Mon. Not. R. Astron. Soc. 000, 1–8 (2007) Printed 6 September 2018 (MN LATEX style file v2.2) The multiplicity of planet host stars — New low-mass companions to planet host stars M. Mugrauer1⋆, A. Seifahrt1,2, R. Neuh¨auser1 † 1Astrophysikalisches Institut, Universit¨at Jena, Schillerg¨aßchen 2-3, 07745 Jena, Germany 2European Southern Observatory, Karl-Schwarzschild-Str. 2, 85748 Garching, Germany Accepted 2007 April 12. Received 2007 April 2; in original form 2007 March 8 ABSTRACT We present new results from our ongoing multiplicity study of exoplanet host stars, carried out with the infrared camera SofI at ESO-NTT. We have identified new low mass companions to the planet host stars HD101930 and HD65216. HD101930AB is a wide binary systems composed of the planet host star HD101930A and its compan- ion HD 101930 B which is a M0 to M1 dwarf with a mass of about 0.7 M⊙ separated from the primary by ∼73arcsec (2200AU projected separation). HD65216 forms a hi- erarchical triple system, with a projected separation of 253 AU (angular separation of about 7arcsec) between the planet host star HD65216A and its close binary compan- ion HD65216BC, whose two components are separated by only ∼0.17arcsec (6AU of projected separation). Two VLT-NACO images separated by 3 years confirm that this system is co-moving to the planet host star. The infrared photometry of HD65216B and C is consistent with a M7 to M8 (0.089 M⊙), and a L2 to L3 dwarf (0.078 M⊙), re- spectively, both close to the sub-stellar limit. An infrared spectrum with VLT-ISAAC of the pair HD 65216 BC, even though not resolved spatially, confirms this late spectral type. Furthermore, we present H- and K-band ISAAC infrared spectra of HD16141B, the recently detected co-moving companion of the planet host star HD16141A. The infrared spectroscopy as well as the apparent infrared photometry of HD16141B are both fully consistent with a M2 to M3 dwarf located at the distance of the planet host star. Key words: stars: individual: HD16141, HD65216, HD101930, stars: low-mass bina- ries: visual, planetary systems arXiv:0704.1767v1 [astro-ph] 13 Apr 2007 1 INTRODUCTION are carried out with seeing limited near infrared imaging (see e.g. Mugrauer et al. 2004a&b, 2005, 2006a) as well as Since the mid nineties of the last century high precision high contrast diffraction limited AO observations (Patience radial-velocity studies revealed more than 200 exoplanet et al. 2002, Luhman & Jayawardhana 2002, Chauvin et al. candidates around mostly solar-like stars. These planet 2006, and most recently Neuh¨auser et al. 2007). In addition, host stars are located in the solar neighborhood and are data from visible and infrared all sky surveys like POSS mostly isolated single stars. However, already among the or 2MASS are used to identify new companions of planet first reported planet detections, three planets were found host stars (see e.g. Bakos et al. 2006 or Raghavan et al. to orbit the brighter component of binary systems, namely 2006). Because of all these efforts, more than 30 planet host 55 Cnc AB, υ AndAB and τ Boo AB (Butler et al. 1997). multiple star systems are known today, suggesting that the Since that time more and more planet host multiple multiplicity of planet host stars is at least 20 %. star systems were identified, most of them being found in multiplicity studies of the planet host stars. These studies Most of the detected stellar companions to planet host stars are low-mass main sequence stars. The projected sep- arations of these companions to the planet host stars range ⋆ E-mail: [email protected] † Based on observations obtained on La Silla in ESO programs from only a few tens of AU up to more than 5000 AU. In a 075.C-0098(A), 077.C-0572(A) and 078.C-0376(A), as well as on few cases the companions themselves turned out to be close Paranal in ESO programs 070.C-0557(A), 076.C-0057(A) and binaries, i.e. these systems are hierarchical triples (see Mu- 078.C-0376(B). grauer et al. 2007 for a summary). Not only main-sequence c 2007 RAS 2 Mugrauer et al. stars but also white dwarfs were revealed as companions We observed HD 65216 and HD 101930 for our multi- of exoplanet host stars, suggesting that planetary systems plicity study of planet host stars, using the infrared camera also exist in evolved multiple star systems. Three white SofI at the ESO-NTT at La Silla observatory. All observa- dwarfs were found so far at close (Gl 86 B, ∼20 AU, see Mu- tions were obtained in the H-band with the SofI small-field grauer & Neuh¨auser 2005, Lagrange et al. 2006), intermedi- camera (147 arcsec×147 arcsec field of view). By comparing ate (HD 27442 B at ∼240 AU, see Chauvin et al. 2006 and two images of the planet host stars taken at different ob- Mugrauer et al. 2007) and at wide separations (HD 147513 B serving epochs we can find co-moving companions of these at ∼5400 AU, see Mayor et al. 2004). Recently, the first di- stars and distinguish them from non- or only slowly moving rectly imaged substellar companion of an exoplanet host background sources. star, the T7.5±0.5 brown dwarf HD 3651 B, was discovered The first epoch observations of HD 65216 and (see Mugrauer et al. 2006b, Burgasser 2006, Liu et al. 2007, HD 101930 were both obtained in June 2005 with follow-up and Luhman et al. 2007). second epoch imaging one year later in June 2006. In order In this letter we present new results of our multiplic- to limit saturation effects of the detector due to the bright ity study carried out with the ESO-NTT at La Silla obser- planet host stars, we always used the shortest available in- vatory. We have discovered new stellar companions to two tegration time (1.2 s), and 50 of these 1.2 s integrations were planet host stars. We show the results of our near infrared averaged to one image. The standard dither (jitter) tech- imaging observations in section 2. Section 3 summarizes the nique was applied to subtract the bright infrared sky back- infrared H- and K-band spectroscopic data obtained for the ground. In total 10 images were taken at 10 different dither new companions presented here, and also for the close planet positions, resulting in a total integration time of 10 min. The host star companion HD 16141 B, for which we have already data reduction, i.e. background estimation and subtraction, presented the astrometric confirmation of companionship in flat-fielding of all images as well as the final shift and add Mugrauer et al. (2005). In the last section we discuss the procedure was done with the ESO data reduction package properties of the newly found companions. ECLIPSE (Devillard 2001). Figure 1 shows our second epoch SofI H-band images of HD 65216 and HD 101930, in which the primaries are located in the center of each image. 2 OBSERVATIONS All images are astrometrically calibrated with sources HD 65216 is a nearby G5 dwarf, located in the south- detected in our SofI images and listed in the 2MASS point ern constellation Carina. Its proper motion and paral- source catalogue (Skrutskie et al. 2006). The astrometric lax (µαcos(δ) = −122.12 ± 0.98 mas/yr, µδ = 145.90 ± calibration of both SofI observing runs is summarized in 0.64 mas/yr, and π = 28.10±0.69 mas) are both determined Tab. 2. by Hipparcos (Perryman et al. 1997), yielding a distance of By comparing our first epoch H-band images with the ∼ 36 pc. According to Santos et al. (2004) HD 65216 has an second epoch images we determined the proper motion of effective temperature of 5666±31 K and a surface gravity of all objects (S/N > 10) detected around the planet host log(g) = 4.53 ± 0.09 cm/s2, as expected for a mid G main- stars within the SofI field of view. As illustrated in Fig. 2, sequence star. The same group also determined the mass of most of the detected objects have a small or even negligi- the star to be 0.94 M⊙. Saffe et al. (2005) derived an up- ble proper motion. However, we have identified two sources per age limit for HD 65216 to be 10.2 Gyr, obtained from which share the proper motion of the planet host stars, indi- the stellar metallicity ([Fe/H]= −0.12). A variation of the cated as boxes in Fig. 2. Thus, these sources are co-moving radial velocity of HD 65216 with a period of 613 day is re- companions of the planet host stars and will be denoted ported by Mayor et al. (2004), indicating that the star is HD 65216 B and HD 101930 B from hereon. orbited by a Jupiter-mass planet (m · sin(i) = 1.21 MJup) We measured the H-band photometry and relative as- on an eccentric (e = 0.41) orbit with a semi-major axis of trometry of both detected co-moving companions in our a = 1.37 AU. SofI H-band images. The results are summarized in Tab. 2. HD 101930 is a nearby (31 pc) dwarf of spectral type HD 101930 B is already too bright and exceeds the linearity early K, located in the southern part of the constella- limit of the SofI detector, hence we cannot give an accu- tion Centaurus, close to the famous constellation South- rate H-band photometry of this object.