr−1. Those ofFHLC stars are almost all in the range 5–15 mas yr−1 (Totten 1998).Certainly, there are no obvious high-velocity stars in the Table 1sample, but in view of the high errors involved, it is not possible tobe conclusive about the status of the stars as determined from theirproper motions.

9 I N D I V I D UA L S TA R S

Some details of the individual stars are given below.

9.1 Candidate RCB stars

9.1.1 RAW 21

This star has all the characteristics of an RCB star as discussedearlier. Its spectrum is dominated by the Swan bands – the first sixmembers of the �ν = −2 sequence between 5900 and 6200 A, thesecond to fifth members of the �ν = −3 sequence between 6475and 6700 A, and the first two members of the �ν = −1 sequenceat the blue end of the spectrum. RAW 21 is extremely variable with�R � 4 mag. In addition to the variation seen in Fig. 5, it should benoted that the star was bright during 2000 January when the good2dF spectrum was obtained and very weak during 2000 June, whenit failed to register any signal in the repeat 2dF observation of thatfield. If confirmed, RAW 21 will become the first RCB star foundin the SMC. Its radial velocity of 139 km s−1 is close to the averagefor carbon stars in the SMC and therefore does not distinguish itfrom the other SMC carbon stars in the way noted in Section 7 forthe LMC RCB-star candidates. RAW 21 is the reddest of the RCB

stars in the near-infrared (see Figs 2 and 3). Not surprisingly, thesedata were obtained (in 1998) when the star was faint in the opticalrange (see Fig. 5).

9.1.2 KDM 2492

This star is the known RCB star HV 5637, though its RCB statushas been questioned by Glass et al. (1994) on the grounds that itsinfrared colours are not those of a normal RCB star and it has un-dergone only one significant decline in brightness, which was muchlonger and slower than is normal for RCB stars. Although its infraredcolours are blue, they are not unlike those of some of the new RCBstars identified by Alcock et al. (2001). The changes in R seen hereare greater than the normal brightness fluctuations of an RCB star atmaximum, but well below that seen during a dust emission episode.The JHK photometry of Glass et al. (1994) shows HV 5637 to bealmost constant in brightness between 1986 and 1994, but fainterin 1979, consistent with the variation in the R data (see Fig. 5).Nor is any brightness variation detected by the MACHO Survey be-tween 1992 and 1999. Similarly, for the slightly earlier period 1976–1982, Reid et al. (1995) find just a weak I-magnitude fluctuation ofσ I = 0.35 around a mean of I = 13.5, which is slightly brighter thanthe values in Fig. 5. However, KDM 2492 (HV 5637) does have thespectrum and photometry of an RCB star. It seems to be a relativelyinactive RCB star, like the Galactic RCB star XX Cam.

9.1.3 KDM 2373

This star is another new candidate RCB star according to its spectrumand photometry; it is also an extremely irregular variable. KDM2373 is also HC 182 which, like KDM 5651 (HC 119), was a rejectedcandidate CH star (Hartwick & Cowley 1988) with no radial velocityquoted. The 2dF radial velocity is low for the LMC, but is stillconsistent with LMC membership.

KDM 2373 is the one star from Table 1 that was observed with2dF in the blue. The spectrum shows the (1, 0) 13C13C λ4752 bandto be absent and the (1, 0) 13C12C λ4744 band to be present but verymuch weaker than the (1, 0) 12C12C λ4737 band, as expected foran RCB star (Lloyd Evans et al. 1991). KDM 2373 seems to showHβ in absorption as a broader feature than in other stars where it isseen, but at a level weak for its blue colour. The G band cannot bedetected, though the spectrum is very weak that far into the blue.Although these two features are usually absent from RCB stars,not all RCB stars show complete hydrogen deficiency (Keenan &Barnbaum 1997). Ba II λ4554 does not stand out as stronger thanthe neighbouring �ν = −2 violet series of CN bands in the way itdoes in N stars and in most dC stars where s-process elements areenhanced (Green & Margon 1994). KDM 2373 also shows a strongbroad absorption feature centred near 4773 A (corrected for radialvelocity). This feature is also seen in DY Per but not in typical Nstars and is thought to be due to C I (Keenan & Barnbaum 1997).

The light curve of KDM 2373 as extracted from the MACHO database is shown in Fig. 8. The data shown are in the R waveband; theB data are similar. The zero-point of the curve was set to match theGSC II R magnitude, which was measured on a plate taken justbefore KDM 2373 became faint. The light curve shows a very largedecline in the form of a series of smaller changes of ∼1 mag followedby a larger steeper decline of 3–4 mag. Although a sequence ofdeclines and small recoveries is not unknown in RCB stars (Clayton1996), the precise form seen here, without a very steep initial decline,is rather unusual. The infrared photometry plotted in Fig. 2 reflectsthis decline; the data from the DENIS Survey were obtained just

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Figure 8. Light curves in R as observed by the MACHO project.

after the star started to fade and are much bluer than those obtainedby the 2-MASS Survey when the star was ∼2 mag fainter in R.

9.1.4 KDM 5651

This star was observed during each of the two observing runs; thetwo spectra show very few differences. The MACHO light curveof KDM 5651 is shown in Fig. 8. Apart from a 1-year fading ofno more than 0.5 mag, the star remained constant throughout theMACHO observing period. KDM 5651 appears to be an RCB staron the basis of its spectrum and photometry, though the form ofits variability (�2 mag; see Fig. 5) is not yet properly known. Itwas observed by Hartwick & Cowley (1988) as a candidate CH star(HC 119) but rejected as such by those authors; no radial velocitywas quoted. Both 2dF spectra yield radial velocities that place KDM5651 at the low-velocity limit of the LMC.

9.1.5 KDM 7101

This star is extremely variable. In addition to the points plotted inFig. 5, the following information has been gleaned from plates inthe UKST Plate Library. KDM 7101 is bright in the 4550–5500 Awaveband on the objective-prism plate YJ15950P (1994 February10) but absent from the similar plate YJ11510P (1986 November25). In I, it is bright on plate I 17209 (1996 September 4), but fainton plate I 2816 (1976 December 30). It appears to be an RCB staron all three tests – spectrum, photometry and variability. Similarlyto KDM 2373, KDM 7101 appears redder in the 2-MASS Surveythan in the DENIS data base. Inspection of the optical data in Fig. 5shows that the DENIS data were obtained while the star was nearmaximum brightness and the (redder) 2-MASS data were obtainedduring a decline.

9.2 The Galactic star KDM 6546

KDM 6546 is far too bright in all wavebands to be a normal LMCmember. Its radial velocity of ∼350 km s−1 is high compared with

the mean LMC value of ∼250 km s−1 and even higher comparedwith ∼220 km s−1, which is the mean velocity of carbon stars atthe same southerly declination. Hence, KDM 6546 is likely to be ofGalactic origin.

KDM 6546 is also CGCS 1138 in the General Catalogue ofCool Carbon Stars (Stephenson 1989); no 438 in the first edition(Stephenson 1973) and no 21 in Mayall (1951). It could also be HC193 (Hartwick & Cowley 1988), in which case it is classed as a CHstar. However, this is unlikely because the positional mismatch withHC 193 is more than usual for these pairings, and because Hartwick& Cowley do not note HC 193 as being especially bright. Unfortu-nately, Hartwick & Cowley do not quote a radial velocity for HC193. The star observed as HC 193 by Feast & Whitelock (1992) isclearly much fainter and these authors do doubt their identification.

The star appears to show some variability (see Fig. 5), but that isdeceptive because it is just the SuperCOSMOS measurements thatare fainter than the others. As explained earlier, the SuperCOSMOSSSS measurements of such a bright star could be inaccurate, andeven the two R measurements made from the short-exposure platesare less accurate for KDM 6546 because it is so much brighter thanthe calibration sequences used. So, there is no strong evidence ofvariability for this star.

The JHK colours are consistent with KDM 6546 being a normalbut blue LMC carbon star, a dC star or an FHLC star. However, asalready noted, it is too bright for the first of these possibilities. Noris it likely to be a dC star for the following reasons. First, there isno evidence of the red isotopic bands. Secondly, with MV ∼ +10(Green & Margon 1994) and negligible reddening, it would be ata distance of 21 pc, which is to close for it to have such a highradial velocity. Finally, there is no evidence of it having a propermotion. Its proper motion, as averaged from the SSS and the UCAC1Catalogue (Zacharias et al. 2000), is (�α, �δ) = (6 ± 14, 15 ± 15)mas yr−1. Although this is a null detection, the upper limit is verysmall and well below the usual proper motions of �40 mas yr−1

measured for dC stars. However, the real errors may be greater thanthe mean errors quoted because the star is bright and has strongdiffraction spikes on the Sky Surveys. Nor did Green et al. (1992)find CGCS 1138 (KDM 6546) to have a proper motion when theystudied all the stars in the General Catalogue of Cool Carbon Stars(Stephenson 1989). With such a large radial velocity, KDM 6546could be a halo CH star (Yamashita 1975). As remarked earlier, CHstars can have weak CN bands due to lower metallicity and/or lowerluminosity. The spectral coverage available is insufficient to confirmthis possibility.

1 0 C O M PA R I S O N W I T H T H E M AC H O S U RV E Y

10.1 Identifications

To date, 13 LMC members have been listed in the literature as RCBstars: three – W Men, HV 12842 and HV 5637 – have been knownfor a long time; and more recently, two and then eight were foundthrough the MACHO project (Alcock et al. 1996, 2001), whichmonitored the brightness of several million stars during the period1992–1999. Descriptions of the spectra of the original three havebeen given by Feast (1972), and the spectra of the others have beenplotted by Alcock et al. (1996, 2001). W Men, HV 12842 and twoof the MACHO identifications have spectra that are characteristicof the warm or hot types of RCB star, showing the (1, 0) and (0,0) C2 bands to be no stronger than a level of ∼10–15 per cent ofthe continuum. None of these stars is in KDMK01 because bandsthat weak would be too difficult to detect in the objective-prism

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spectra on which the catalogue was based. Three of the remain-ing nine stars (the cool RCB stars) are in KDMK01 – HV 5637(KDM 2492) plus two which were not included in the 2dF observa-tions. These stars are: MACHO∗05:11:31.4–67:55:52 ≡ HV 5637≡ KDM 2492; MACHO∗05:01:00.2–69:03:43 ≡ KDM 1424; MA-CHO∗05:26:33.8−69:07:33 ≡ KDM 4031. Five of the six MACHOidentifications that are absent from KDMK01 lie in crowded partsof the LMC where the catalogue is far from complete. An alterna-tive explanation for their absence from KDMK01 is that they mighthave been at minimum light when the search plates were taken; forexample, see the note on KDM 7101 in Section 9.1.5.

Three of the four DY-Per-like stars listed by Alcock et al. (2001)are found in the KDMK01 catalogue: they are MACHO∗05:16:51.8–68:45:17 ≡ KDM 3071, MACHO∗05:46:13.0–71:07:40 ≡ KDM 6099 and MACHO∗05:19:56.0–69:48:06 ≡KDM 3353, but none was observed with 2dF. The one absent fromKDMK01 is in a crowded field but could also have been missedthrough its variability. Of course, the selection processes describedearlier would not have led to the identification of any DY-Per-likestars because their spectra are indistinguishable, superficially atleast, from those of the N stars. To summarize: HV 5637 was theonly known RCB or DY-Per-like star observed with 2dF.

There remains the question of why the new candidate RCB starsof Table 1 were not detected in the MACHO survey. KDM 7101lies outside the main MACHO 22-field area, but KDM 5651 andKDM 2373 lie within it and could have been identified. As seenin Fig. 8, KDM 2373 does undergo an enormous decline towardsthe end of the MACHO observing period. It could be that KDM2373 was not selected directly from the MACHO data because thesteady series of relatively small declines nullified the requirement ofa rapid decline. KDM 5651 does not show any significant variationduring the period of the MACHO observations (see Fig. 8) and sowould not have been selected. However, it does show a decline of∼2 mag or more outside that period. Given the fact that some RCBstars such as HV 5637 (KDM 2492) and R CrB itself (Alcock et al.2001) can remain at maximum light for longer than the 7 years ofthe MACHO survey, it must be assumed that KDM 5651 falls into this category. Alcock et al. (2001) estimate that they would havefailed to detect 25 per cent of the RCB stars for this reason. No RCBstars were found by Alcock et al. (2001) in their six-field area of theSMC. However, RAW 21 is not in that area.

10.2 Statistics

Three LMC stars out of ∼1500 observed with 2dF were identifiedas candidate RCB stars and one from 46 observed with FLAIR. Inthe SMC, the number is one from ∼730. The FLAIR sample is toosmall to be statistically useful, but the 2dF samples yield identifi-cation levels of 0.20 and 0.14 per cent, respectively. The detectionrate is lower than the true rate because the stars are so variable. Ifan RCB star spends one-third of its time at a faint magnitude thenthe number of detected stars would be about one-third that of thetrue sample because the star could have been too faint at the time ofobservation appropriate to the catalogue compilation, the 2dF spec-troscopy or the photometry (very faint stars were excluded from the2dF observations). However, this figure could be a little low becauseparts of the LMC were covered twice (or more) by KDKM01 andone 2dF field was observed twice.

Alcock et al. (2001) estimate that there are 85 RCB stars within4 kpc of the LMC centre. Of these, 59 should be cool starswith strong C2 bands. Interpolating the calculations of Blanco &McCarthy (1983), one would expect ∼9000 carbon stars within the

same area. Hence, the expected number of RCB stars would be 54if the 2dF results were similarly extrapolated to the same area. Thisnumber is consistent with the value of 59 derived from the MACHOdata given the uncertainties associated with all of these numbers andthe usual problems with low number statistics.

1 1 S U M M A RY

The red CN bands are almost completely absent from five out of∼2300 carbon stars observed with 2dF and FLAIR. The spectraof these five stars are similar to the spectra of cool RCB stars.One was already known as an RCB star. All five stars have near-infrared colours like those of the RCB stars. Three of the newlyidentified stars exhibit deep minima of at least 3–4 mag and anotherhas a moderately deep minimum of least ∼2 mag. Thus, it is highlylikely that these four are new RCB stars. The final proof will requireproperly sampled light curves. One is an SMC member and becomesthe first RCB star known in that galaxy.

The radial velocities of the LMC stars lie close to the lower limitof the velocities of the N stars and are a better match with velocitiesof the planetary nebulae, thereby supporting the view that RCBstars evolved from planetary nebulae by means of a final heliumshell flash. However, the statistics are too poor for this to be a firmconclusion. The SMC RCB candidate has a velocity close to themean for that galaxy.

A sixth star with weak CN bands is a Galactic star. Although itsluminosity is not yet determined, it is unlikely to be a dC star. It hasa large radial velocity and is perhaps a halo CH star.

AC K N OW L E D G M E N T S

The authors are grateful to the staff of the Anglo-Australian Obser-vatory for assistance with the observations, to the Australian TimeAssignment Committee for the allocation of telescope time, to the2-MASS and DENIS project teams for making the infrared pho-tometry available and to the MACHO project team for providingthe MACHO light curves. Thanks are also given to members of theUKST and WFAU staffs at the AAO and Edinburgh for providingthe photographic plates and measurements. Finally, the referee isthanked for helpful comments and for directing the authors to theMACHO data base.

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