arXiv subjects

Guillermo Torres

Publications and source records attributed to Guillermo Torres.

240 records · Page 14Linked to original sources

arXiv2001

Spectroscopic binaries in a sample of ROSAT X-ray sources south of the Taurus molecular clouds

We report the results of our radial-velocity monitoring of spectroscopic binary systems in a sample of X-ray sources from the ROSAT All Sky Survey south of the Taurus-Auriga star-forming region. The original sample of approximately 120 sources by Neuhaeuser et al. was selected on the basis of their X-ray properties and the visual magnitude of the nearest optical counterpart, in such a way as to promote the inclusion of young objects. Roughly 20% of those sources have previously been confirmed to be very young. We focus here on the subset of the original sample that shows variable radial velocities (43 objects), a few of which have also been flagged previously as being young. New spectroscopic orbits are presented for 42 of those systems. Two of the binaries, RXJ0528.9+1046 and RXJ0529.3+1210, are indeed weak-lined T Tauri stars likely to be associated with the Lambda Orionis region. Most of the other binaries are active objects of the RS CVn-type, including several W UMa and Algol systems. We detect a strong excess of short-period binaries compared to the field, and an unusually large fraction of double-lined systems, as well as an overall high frequency of binaries out of the original sample. These results can be understood as selection effects. A short description of the physical properties of each binary is provided, and a comparison with evolutionary tracks is made using the stellar density as a distance-independent measure of evolution (abridged).

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arXiv2001

Absolute dimensions of the M-type eclipsing binary YY Geminorum (Castor C): a challenge to evolutionary models in the lower main-sequence

We present new spectroscopic observations of the detached late-type double-lined eclipsing binary YY Gem, a member of the Castor sextuple system and one of the benchmarks for the comparison between observations and stellar evolution theory in the lower main-sequence. In addition, we have re-analyzed existing light curves in several passbands using modern techniques that account for the conspicuous presence of spots. This, combined with the spectroscopy, has yielded a very precise determination of the absolute dimensions of the components, which are virtually identical to each other. We obtain mean values of M=0.5992+/-0.0047 Mo, R=0.6191+/-0.0057 Ro, and Teff=3820+/-100 K. Both the mass and the radius determinations are good to better than 1%. A re-analysis of the Hipparcos transit data for Castor AB has yielded an improved parallax for the system of 66.90+/-0.63 mas. With this, we have estimated the age (~370 Myr) and metal abundance ([Fe/H]~0.0) of YY Gem from isochrone fits to Castor A and B. We have compared the observations of YY Gem with a large number of recent theoretical calculations, and we show that all models underestimate the radius by up to 20%, and most overestimate the effective temperature by 150 K or more. Both of these trends are confirmed by observations of another similar system in the Hyades cluster (V818 Tau). Consequently, theoretical ages for relatively low-mass objects such as T Tauri stars derived from the H-R diagram may be considerably biased. If the radius is used directly as a measure of evolution, ages could be underestimated by as much as a factor of 10 in this mass regime. In view of these discrepancies, absolute ages from essentially all current models for the lower main sequence must be viewed with at least some measure of skepticism. (abridged)

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arXiv2001

Radial-Velocity Monitoring of Members and Candidate Members of the TW Hydrae Association

We present our spectroscopic measurements of the radial velocity, effective temperature, and projected rotational velocity of several of the known members of the TW Hya association, as well as measurements for candidate members selected on the basis of their X-ray or kinematic properties. A number of our targets turn out to be binaries, but most are non-members. The radial velocities for some of the other candidates support the conclusion that they are kinematically associated with the group, although further observations are required to show that they are indeed pre-main sequence objects.

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arXiv2001

Studies of multiple stellar systems - IV. The triple-lined spectroscopic system Gliese 644

We present a radial-velocity study of the triple-lined system Gliese 644 and derive spectroscopic elements for the inner and outer orbits with periods of 2.9655 and 627 days. We also utilize old visual data, as well as modern speckle and adaptive optics observations, to derive a new astrometric solution for the outer orbit. These two orbits together allow us to derive masses for each of the three components in the system: M_A = 0.410 +/- 0.028 (6.9%), M_Ba = 0.336 +/- 0.016 (4.7%), and $M_Bb = 0.304 +/- 0.014 (4.7%) M_solar. We suggest that the relative inclination of the two orbits is very small. Our individual masses and spectroscopic light ratios for the three M stars in the Gliese 644 system provide three points for the mass-luminosity relation near the bottom of the Main Sequence, where the relation is poorly determined. These three points agree well with theoretical models for solar metallicity and an age of 5 Gyr. Our radial velocities for Gliese 643 and vB 8, two common-proper-motion companions of Gliese 644, support the interpretation that all five M stars are moving together in a physically bound group. We discuss possible scenarios for the formation and evolution of this configuration, such as the formation of all five stars in a sequence of fragmentation events leading directly to the hierarchical configuration now observed, versus formation in a small N cluster with subsequent dynamical evolution into the present hierarchical configuration.

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arXiv2000

The change in the inclination angle of the non-eclipsing binary SS Lacertae: future eclipses

Eclipses in the 14.4-day period double-lined binary SS Lac were observed photographically and visually early in the 20th century, but stopped some 50 or 60 years ago. This has been explained by the presence of a distant third star in the system, which has now been detected spectroscopically with a period of 679 days. The plane of the orbit of the binary is changing relative to the line of sight in response to perturbations from this third object. A recent analysis by Milone et al. (M00) of all photometric material available for the system, including a re-measurement of original Harvard plates, has confirmed earlier reports of changes in the depth of the eclipses as a function of time, which are due to the third star. In this paper we discuss our detailed analysis of the eclipse amplitude measurements, and extract from them information on the change in the inclination angle of the binary over the last century. Our use of a much improved ephemeris for the system by Torres & Stefanik was found to be crucial, and prompted us to re-determine all the amplitudes from the historical data at our disposal, including the Harvard material used by M00. Systematically lower measurements on the branches of the minima were properly accounted for, and we made use of both a linear approximation to the time variation of the inclination angle and a more realistic model based on the theory of three-body interactions ("regression of the nodes" effect). The nodal cycle is found to be about 600 yr, within which TWO eclipse "seasons" occur, each lasting about 100 yr. The non-eclipsing status of the system is expected to continue until the beginning of the 23rd century.

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arXiv2000

Studies of multiple stellar systems - III. Modulation of orbital elements in the triple-lined system HD 109648

The triple-lined spectroscopic triple system HD 109648 has one of the shortest periods known for the outer orbit in a late-type triple, 120.5 days, and the ratio between the periods of the outer and inner orbits is small, 22:1. With such extreme values, this system should show orbital element variations over a timescale of about a decade. We have monitored the radial velocities of HD 109648 with the CfA Digital Speedometers for eight years, and have found evidence for modulation of some orbital elements. While we see no definite evidence for modulation of the inner binary eccentricity, we clearly observe variations in the inner and outer longitudes of periastron, as well as in the radial velocity amplitudes of the three components. The observational results, combined with numerical simulations, allow us to put constraints on the orientation of the orbits.

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