Determining Age on Two Cooling Sequences: Sub-Stellar Brown Dwarfs and Evolved White Dwarfs

Sandy Leggett1, Pierre Bergeron2, Ben Burningham3, Mike Cushing4, Nicolas Lodieu5,

Mark Marley6, Atsuko Nitta1, David Pinfield3, Didier Saumon7, Steve Warren8

1Gemini Observatory, 2University of Montreal CA, 3University of Hertfordshire UK, 4University of Hawaii US, 5IAC Spain, 6NASA-Ames US, 7LANL US, 8Imperial College London UKMauna Kea Summit:

UKIRT, UH 88”, Gemini and CFHT

The Wide Field Camera on UKIRT

Teff K log g (cgs)

[M/H] R solar

Mass Jupiter

Age Gyr Distance pc

ULAS 0034

550 4.48 +0.3 0.109 15 1 - 2 14

550 5.00 +0.3 0.091 30 4 - 10 10

600 4.00 +0.3 0.125 5 0.1 - 0.2 17

600 4.48 0 0.109 15 1 - 2 14

600 4.48 +0.3 0.110 15 1 - 2 14

600 5.00 +0.3 0.092 30 4 - 10 12

ULAS 1335

550 5.00 +0.3 0.091 30 4 - 10 9

550 5.30 +0.3 0.083 50 10 8

600 4.48 0 0.109 15 1 - 2 12

CFBD 0059

550 5.00 +0.3 0.091 30 4 - 10 9

550 5.30 +0.3 0.083 50 10 8

600 5.00 +0.3 0.092 30 4 - 10 12

600 5.30 +0.3 0.084 50 10 11

REFERENCES

Adelman-McCarthy et al. 2007, ApJS 172, 634Bergeron et al. 1992, ApJ 394, 228Bergeron et al. 2005, ApJ 625, 838Burgasser et al. 2006, ApJ 637, 1067Burningham et al. 2008, MNRAS in press, arXiv:0806.0067Catalan et al. 2008, MNRAS, 387, 1693Delorme et al. 2008, A&A 482, 961Fontaine et al. 2001, PASP 113, 409Hall et al. 2008, AJ 136, 76Holberg & Bergeron 2006, AJ 132, 1221Kalirai et al. 2008, ApJ 767, 594Kepler et al. 2008, MNRAS 375, 1315Kilic et al. 2006, AJ 131, 582Knapp et al. 2004, AJ 127, 3553Leggett et al. 1998, ApJ 497, 294Lodieu et al. 2008, ApJ submittedMcLean et al. 2003, ApJ 596, 561Saumon et al. 2007, ApJ 662, 1245Saumon & Marley 2008, ApJ in press, arXiv:0808.2611 Warren et al. 2007, MNRAS 381, 1400

The LAS and the far-red CFHT brown dwarf survey (CFBDS) have discovered three very low- temperature dwarfs that are similar to each other, but are cooler than any previously known brown dwarf: ULAS J003402.77-005206.7 (Warren et al. 2007), ULAS J133553.45+113005.2 (Burningham et al. 2008), and CFBDS J005910.82-011401.3 (Delorme et al. 2008). The objects were identified by their extremely red far- red colors and blue near-IR colors, the latter due to strong absorption bands. The near- IR spectra of these three dwarfs are shown in Figure 1. The spectrum of the T8 spectral standard 2MASS J04151954-0935066 (Burgasser et al. 2006, Knapp et al. 2004, McLean et al. 2003) is also shown; this dwarf has an effective temperature (Teff) of 750K (Saumon et al. 2007). Little flux remains to be absorbed in the troughs of the H2O and CH4 bands, but ULAS 0034 and 1335, and CFBD 0059, have narrower J and H (1.2 and 1.6um) flux peaks, indicating Teff<750K. There is a suggestion of NH3 absorption in the blue wing of the H band. Burningham et al. (2008) suggest a type of T9 for these dwarfs, while Delorme et al. (2008) suggest they may be prototype Y0 dwarfs. Pending the discovery of even cooler dwarfs to define the Y class, we adopt T9 here.

We have obtained 7.6 - 15.2um spectra for ULAS 0034 and 1335 using the Infrared Spectrograph (IRS) on the Spitzer space telescope (Leggett et al. in prep.). Figure 2 shows the observed spectra for ULAS 0034, and a selection calculated by Marley & Saumon. These models include vertical transport of CO and N2 gas, which is required to reproduce the shape of the 8 - 12um spectrum. The mechanism is parameterized by an eddy diffusion coefficient Kzz cm2s-1. We find that the combination of the near- and mid-IR spectra and photometry tightly constrains the range of Teff and also constrains gravity and metallicity, although these two parameters both affect the 2.0um and 4.5um regions and are difficult to separate (Figure 2). We find that ULAS 0034 has Teff = 550 - 600K, surface gravity g = 100 - 1000 ms-2 and metallicity [m/H] = 0 - 0.3. Similar comparisons to near- and mid-IR data for ULAS 1335, and to near-IR data only for CFBD 0059, indicate that: ULAS 1335 has Teff = 550 - 600K, g = 300 - 2000 ms-2, [m/H] = 0 - 0.3; and CFBD 0059 has Teff = 550 - 600K, g = 1000 - 2000 ms-2, [m/H] = 0.3. There is excess model flux at 1.05um and 1.25um. This may be due to: the CH4 opacity being too weak in this region, the absence of NH3 opacity at wavelengths shorter than 1.4um, errors in the 1um pressure-induced H2 opacity, or errors in the red wing of the 0.77um KI line.

Figure 3 shows evolutionary sequences for brown dwarfs by Saumon & Marley (2008). It can be seen that if both Teff and gravity are known then mass and age are also determined. Table 1 gives these properties for the T9 dwarfs, as well as distance, derived from the ratio of the observed and modelled flux, and the radius of the dwarf from evolutionary models. The distances derived for ULAS 0034 range from 10 to 17 pc, in agreement with a preliminary parallax (Smart, priv. comm.). If it is at 10 pc, it is a 4 - 10 Gyr old 30 MJup dwarf, at 15 pc it is a 1 - 2 Gyr old 15 MJup dwarf, and at 17 pc it is a 150 Myr old 6 MJup object. ULAS 1335 is either a 4 - 10 Gyr old 40 MJup dwarf at ~8 pc, or a 1 - 2 Gyr old 15 MJup object at ~12 pc. Both these objects have been imaged at high spatial resolution and are unresolved (Liu, Lodieu, priv. comm.). CFBD 0059 appears to be the most massive and oldest of the three: a 4 - 10 Gyr old 40 MJup object at ~10 pc.

Trigonometric parallaxes combined with analyses of the near- through mid-IR spectral energy distributions will determine mass and age for these cooling sub-stellar objects.

FIGURE 1: Near-IR spectra of the three T9 dwarfs ULAS 0034, ULAS 1335 and CFBD 0059, and the T8 2MASS 0415.

FIGURE 2: Observed near- and mid-IR spectra for ULAS 0034 (black line) and synthetic spectra from Marley & Saumon (red line) for a range of possible properties. Four pairs of lines indicate the observed (black) and calculated (red) IRAC fluxes.

Abstract: We have identified low-mass cool brown dwarfs and high-velocity evolved white dwarfs in the Large Area Survey (LAS) component of the UKIRT Infrared Deep Sky Survey (UKIDSS; Burningham et al. 2008, Lodieu et al. 2008). Both types of object cool with time, and age is a crucial parameter for full characterization.

Brown Dwarfs White Dwarfs

TABLE 1: Range of Possible Properties for the T9 Dwarfs

FIGURE 3: Evolutionary sequences (Saumon & Marley 2008) for cloudless brown dwarfs with masses: 0.08, 0.077, 0.075, 0.07, 0.06, 0.05, 0.04, 0.03, 0.02, 0.01 MSun (black lines, upper right to lower left). Isochrones are shown in blue for: 10, 4, 2, 1, 0.4, 0.2, 0.1, 0.04, 0.02, 0.01 Gyr (left to right).

Name Com-position

Teff (err) K Cooling Age Gyr

Distance (err) pc

Vtan (err) km/s

ULAS 0049 H 6380(140) 1.3 - 3.7 140(25) 80(15)

ULAS 0142 H 5950(140) 1.5 - 4.4 140(25) 70(15)

ULAS 0226 H 5670(160) 1.7 - 5.2 150(30) 80(15)

ULAS 0302 He 5720(160) 1.9 - 5.4 180(35) 100(20)

ULAS 1522 H 5390(180) 2.0 - 6.6 150(30) 70(15)

ULAS 1528 unknown 6110(250) 1.4 - 4.6 200(40) 100(20)

ULAS 1554 unknown 6270(200) 1.3 - 4.2 160(30) 100(20)

SDSS 2242 H=He 3820(100) 7.6 - 9.1 37(6) 29(5)

ULAS 2331 H 6340(220) 1.3 - 3.7 180(20) 120(20)

ULAS 2339 H 6590(240) 1.2 - 3.3 190(35) 90(20)

FIGURE 4: Color-color (left panels) and Reduced Proper Motion (right) plots showing the LAS white dwarfs (red), other white dwarfs (green), the main sequence, selection before motion cut (large black dots), and search regions (red lines). Model sequences for H and He dwarfs are shown as blue solid and dashed lines with points every 250K. Errors in the colors are shown. Sequences for the disk and halo, and subdwarfs (open triangles), are shown in the RPM plot.

TABLE 2: Properties of the LAS White Dwarfs

FIGURE 5: An example fit to the spectral energy distribution and hydrogen line profiles of one of the ~6000K LAS white dwarfs (left two panels), and our fit to the mixed-composition 3800K SDSS white dwarf recovered in our search (right). Error bars represent observed photometry while open and filled circles are model fits for composition as indicated.

As the distances and masses of our sample are unknown, we have adopted the canonical white dwarf mass and gravity of 0.6MSun and log g=8.0 (e.g. Bergeron et al. 1992, Kepler et al. 2008), and allowed a plausible range in mass of 0.4 - 0.8MSun when deriving the uncertainty in their properties. We used synthetic photometry (Holberg & Bergeron 2006) and the evolutionary sequences of Fontaine et al. (2001) to derive Teff, cooling age and distance for each object, for both pure-H and pure-He atmospheres (Figure 5). The nine LAS dwarfs are relatively warm with Teff~6000K; pushing the LAS to faint limits led to such objects being scattered into our catalog selection. Spectroscopy shows six of these to be H-rich due to the presence of strong hydrogen lines, and a seventh to be He-rich, as the spectrum is featureless. The recovered SDSS white dwarf is much cooler and is fit with a mixed composition atmosphere (Figure 5).

We have paired the second Data Release of 282 deg2 of the LAS with the fifth of the Sloan Digital Sky Survey (SDSS; Adelman-McCarthy et al. 2007) to search for cool hydrogen-rich white dwarfs (Lodieu et al. 2008). Cool white dwarfs are old, and can constrain the age of the Galaxy (e.g. Hall et al. 2008, Leggett et al. 1998); their cool high- pressure atmospheres are also of intrinsic interest. Based on modelled colors (Bergeron et al. 2005), as well as observed colors of known white dwarfs, we searched for objects that are neutral to red in the optical, and blue in the near-IR, indicating the presence of pressure-induced H2 opacity (Figure 4). The search based on colors only produced a candidate list of 586 objects. We reduced the sample to 10 by also requiring a proper motion > 0.1”yr-1. This puts our gAB~20 - 21 targets into the lower region of the Reduced Proper Motion diagram (RPM, Figure 4) which acts as a proxy for absolute magnitude for a sample with similar kinematics. One of these ten is a previously known very cool SDSS white dwarf (Kilic et al. 2006), and we obtained optical spectra for seven of the remaining nine candidates. All seven are confirmed by the spectra to be white dwarfs. We suggest the remaining two faint high proper motion objects are also white dwarfs.

Table 2 lists the properties of our sample, including tangential velocity which can be derived from the distance and proper motion. The uncertainty in Teff is due to photometric scatter; that in cooling age, distance and velocity is due to the uncertainty in gravity (or mass). The LAS dwarfs are warmer than the SDSS dwarf, with a shorter cooling age. However they are also more distant, with high velocities typical of the thick disk or halo. While it is clear that the SDSS white dwarf is older than 9 Gyr, the total age of the LAS dwarfs is difficult to constrain. If they are 0.6MSun objects their main-sequence lifetime would be ~2 Gyr and the total age ~5 Gyr; however if they are less massive they could be the descendants of solar-mass stars with a much larger total age (e.g. Catalan et al. 2008, Kalirai et al. 2008). Hence while the SDSS white dwarf is an old member of the disk of the Galaxy, the warmer dwarfs may be either relatively young thin disk objects with unusually high space motion, or they may be old remnants of low-mass stars. Parallaxes will be difficult to obtain for these distant sources, and so this puzzle is unlikely to be solved. We will continue to search larger-area releases of the LAS for white dwarfs cooler than 4500K, which will necessarily be older than 8 Gyr.

Spitzer Space Telescope