arXiv subjects

Markus Mugrauer

Publications and source records attributed to Markus Mugrauer.

27 records · Page 2Linked to original sources

arXiv2008

A search for wide brown dwarf companions to stars within 10pc

We present the first results of a large imaging survey to identify wide brown dwarf companions to stars within 10 pc. We have performed a deep (H-band limit ~ 19.0 mag), wide field (up to 16x16 arcmin) astrometric imaging campaign in two epochs around more than 230 nearby stars. Preliminary results show that the wide low-mass companion fraction is far lower than expected, indicating that interactions with the galactic disk may have removed the weakly bound wide companions around old stars.

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arXiv2008

Astrometric and photometric monitoring of GQ Lup and its sub-stellar companion

Neuhaeuser et al. (2005) presented direct imaging evidence for a sub-stellar companion to the young T Tauri star GQ Lup. Common proper motion was highly significant, but no orbital motion was detected. Faint luminosity, low gravity, and a late-M/early-L spectral type indicated that the companion is either a planet or a brown dwarf. We have monitored GQ Lup and its companion in order to detect orbital and parallactic motion and variability in its brightness. We also search for closer and fainter companions. We have taken six more images with the VLT Adaptive Optics instrument NACO from May 2005 to Feb 2007, always with the same calibration binary from Hipparcos for both astrometric and photometric calibration. By adding up all the images taken so far, we search for additional companions. The position of GQ Lup A and its companion compared to a nearby non-moving background object varies as expected for parallactic motion by about one pixel (2 πwith parallax π). We could not find evidence for variability of the GQ Lup companion in the K-band (standard deviation being \pm 0.08 mag), which may be due to large error bars. No additional companions are found with deep imaging. There is now exceedingly high significance for common proper motion of GQ Lup A and its companion. In addition, we see for the first time an indication for orbital motion (about 2 to 3 mas/yr decrease in separation, but no significant change in the position angle), consistent with a near edge-on or highly eccentric orbit. We measured the parallax for GQ Lup A to be π= 6.4 \pm 1.9 mas (i.e. 156 \pm 50 pc) and for the GQ Lup companion to be 7.2 \pm 2.1 mas (i.e. 139 \pm 45 pc), both consistent with being in the Lupus I cloud and bound to each other.

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arXiv2006

Direct detection of exoplanet host star companion gamma Cep B and revised masses for both stars and the sub-stellar object

The star gamma Cep is known as a single-lined spectroscopic triple system at a distance of 13.8 pc, composed of a K1 III-IV primary star with V = 3.2 mag, a stellar-mass companion in a 66--67 year orbit (Torres 2006), and a substellar companion with M_p sin i = 1.7 M_Jup that is most likely a planet (Hatzes et al. 2003). We aim to obtain a first direct detection of the stellar companion, to determine its current orbital position (for comparison with the spectroscopic and astrometric data), its infrared magnitude and, hence, mass. We use the Adaptive Optics camera CIAO at the Japanese 8m telescope Subaru on Mauna Kea, Hawaii, with the semi-transparent coronograph to block most of the light from the bright primary gamma Cep A, and to detect at the same time the faint companion B. In addition, we also used the IR camera Omega-Cass at the Calar Alto 3.5m telescope, Spain, to image gamma Cep A and B by adding up many very short integrations (without AO). gamma Cep B is clearly detected on our CIAO and Omega-Cass images. We use a photometric standard star to determine the magnitude of B after PSF subtraction in the Subaru image, and the magnitude difference between A and B in the Calar Alto images, and find an average value of K = 7.3 \pm 0.2 mag. The separations and position angles between A and B are measured on 15 July 2006 and 11 and 12 Sept 2006, B is slightly south of west of A. By combining the radial velocity, astrometric, and imaging data, we have refined the binary orbit and determined the dynamical masses of the two stars in the gamma Cep system, namely 1.40 \pm 0.12 M_sun for the primary and 0.409 \pm 0.018 M_sun for the secondary (consistent with being a M4 dwarf). We also determine the minimum mass of the sub-stellar companion to be M_p sin i = 1.60 \pm 0.13 M_Jup.

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arXiv2006

Detectability of planets in wide binaries by ground-based relative astrometry with AO

Many planet candidates have been detected by radial velocity variations of the primary star; they are planet candidates, because of the unknown orbit inclination. Detection of the wobble in the two other dimensions, to be measured by astrometry, would yield the inclination and, hence, true mass of the companions. We aim to show that planets can be confirmed or discovered in a close visual stellar binary system by measuring the astrometric wobble of the exoplanet host star as periodic variation of the separation, even from the ground. We test the feasibility with HD 19994, a visual binary with one radial velocity planet candidate. We use the adaptive optics camera NACO at the VLT with its smallest pixel scale (13 mas) for high precision astrometric measurements. The separations measured in single 120 images taken within one night are shown to follow white noise, so that the standard deviation can be devided by the square root of the number of images to obtain the precision. In this paper we present the first results and investigate the achievable precision in relative astrometry with adaptive optics. With a careful data reduction it is possible to achieve a relative astrometric precision as low as 50 micro arc sec for a 0.6 arc sec, binary with VLT/NACO observations in one hour, the best relative astrometric precision ever achieved with a single telescope from the ground. The relative astrometric precision demonstrated here with AO at an 8-m mirror is sufficient to detect the astrometric signal of the planet HD 19994 Ab as periodic variation of the separation between HD 19994 A and B.

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arXiv2006

A Search for wide visual companions of exoplanet host stars - The Calar Alto Survey

We have carried out a search for co-moving stellar and substellar companions around 18 exoplanet host stars with the infrared camera MAGIC at the 2.2m Calar Alto telescope, by comparing our images with images from the all sky surveys 2MASS, POSS I and II. Four stars of the sample namely HD80606, 55Cnc, HD46375 and BD-103166, are listed as binaries in the Washington Visual Double Star Catalogue (WDS). The binary nature of HD80606, 55Cnc, and HD46375 is confirmed with both astrometry as well as photometry, thereby the proper motion of the companion of HD46375 was determined here for the first time. We derived the companion masses as well as the longterm stability regions for additional companions in these three binary systems. We can rule out further stellar companions around all stars in the sample with projected separations between 270AU and 2500AU, being sensitive to substellar companions with masses down to \~60MJup (S/N=3). Furthermore we present evidence that the two components of the WDS binary BD-103166 are unrelated stars, i.e this system is a visual pair. The spectrophotometric distance of the primary (a K0 dwarf) is ~67pc, whereas the presumable secondary BD-103166B (a M4 to M5 dwarf) is located at a distance of 13pc in the foreground.

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arXiv2005

New brown dwarf companions to dM(e) stars

We present new astrometric and spectroscopic data to confirm two new M/L dwarf systems, G124-62 and LHS5166, and discuss the nature of a third system (LP261-75). Age and thus mass determinations of the L dwarf companions are discussed based on various activity-age relationships of the M dwarf primaries. This publication will update the list of widely separated substellar companions to nearby stars.

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arXiv2004

Astrometric proof of companionship for the L dwarf companion candidate GJ 1048B

Gizis et al. (2001) reported a companion candidate of spectral type L1 near the K2 dwarf GJ 1048 using the Two Micron All-Sky Survey (2MASS). At that time it was not possible to verify companionship astrometrically using the 2MASS data alone due to the small proper motion of GJ 1048. We now show that both objects share the same proper motion by using data from the UK Schmidt Telescope Near-infrared (IVN) Southern Survey as the first epoch and data from 2MASS as the second epoch. Our technique of subtracting the PSF of the primary from the SuperCOSMOS I scans of the Southern Survey enables the astrometry of the companion candidate to be measured directly.

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arXiv2002

Infrared imaging and spectroscopy of companion candidates near the young stars HD 199143 and HD 358623 in Capricornius

We present JHK images of the young (~20 Myrs) nearby (~48 pc) stars HD 199143 and HD 358623 to search for (sub-)stellar companions around them. The images were obtained in JHK with the speckle camera SHARP-I in July 2001 and in H with SofI in Dec 2000, July 2001, and Dec 2001, all at the ESO 3.5m NTT. We present a companion candidate with a 2 arc sec offset being almost 2 mag fainter than HD 358623 with proper motion consistent with the known proper motion of the primary star HD 358623 A. Then, we obtained a spectrum in the H-band (with SofI at the NTT) of this companion showing that it has spectral type M1-3, consistent with being a companion of HD 358623 A (K7-M0). Also, a companion candidate with a 1 arcsecond offset being 2 mag fainter than HD 199143 is detected. The JHK colors of this candidate are consistent with a spectral type M0-2. We determine the ages and masses of all four objects from theoretical tracks and isochrones, all four stars appear to be co-eval with ~20 Myrs. Our limits for additional detectable but undetected companions are such that we would have detected all stellar companions with separations > 0.5 arc sec (24 AU at 48 pc).

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arXiv2002

Imaging and spectroscopy of DENIS-P J104814-395606

We obtained deep H- and K-band images of DENIS-P J104814-395606 using SofI and the speckle camera SHARP-I at the ESO-3.5m-NTT as well as QUIRC at the Mauna Kea 2.2m telescope between December 2000 and June 2001. The target was recently discovered as nearby M9-dwarf among DENIS sources (Delfosse et al. 2001). We detect parallactic motion on our images and determine the distance to be 4.6 pm 0.3 pc, more precise than previously known. From the available colors, the distance, and the spectral type, we conclude from theoretical models that the star has a mass of ~0.075 to 0.09 M_sun and an age of ~1 to 2 Gyrs. We also obtained H- and K-band spectra of this star with ISAAC at the VLT. A faint companion candidate is detected 6 arc sec NNW of the star, which is 6.4 pm 0.5 mag fainter in H. However, according to another image taken several month later, the companion candidate is not co-moving with the M9-dwarf. Instead, it is a non-moving background object. Limits for undetected companion candidates are such that we can exclude any stellar companions outside of ~0.25 arc sec (1 AU), any brown dwarf companions (above the deuterium burning mass limit) outside of ~2 arc sec (9 AU), and also any companion down to ~40 M_jup with \ge 0.15 arc sec (0.7 AU) separation, all calculated for an age of 2 Gyrs. Our observations show that direct imaging of sub-stellar companions near the deuterium burning mass limit in orbit around nearby ultra-cool dwarfs is possible, even with separations that are smaller than the semi-major axis of the outermost planet in our solar system, namely a few tens of AU.

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