Spectral Types of Planetary Host Star Candidates Two New Transiting Planets

a r X i v :a s t r o -p h /0207192v 1 9 J u l 2002

1

Abstract.Recently,46low-luminosity object transits were re-ported from the Optical Gravitational Lensing Experiment.Our follow-up spectroscopy of the 16most promising candidates provides a spectral classi?cation of the primary.Together with the radius ratio from the transit measurements,we derived the radii of the low-luminosity companions.This allows to exam-ine the possible sub-stellar nature of these objects.Fourteen of them can be clearly identi?ed as low-mass stars.Two objects,OGLE-TR-03and OGLE-TR-10have companions with radii of 0.15R ⊙which is very similar to the radius of the transiting planet HD 209458B.The planetary nature of these two objects should therefore be con?rmed by dynamical mass determina-tions.

Key words:binaries:eclipsing -stars:low-mass -stars:brown dwarfs -stars:planetary systems

A&A manuscript no.

(will be inserted by hand later)

ASTRONOMY

AND ASTROPHYSICS

Send offprint requests to:Dreizler,

dreizler@astro.uni-tuebingen.de the radii of the visible primaries and of the invisible secon-daries were derived.Up to now,no spectroscopic information of the primary is available.The goal of this project is to provide this information and to infer the nature of these low-luminosity companions.

We will describe the observations,data reduction and dis-cuss the determination of the spectral types of the primaries in Sect.2.The results are discussed in Sect.3.

2.Observations,Data Reduction,and Spectral Types of

the Primary Stars

We selected16candidates from the list of Udalski et al.(2002), 13of these have the smallest predicted companion radii.The spectra were obtained as back-up program by one of us(T.R.) at the SAAO1.9m telescope using the Grating Spectrograph equipped with a266×1798SITe chip.This follow-up will be continued to complete the whole list of Udalski et al.(2002). The grating7provides a spectral resolution of5?A,exposure times were set to1800sec for all objects.Standard data reduc-tion of these long-slit spectra was performed using IRAF1and included bias subtraction,?at-?eld correction as well as wave-length and?ux https://www.sodocs.net/doc/2c5951668.html,paring the targets from the list of Udalski et al.(2002),we note that OGLE-TR-08is identical to OGLE-TR-29.

The obtained spectra are compared to the spectral library of Silva&Cornell(1992)which provides templates in steps of about0.3spectral classes.We use them without interpola-tion within the library.The quality of the match between ob-served and template spectrum is determined with aχ2test.Re-binning the observed spectra and the templates to a common wavelength grid with590spectral bins,we obtain reducedχ2 close to unity for the best?ts.Deviation in theχ2from the best?t to the neighboring templates corresponds to deviations of more than3σ.The?tting therefore provides a

classi?cation

Dreizler et al.:Spectral Types of Planetary Host Star Candidates:Two New Transiting Planets?3

400045005000

550060006500

wavelength/?

10

20

30

r e l a t i v e f l u x

OGLE?TR?09 A3V OGLE?TR?02 A7V

OGLE?TR?38 A8V OGLE?TR?04 F0V OGLE?TR?05 F0V OGLE?TR?14 F0V OGLE?TR?32 F0V

OGLE?TR?40 F0V OGLE?TR?45 F7V OGLE?TR?03 F9V OGLE?TR?08 F9V OGLE?TR?12 F9V

OGLE?TR?35 F9V OGLE?TR?10 G2V OGLE?TR?06 G2V OGLE?TR?19 K4V

Fig.2.Spectra of our target stars with our spectral classi?cation.The spectral types cover stars from the maximum down to vanishing Balmer lines.Also visible is the maximum strength of the Ca H and K doublet as well as the increasing strength of the G-band.We do not ?nd spectral signature of the low-luminosity companion.

better than half a spectral class.In Fig.1we compare the most promising candidate OGLE-TR-10with the best matching tem-plate and the next earlier and later library spectrum.While e.g.hydrogen Balmer lines become too shallow in the G6V tem-plate compared to the target star,they are too strong in the F9V template.We restricted the classi?cation to the luminosity class V since the observed orbital periods indicate an orbital separa-tion of the order of ten solar radii and therefore prohibit the presence of a larger star.The spectral classi?cations of all ob-jects are displayed in Fig.2.The presence of the companion could not be detected from our data,neither from double lined spectra nor from the ?ux distribution.

We then used the derived spectral classes to estimate the stellar radii of the primary stars (Tab.1)using the tabulated val-ues from Cox (2000).The photometric monitoring of Udalski et al.(2002)provides the brightness variation during eclipses.Assuming a negligible radiation from the secondary and a cen-tral passage in front of the primary this brightness variation is directly proportional to the radius ratio.Multiplied with the

primary radius it yields the radius of the secondary.Finally,we used the evolutionary models for low mass stars (thick lines Fig.3)to obtain the mass of the secondary assuming it to be a low-mass star.The more sophisticated approach towards ra-dius ratios,i.e.to model the eclipse light curves with the de-rived primary radii as constraint seems to be unnecessary with the current data set,because the error for the companion ra-dius is dominated by the uncertainty of spectral classi?cation of the primaries and of the tabulated radii of the spectral type standards.Table 1additionally provides an estimate of the un-certainties in the companion radius introduced by our spectral classi?cation.This error is small enough to obtain a quite clear picture of the nature of the secondary star.3.Discussion

The range of secondary radii is displayed in the Hertzsprung-Russell-Diagram (Fig.3)together with evolutionary tracks of Baraffe et al.(1998),Chabrier et al.(2002),and Baraffe et al.

4Dreizler et al.:Spectral Types of Planetary Host Star Candidates:Two New Transiting Planets?

4000

4500

5000550060006500

wavelength/?

012

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r e l a t i v e f l u x

OGLE?TR?10

F9V

G2V

G6V

Fig.1.OGLE-TR-10(bottom)compared to three template spectra with the best matching one in the middle.Main differ-ences are the strengths of the Balmer lines and of the G-Band.Table 1.Light-curve variations (Udalski et al.2002),spectral types (SP),and derived quantities,i.e.primary radius,compan-ion radius ratio,companion radius and mass in solar units,as well as mass ratio.The companion mass is derived assuming it is a low-mass star (thick lines Fig.3).Typical error estimates for derived companion radius and mass in the bottom part.See text for details.

219A7 1.620.130.210.200.11319F9 1.140.130.150.130.12465F0 1.500.240.360.370.23543F0 1.500.200.300.300.19653G2 1.000.220.220.220.22848F9 1.140.210.240.230.21948A3 1.980.210.410.430.181022G2 1.000.140.150.130.131238F9 1.140.190.210.200.181434F0 1.500.180.260.260.161965K40.750.240.180.170.243234F0 1.500.180.260.260.163530F9 1.140.170.190.180.163848A8 1.580.210.330.340.194026F0 1.500.150.230.220.144562F7 1.220.240.290.290.231022F9 1.140.140.160.150.131022G2 1.000.140.150.130.1310

22

G6

0.91

0.14

0.13

0.11

0.12

Dreizler et al.:Spectral Types of Planetary Host Star Candidates:Two New Transiting Planets?5

3.6 3.5 3.4

3.3 3.2 3.1 3.0

log T eff /K

-5

-4

-3

-2-1

l o g L /L O ?

0.50.40.30.20.150.10.08

0.05

0.02

0.01

0.005

0.002

0.4

0.350.30.250.20.150.1

5Gyr

0.100.150.20

0.25

0.300.35

R/R O ?

0.00.10.20.30.4M /M O ?

https://www.sodocs.net/doc/2c5951668.html,panion radii compared to evolutionary tracks of Baraffe et al.(1998),Chabrier et al.(2002),and Baraffe et al.(2002)in the HRD.Thick lines:stellar models,dashed lines:brown-dwarf models,dotted lines:gas-giant models.Note that the sub-stellar models are for isolated objects.Masses and radii are given in solar units.The inset ?gure shows the mass-radius relation for low-mass stars at an age of 5Gyr.

candidates could be identi?ed as low-mass stars.Two objects did,however,pass this spectroscopic test and therefore con-tinue to qualify as planetary candidates.The ultimate determi-nation of their nature does require a detailed study of radial ve-locity variations with very high precision.Dynamical mass de-termination of the secondaries with less demanding instrumen-tal requirement will provide more insight in the mass-radius relation at the lower end of the main sequence.

Acknowledgements.We use observations made at the South African Astronomical Observatory (SAAO).T.R.acknowledges a travel grant from the DFG (RA 733/11-1).This research was supported by the DLR under grant 50OR 0201(T¨u bingen).

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