arXiv subjects

A. P. Hatzes

Publications and source records attributed to A. P. Hatzes.

143 records · Page 8Linked to original sources

arXiv2007

Evolved stars hint to an external origin of enhanced metallicity in planet-hosting stars

Exo-planets are preferentially found around high metallicity main sequence stars. We aim at investigating whether evolved stars share this property, and what this tells about planet formation. Statistical tools and the basic concepts of stellar evolution theory are applied to published results as well as our own radial velocity and chemical analyses of evolved stars. We show that the metal distributions of planet-hosting (P-H) dwarfs and giants are different, and that the latter do not favor metal-rich systems. Rather, these stars follow the same age-metallicity relation as the giants without planets in our sample. The straightforward explanation is to attribute the difference between dwarfs and giants to the much larger masses of giants' convective envelopes. If the metal excess on the main sequence is due to pollution, the effects of dilution naturally explains why it is not observed among evolved stars. Although we cannot exclude other explanations, the lack of any preference for metal-rich systems among P-H giants could be a strong indication of the accretion of metal-rich material. We discuss further tests, as well as some predictions and consequences of this hypothesis.

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arXiv2007

Planets around active stars

We present the results of radial velocity measurements of two samples of active stars. The first sample contains field G and K giants across the Red Giant Branch, whereas the second sample consists of nearby young stars (d < 150 pc) with ages between 10 - 300 Myrs. The radial velocity monitoring program has been carried out with FEROS at 1.52 m ESO telescope (1999 - 2002) and continued since 2003 at 2.2 m MPG/ESO telescope. We observed stellar radial velocity variations which originate either from the stellar activity or the presence of stellar/substellar companions. By means of a bisector technique we are able to distinguish the sources of the radial velocity variation. Among them we found few candidates of planetary companions, both of young stars and G-K giants sample.

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arXiv2007

Physical parameters of evolved stars in clusters and in the field from line-depth ratios

We present a high-resolution spectroscopic analysis of two samples of evolved stars selected in the field and in the intermediate-age open cluster IC 4651, for which detailed measurements of chemical composition were made in the last few years. Applying the Gray's method based on ratios of line depths, we determine the effective temperature and compare our results with previous ones obtained by means of the curves of growth of iron lines. The knowledge of the temperature enables us to estimate other fundamental stellar parameters, such as color excess, age, and mass.

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arXiv2007

Discovery of a planet around the K giant star 4 UMa

Context: For the past 3 years we have been monitoring a sample of 62 K giant stars using precise stellar radial velocity measurements taken at the Thueringer Landessternwarte Tautenburg. Aims: To search for sub-stellar companions to giant stars and to understand the nature of the diverse radial velocity variations exhibited by K giant stars. Methods: We present precise stellar radial velocity measurements of the K1III giant star 4 UMa (HD 73108). These were obtained using the coude echelle spectrograph of 2-m Alfred Jensch Telescope. The wavelength reference for the radial velocity measurements was provided by an iodine absorption cell. Results: Our measurements reveal that the radial velocity of 4 UMa exhibits a periodic variation of 269.3 days with a semiamplitude K = 216.8 m/s. A Keplerian orbit with an eccentricity, e = 0.43 +/- 0.02 is the most reasonable explanation for the radial velocity variations. The orbit yields a mass function, f(m) = (2.05 +/- 0.24) x 10^(- 7) M_sun. From our high resolution spectra we calculate a metallicity of -0.25 +/- 0.05 and derive a stellar mass of 1.23 M_sun +/- 0.15 for the host star. Conclusions: The K giant star 4 UMa hosts a substellar companion with minimum mass m sin i = 7.1 +/- 1.6 M_Jupiter.

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arXiv2006

Confirmation of the Planet Hypothesis for the Long-period Radial Velocity Variations of Beta Geminorum

We present precise stellar radial velocity measurements for the K giant star Beta Gem spanning over 25 years. These data show that the long period low amplitude radial velocity variations found by Hatzes & Cochran (1993) are long-lived and coherent. An examination of the Ca II K emission, spectral line shapes from high resolution data (R = 210,000), and Hipparcos photometry show no significant variations of these quantities with the RV period. These data confirm the planetary companion hypothesis suggested by Hatzes & Cochran (1993). An orbital solution assuming a stellar mass of 1.7 M_sun yields a period, P = 589.6 days, a minimum mass of 2.3 M_Jupiter, and a semi-major axis, and a = 1.6 AU. The orbit is nearly circular (e = 0.02). Beta Gem is the seventh intermediate mass star shown to host a sub-stellar companion and suggests that planet-formation around stars much more massive than the sun may common.

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arXiv2005

Hyades Oxygen Abundances from the Lambda 6300 [O I] Line: The Giant-Dwarf Discrepancy Revisited

We present the results of our abundance analysis of Fe, Ni, and O in high S/N, high-resolution VLT/UVES and McDonald/2dcoude spectra of nine dwarfs and three giants in the Hyades open cluster. The difference in Fe abundances derived from Fe II and Fe I lines ([Fe II/H] - [Fe I/H]) and Ni I abundances derived from moderately high-excitation (Chi ~ 4.20 eV) lines are found to increase with decreasing T_eff for the dwarfs. Both of these findings are in concordance with previous results of over-excitation/ionization in cool young dwarfs. Oxygen abundances are derived from the Lambda 6300 [O I] line, with careful attention given to the Ni I blend. The dwarf O abundances are in star-to-star agreement within uncertainties, but the abundances of the three coolest dwarfs (4573 < T_eff < 4834 K) evince an increase with decreasing T_eff. Possible causes for the apparent trend are considered, including the effects of over-dissociation of O-containing molecules. O abundances are derived from the near-UV Lambda 3167 OH line in high-quality Keck/HIRES spectra, and no such effects are found- indeed, the OH-based abundances show an increase with decreasing T_eff, leaving the nature and reality of the cool dwarf [O I]-based O trend uncertain. The mean relative O abundance of the six warmest dwarfs (5075 < T_eff < 5978 K) is [O/H] = +0.14 +/- 0.02, and we find a mean abundance of [O/H] = +0.08 +/- 0.02 for the giants. Thus, our updated analysis of the Lambda 6300 [O I] line does not confirm the Hyades giant-dwarf oxygen discrepancy initially reported by King & Hiltgen (1996), suggesting the discrepancy was a consequence of analysis-related systematic errors....

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arXiv2005

Planets of young stars: the TLS radial velocity survey

We report on the search for planets orbiting 46 nearby young stars performed at the State Observatory of Turingia (TLS) by means of a radial velocity survey. The aim of this program is to test the theories of formation/evolution of planetary systems. For 19(8) stars we can exclude planets with Msini > 1 MJ (5 MJ) and P < 10 days; we find 1 short period binary and 5 stars with long period RV-trend. One good young exo-planet candidate is presented.

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arXiv2005

Low-mass companions to Hyades stars

It is now well established that a large fraction of the low-mass stars are binaries or higher order multiples. Similarly a sizable fraction have giant planets. In contrast to these, the situation for brown dwarf companions is complicated: While close systems seem to be extremely rare, wide systems are possibly more common. In this paper, we present new results on a survey for low-mass companions in the Hyades. After measuring precisely the radial velocity of 98 Hyades dwarf stars for 5 years, we have selected all stars that show low-amplitude long-period trends. With AO-observations of these 14 stars we found companion candidates around nine of them, where one star has two companions. The two companions of HIP 16548 have masses between 0.07 to 0.08 Mo, and are thus either brown dwarfs or very low mass stars. In the case of HAN 172 we found a companion with a mass between 0.08 to 0.10 Mo, which is again between a star and a brown dwarf. The other seven stars all have stellar companions. In two additional cases, the RV-variations are presumably caused by stellar activity, and in another case the companion could be a short-period binary. The images of the remaining two stars are slightly elongated, which might imply that even these are binaries. Because at least 12 of the 14 stars showing low-amplitude RV trends turn out to have companions with a mass greater than 70 MJupiter, or are just active, we finally estimate the number of companions with masses between 10 MJupiter and 70 MJupiter within 8 AU of the host stars in the Hyades as less equal 2%.

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arXiv2005

A substellar companion around the intermediate-mass giant star HD 11977

We report the discovery of a substellar companion to the intermediate-mass star HD 11977 (G5 III). Radial velocities of this star have been monitored for five years with FEROS at the 1.52-m ESO and later at the 2.2-m MPG/ESO telescope in La Silla, Chile. Based on the collected data we calculated an orbital solution with a period of P=711 days, a semi-amplitude of K1=105 m/s, and an eccentricity of e=0.4. The period of the radial-velocity variation is longer than that of the estimated stellar rotation, rendering it unlikely that rotational modulation is the source of the variation in the radial velocity. This hypothesis is supported by the absence of a correlation between stellar activity indicators and radial-velocity variation. By determining a primary stellar mass of Mstar=1.91 Msun, the best-fit minimum mass of the companion and semi-major axis of the orbit are m2sini=6.54 Mjup and a2=1.93 AU, respectively. An upper limit for the mass of the companion of m2 < 65.5 Mjup has been calculated from Hipparcos astrometric measurements. Although the possibility of a brown-dwarf companion cannot be excluded, HD 11977 B is one of the few planet candidates detected around an intermediate-mass star. The progenitor main-sequence star of HD 11977 is probably an A-type star. This discovery gives an indirect evidence for planetary companions around early type main-sequence stars.

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arXiv2005

The young, active binary star EK Draconis

EK Dra (HD 129333) is a young, active, nearby star that is orbited by a low mass companion. By combining new speckle observations with old and new radial velocity measurements we find that the orbit is highly eccentric with e=0.82\pm0.03, and we derive the true masses of both components. The masses are $0.9\pm0.1 {\rm M}_\odot$ and $0.5\pm0.1 {\rm M}_\odot$, for the primary and secondary, respectively. From high resolution spectra we derive a new $T_{\rm eff}$ of $5700\pm70$ K, and a $\log g$ of $4.37\pm0.10$, which is different to previous estimates. However, the new spectroscopic distance differs by only 5.8% to the distance derived by parallax measurement of the Hipparcos satellite and thus the stellar parameters are presumably more realistic than older determinations. We derive a somewhat higher value for the metallicity of $[Fe/H]=-0.16\pm0.07$. EK Dra turns out to be one of the few nearby young stars which will evolve similar to the sun. The precise radial velocity measurements taken in the course of this program also allows us to shed more light on to the activity of this star. In 2001 and 2002 we find a periodic signal of the radial velocity variations with a period of $2.767\pm0.005$ days which we interpret as the rotation period. This signal vanishes in 2003. However the signal can be recovered if only the spectra in which the photospheric lines are asymmetric are used. On the other hand, we do not find a close correlation between the asymmetry of photospheric lines and the radial velocity.

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arXiv2004

Searching for Planets in the Hyades V: Limits on Planet Detection in the Presence of Stellar Activity

We present the results of a radial velocity survey of a sample of Hyades stars, and discuss the effects of stellar activity on radial velocity measurements. The level of radial velocity scatter due to rotational modulation of stellar surface features for the Hyades is in agreement with the predictions of Saar & Donahue (1997)- the maximum radial velocity rms of up to ~50 m/s, with an average rms of ~16 m/s. In this sample of 94 stars, we find 1 new binary, 2 stars with linear trends indicative of binary companions, and no close-in giant planets. We discuss the limits on extrasolar planet detection in the Hyades and the constraints imposed on radial velocity surveys of young stars.

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arXiv2004

HD 77407 and GJ 577: two new young stellar binaries detected with the Calar Alto Adaptive Optics system ALFA

We present the first results from our search for close stellar and sub-stellar companions to young nearby stars on the northern sky. Our infrared imaging observations are obtained with the 3.5 m Calar Alto telescope and the AO system ALFA. With two epoch observations which were separated by about one year, we found two co-moving companion candidates, one close to HD 77407 and one close to GJ 577. For the companion candidate near GJ 577, we obtained an optical spectrum showing spectral type M4.5; this candidate is a bound low-mass stellar companion confirmed by both proper motion and spectroscopy. We estimate the masses for HD 77407 B and GJ 577 B to be ~0.3 to 0.5 Msun and ~0.16 to 0.2 Msun, respectively. Compared to Siess al.(2000) models, each of the two pairs appears co-eval with HD 77407 A,B being 10 to 40 Myrs old and GJ 577 A,B being older than 100 Myrs. We also took multi-epoch high-resolution spectra of HD 77407 to search for sub-stellar companions, but did not find any with 3 Mjup as upper mass (msin(i)) limit (for up to 4 year orbits); however, we detected a long-term radial velocity trend in HD 77407 A, consistent with a ~ 0.3 Msun companion at ~ 50 AU separation, i.e. the one detected by the imaging. Hence, HD 77407 B is confirmed to be a bound companion to HD 77407 A. We also present limits for undetected, but detectable companions using a deep image of HD 77407 A and B, also observed with the Keck NIRC2 AO system; any brown dwarfs were detectable outside of 0.5 arcsec (17 AU at HD 77407), giant planets with masses from ~ 6.5 to 12 Mjup were detectable at > 1.5 arcsec.

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arXiv2003

The low-level radial velocity variability in Barnard's star (=GJ 699). Secular acceleration, indications for convective redshift, and planet mass limits

We report results from 2 1/2 yr of high precision radial velocity (RV) monitoring of Barnard's star. The high RV measurement precision of the VLT-UT2+UVES of 2.65 m/s made the following findings possible. (1) The first detection of the change in the RV of a star caused by its space motion (RV secular acceleration). (2) An anti-correlation of the measured RV with the strength of the filling-in of the Halpha line by emission. (3) Very stringent mass upper limits to planetary companions. Using only data from the first 2 years, we obtain a best-fit value for the RV secular acceleration of 5.15+/-0.89 m/s/yr. This agrees with the predicted value of 4.50 m/s/yr based on the Hipparcos proper motion and parallax combined with the known absolute radial velocity of the star. When the RV data of the last half-year are added the best-fit slope is strongly reduced to 2.97+/-0.51 m/s/yr suggesting the presence of additional RV variability in the star. Part of it can be attributed to stellar activity as we demonstrate by correlating the residual RVs with an index that describes the filling-in of the Halpha line by emission. A correlation coefficient of -0.50 indicates that the appearance of active regions causes a blueshift of photospheric absorption lines. Assuming that active regions basically inhibit convection we discuss the possibility that the fundamental (inactive) convection pattern in this M4V star produces a convective redshift. We also determine upper limits to the projected mass msini and to the true mass m of hypothetical planetary companions in circular orbits. For the separation range 0.017-0.98 AU we exclude any planet with msini>0.12 Mjupiter and m>0.86 Mjupiter. Throughout the habitable zone, i.e. 0.034-0.082 AU, we exclude planets with msini>7.5 Mearth and m>3.1 Mneptune.

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arXiv2002

The planet search program at the ESO Coude Echelle spectrometer. III. The complete Long Camera survey results

We present the complete results of the planet search program carried out at the ESO Coude Echelle Spectrometer (CES) on La Silla, using the Long Camera from Nov. 1992 to April 1998. The CES survey has monitored 37 late-type (F8V - M5V) stars in the southern hemisphere for variations in their differential radial velocities (RV) in order to detect Doppler reflex motions caused by planetary companions. This led to the discovery of the first extrasolar planet in an Earth-like orbit around the young (ZAMS) and active G0V star iota Horologii (Kuerster et al. 2000). Here we present the RV results for all survey stars and perform a statistical examination of the whole data-set. Each star is tested for RV variability, RV trends (linear and non-linear) and significant periodic signals. Beta Hyi and eps Ind are identified as long-term, low-amplitude RV variables. Furthermore, for 30 CES survey stars we determine quantitative upper mass-limits for giant planets based on our long-term RV results. We find that the CES Long Camera survey would have detected short-period (51 Peg-type) planets around all 30 stars but no planets with m sin i < 1 M_Jup at orbital separations larger than 2 AU. Finally, we demonstrate that the CES planet search can be continued without applying velocity corrections to the RV results coming from the currently installed Very Long Camera at the CES.

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arXiv1999

Precise radial velocities of Proxima Centauri

We present differential radial velocity measurements of Proxima Centauri collected over 4 years with the ESO CES with a mean precision of $54 ms^{-1}$. We find no evidence of a periodic signal that could corroborate the existence of a sub-stellar companion. We put upper limits (97% confidence) to the companion mass ranging from 1.1 to 22 M_{Jup} at orbital periods of 0.75 to 3000 d, i.e. separations 0.008-2 AU from Prox Cen. Our mass limits concur with limits found by precise astrometry (Benedict et al. 1998a and priv. comm.) which strongly constrain the period range 50-600 d to 1.1-0.22 M_{Jup}. Combining both results we exclude a brown dwarf or supermassive planet at separations 0.008-0.69 AU from Prox Cen. We also find that, at the level of our precision, the RV data are not affected by stellar activity.

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arXiv1998

Radial Velocity Studies of roAp Stars

We present recent results from our radial velocity (RV) studies of rapidly oscillating Ap (roAp) stars. Our measurements reveal that the pulsational amplitude of these stars depends on the spectral region that is examined. For one star, gamma Equ, the pulsational RV amplitude depends on both line strength and atomic species. The elemental difference is most likely related to the abundance patches on the surface of these stars. The line strength difference is interpreted as arising form a height effect in the atmosphere. Ultimately, these measurements may provide valuable diagnostics of the atmospheres of these stars.

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arXiv1996

The Discovery of a Planetary Companion to 16 Cygni B

High precision radial velocity observations of the solar-type star 16 Cygni B taken at McDonald Observatory and at Lick Observatory, have each independently discovered periodic radial-velocity variations indicating the presence of a Jovian-mass companion to this star. The orbital fit to the combined data gives a period of 800.8 days, a velocity amplitude of 43.9 m/s, and an eccentricity of 0.63. This is the largest eccentricity of any planetary system discovered so far. Assuming that 16 Cygni B has a mass of 1.0 Msun, this implies a mass for the companion of 1.5 sin i Jupiter masses. While the mass of this object is well within the range expected for planets, the large orbital eccentricity cannot be explained simply by the standard model of growth of planets in a protostellar disk. It is possible that this object was formed in the normal manner with a low eccentricity orbit, and has undergone post-formational orbital evolution, either through the same process which formed the ``massive eccentric'' planets around 70 Virginis and HD114762, or by gravitational interactions with the companion star 16 Cygni A. It is also possible that the object is an extremely low mass brown dwarf, formed through fragmentation of the collapsing protostar. We explore a possible connection between stellar photospheric Li depletion, pre-main sequence stellar rotation, the presence of a massive proto-planetary disk, and the formation of a planetary companion.

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