Ground-based observationsof Kepler asteroseismic targets

Joanna Molenda-Żakowicz

Instytut Astronomiczny Uniwersytetu WrocławskiegoPOLAND

Kepler asteroseismic targets

what are these objects? pulsating, preferably solar-type stars that will be observed by

the Kepler space telescope for what reason?

to study stellar interiors via asteroseismic methods

what this study will result in? precise radius and mass of the stars can yield precise

parameters of their planetary systems providing that the dedicated asteroseismic models of the stars are computed

Ground-based observations

of which objects? stars that are candidates for Kepler asteroseismic targets

for what reason?

to determine their atmospheric parameters: Teff

, logg, and [Fe/H], and to measure their radial velocity, v

r ,and projected

rotational velocity, v sin i

what this study will result in? it will allow to compute dedicated asteroseismic and

evolutionary models of Kepler asteroseismic targets

Observing sites

Harvard-Smithsonian Center for Astrophysics, USA

Oak Ridge Observatory, Harvard Massachusetts: 1.5-m Wyeth reflector

Fred Lawrence Whipple Observatory, Mount Hopkins, Arizona: 1.5-m Tillinghast reflector

Multiple Mirror Telescope (before it was converted to the monolithic 6.5-m mirror)

Nordic Optical Telescope

Location: Canary Islands, Spain

Altitude: 2,382 m.a.s.l.

Targets:

the faintest candidtes for Kepler asteroseismic targets

stars from open clusters

Photo: Michael J.D. Linden-Vørnle and Bob Tubbs

Nordic Optical Telescope

2.5-m telescope

FIES instrument

a cross-dispersed high-resolution echelle spectrograph

maximum spectral resolution: R = 65 000

the spectral range: 370-740 nm

Photo: Michael J.D. Linden-Vørnle and Bob Tubbs

Wrocław University Observatory

Location: Astrophysical Observatory of the University of Wrocław, Białków, Poland

Targets: open clusters

In the figures: the dome and the 60 cm Cassegrain telescope in Białków

Czech Academy of Sciences Observatory

Location: Ondrejov (Czech Republic)

Altitude: 500 m.a.s.l.

2-m telescope used for high-dispersion coude spectroscopy

Targets: selected binaries from the list of candidates for Kepler asteroseismic targets

Photo: Josef Havelka and Aleš Kolář

Slovak Academy of Sciences Observatory

Location: Tatranska Lomnica (Slovak Republic)

In the figures: the dome and the 60-cm Cassegrain telescope in Tatranska Lomnica

Catania Astrophysical Observatory

Location: Fracastoro Mountain Station, Mt. Etna. Italy

elevation 1,735 m a.s.l

> 200 clear nights per year

occasional breaks in observations due to the activity of Etna

Catania Astrophysical Observatory

Instruments

Telescope

Optical configuration: Cassegrain

Main mirror: 91-cm, paraboloid

Secondary mirror: 24-cm

Mount type: German (see the next figure)

Photometer

Single channel photometer

Filters: Johnson system: U B V Strömgren system: u b v y

H (narrow and wide) Comet narrow band IHW

system

In the figure: the photometer and additional equipment in the Catania astrophysical laboratory.

Spectrograph

Fiber-optics Reosc Echelle Spectrograph of Catania Observatory, FRESCO

Gratings

echellette (cross-disperser), reflection grating of 160x106 mm with 300 l/mm

blazed at 4.3 deg

maximum efficiency 80% at the blaze wavelength 5000 A

Spectrograph

Dispersion

varies from 3.5 A/mm at H

to 6.8 A/mm at H (R=21,000)

The spectral range covered in one exposure is about 2500 A in 19 orders

Spectrograph

Performances

radial velocity measurements precision v < 0.3 km/s rms

S/N at H 100 with Texp

= 10 s for V=6 mag star

limiting magnitude V=11 with S/N =30 and T

exp = 1 h

Calibration lamps

halogen flat field lamp at about 2,600oC

Thorium-Argon hollow cathode lamp

Methodology of observations

Calibration images - Bias

measured at the beginning and the end of each night (typically six measurements in total)

the mean is subtracted from flat fields, calibration lamps and stellar spectra

Calibration images - Flat Field

measured at the beginning and the end of each night (typically six measurements in total)

needed for correction for the shape of the blaze function

Calibration images - Flat Field

each spectrum (calibration lamps and stellar spectra) is divided, order by order, by the fit to the mean flat field

in the figure - the second order of the fit to the mean flat field

Calibration images - Thorium-Argon Lamp

measured 2-3 times per night

needed to place the stellar spectra on the Angstrom scale

Calibration images - Thorium-Argon Lamp

in the figure: emission lines in the spectrum of Thorium-Argon lamp

the emission lines have to be identified in each order

Stars: Oph (K2III)

radial velocity standard

needed for measuring radial velocity of program stars

observed each night

Oph (K2III)

Targets of observations

Targets

standard stars radial velocity standards, e.g,. Ophiuchi stars with well-known spectral types needed for MK

classification fast rotating stars, e.g., Altair needed for the removal of

telluric lines program stars

all the candidates for Kepler asteroseismic targets at least two spectra per star

Primary asteroseismic targets

15 stars which fall onto active pixels of Kepler CCDs

V = 9-11 mag

have precise Hipparcos parallax so that their luminosity can be computed from it

Secondary asteroseismic targets

44 stars which fall onto active pixels of Kepler CCDs

V = 9-11 mag

the Hipparcos parallax are not precise so that the star's luminosity can not be computed from it

Brightest asteroseismic targets

34 stars which fall onto active pixels of Kepler CCDs

V = 8-9 mag

have precise Hipparcos parallax – star's distance and luminosity can be computed

NGC 6811

the candidates for Kepler asteroseismic targets are plotted with green symbols

stars are labeled with WEBDA numbers or with running numbers

red rectangles show the fields observed in Tatranska Lomnica

NGC 6866

the candidates for Kepler asteroseismic targets are plotted with green symbols

stars are labeled with WEBDA numbers or with running numbers

red rectangles show the fields observed in Tatranska Lomnica

Results

Radial velocity measurements

The method: the cross-correlation; the template - Oph

The tool: iraf software

HIP 94734 – SB1

discovered in the ground-based data to be a single-lined spectroscopic binary (see Molenda-Żakowicz et al. 2007 AcA 57, 301)

SB2 stars

show double peak in the cross-correlation function (here: an SB2 star HIP 94335)

SB2 stars – HIP 94335

radial velocity of the primary (red) and secondary (blue) component of the SB2 Algol-type system HIP 94335

Measurements of v sin i

measured with the use of a grid of Kurucz model spectra

and with the Full Width Half Maximum method

in the figure: determination of of v sin i of both components of HIP 94335

Determination of atmospheric parameters

measured by comparison with the grid of spectra of reference stars (see Frasca et al. 2003 A&A 405, 149, Frasca et al. 2006 A&A 454, 301)

the method allows simultaneous and fast determination of logTeff, log g and [Fe/H] even for stars which spectra have low signal-to-noise ratio or limited resolution

requires a dense grid of template spectra of stars with precisely determined atmospheric parameters

in the figure: the reference stars in the logTeff – log g – [Fe/H] space

How this method works

the spectrum of the program star is compared with all template spectra

the best-fitting five template spectra are selected

adopted are weighted means of Teff, log g and [Fe/H] of the five templates that have spectra most similar to the spectrum of the program star

log Teff – log g diagram for Kepler primary asteroseismic targets

Evolutionary and asteroseismic models – HIP 94734

model computed with the use of Monte Carlo Markov Chains. On the right: marginal distributions of model parameters: age and mass. (Bazot et al. in preparation)

mass = 1.114±0.023 M

age = 7.070 ±0.79 Gyr

large separation of solar-like oscillations, = 106.5 ± 3.8 Hz