arXiv subjects

Guillermo Torres

Publications and source records attributed to Guillermo Torres.

At least 235 records · Page 13Linked to original sources

arXiv2005

The Multiple System HD 27638

We report spectroscopic observations of HD 27638B, the secondary in a visual binary in which the physically associated primary (separation approximately 19 arcsec) is a B9V star. The secondary shows strong Li 6708 absorption suggesting youth, and has attracted attention in the past as a candidate post-T Tauri star although this has subsequently been ruled out. It was previously known to be a double-lined spectroscopic binary (F8+G6) with a period of 17.6 days, and to show velocity residuals indicating a more distant massive third companion with a period of at least 8 years. Based on our radial velocity measurements covering more than two cycles of the outer orbit, along with other measurements, we derive an accurate triple orbital solution giving an outer period of 9.447 +/- 0.017 yr. The third object is more massive than either of the other two components of HD 27638B, but is not apparent in the spectra. We derive absolute visual magnitudes and effective temperatures for the three visible stars in HD 27638. Isochrone fitting based on those properties gives an age of 200 +/- 50 Myr for the system. We infer also an inclination angle of about 53.3 degrees for the inner orbit of HD 27638B. We detect a near-infrared excess in HD 27638B which we attribute to the third star being a close binary composed of late-type stars. This explains its large mass and lack of a visible signature. Modeling of this excess allows us to infer not only the masses of the components of the unseen companion, but also the inclination angle of the outer orbit (approximately 73 deg). The HD 27638 system is thus at least quintuple.

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arXiv2005

The Eclipsing Binary V1061 Cygni: Confronting Stellar Evolution Models for Active and Inactive Solar-Type Stars

(Abridged) We present spectroscopic and photometric observations of the eclipsing system V1061 Cyg (P = 2.35 days). We show that it is a hierarchical triple in which the third star is visible in the spectra. We combine the radial velocities for the three stars, times of eclipse, and intermediate astrometric data from the HIPPARCOS mission (abscissa residuals) to establish the elements of the outer orbit (P = 15.8 yr) and accurate values for the masses and radii (1-2% errors) and the effective temperatures of the binary components. Both stars are rotating rapidly and have their rotation synchronized with the orbital motion. There are signs of activity including strong X-ray emission and possibly spots. Current stellar evolution models agree well with the properties of the primary, but show a very large discrepancy in the radius of the secondary: the predicted values are about 10% smaller than observed (a 5-sigma effect). Also, the temperature is cooler than predicted by some 200 K. These discrepancies are quite remarkable given that the star is only 7% less massive than the Sun, the calibration point of all stellar models. We identify the chromospheric activity as the likely cause of the effect. Inactive stars agree very well with the models, while active ones such as V1061 Cyg Ab appear systematically too large and too cool. Theory provides an understanding of this in terms of the strong magnetic fields commonly associated with stellar activity, which tend to inhibit convective heat transport.

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arXiv2005

The Astrometric-Spectroscopic Binary System HIP 50796: An Overmassive Companion

We report spectroscopic observations of the star HIP 50796, previously considered (but later rejected) as a candidate member of the TW Hya association. Our measurements reveal it to be a single-lined binary with an orbital period of 570 days and an eccentricity of e = 0.61. The astrometric signature of this orbit was previously detected by the HIPPARCOS satellite in the form of curvature in the proper motion components, although the period was unknown at the time. By combining our radial velocity measurements with the HIPPARCOS intermediate data (abscissae residuals) we are able to derive the full three-dimensional orbit, and determine the dynamical mass of the unseen companion as well as a revised trigonometric parallax that accounts for the orbital motion. Given our primary mass estimate of 0.73 solar masses (mid-K dwarf), the companion mass is determined to be 0.89 solar masses, or about 20% larger than the primary. The likely explanation for the larger mass without any apparent contribution to the light is that the companion is itself a closer binary composed of M dwarfs. The near-infrared excess and X-ray emission displayed by HIP 50796 support this. Our photometric modeling of the excess leads to a lower limit to the mass ratio of the close binary of about q = 0.8, and individual masses of 0.44-0.48 solar masses and 0.41-0.44 solar masses. The new parallax (20.6 +/- 1.9 mas) is significantly smaller than the original HIPPARCOS value, and more precise.

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arXiv2005

Detection of Thermal Emission from an Extrasolar Planet

We present Spitzer Space Telescope infrared photometric time series of the transiting extrasolar planet system TrES-1. The data span a predicted time of secondary eclipse, corresponding to the passage of the planet behind the star. In both bands of our observations, we detect a flux decrement with a timing, amplitude, and duration as predicted by published parameters of the system. This signal represents the first direct detection of (i.e. the observation of photons emitted by) a planet orbiting another star. The observed eclipse depths (in units of relative flux) are 0.00066 +/- 0.00013 at 4.5um and 0.00225 +/- 0.00036 at 8.0um. These estimates provide the first observational constraints on models of the thermal emission of hot Jupiters. Assuming that the planet emits as a blackbody, we estimate an effective temperature of T_p=1060 +/- 50 K. Under the additional assumptions that the planet is in thermal equilibrium with the radiation from the star and emits isotropically, we find a Bond albedo of A = 0.31 +/- 0.14. This would imply that the planet absorbs the majority of stellar radiation incident upon it, a conclusion of significant impact to atmospheric models of these objects. We compare our data to a previously-published model of the planetary thermal emission, which predicts prominent spectral features in our observational bands due to water and carbon monoxide. Based on the time of secondary eclipse, we present an upper limit on the orbital eccentricity that is sufficiently small that we conclude that tidal dissipation is unlikely to provide a significant source of energy interior to the planet.(abridged)

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arXiv2005

Testing Stellar Models With An Improved Physical Orbit for 12 Bootis

We report on a significantly improved determination of the physical orbit of the double-lined spectroscopic binary system 12 Boo. We have a 12 Boo interferometry dataset spanning six years with the Palomar Testbed Interferometer, a smaller amount of data from the Navy Prototype Optical Interferometer, and a radial velocity dataset spanning 14 years from the Harvard-Smithsonian Center for Astrophysics. We have updated the 12 Boo physical orbit model with our expanded interferometric and radial velocity datasets. The revised orbit is in good agreement with previous results, and the physical parameters implied by a combined fit to our visibility and radial velocity data result in precise component masses and luminosities. In particular, the orbital parallax of the system is determined to be 27.74 $\pm$ 0.15 mas, and masses of the two components are determined to be 1.4160 $\pm$ 0.0049 M$_{\sun}$ and 1.3740 $\pm$ 0.0045 M$_{\sun}$, respectively. Based on theoretical models we can estimate a system age of approximately 3.2 Gyr. Comparisons with stellar models suggest that the 12 Boo primary may be just entering the Hertzsprung gap, but that conclusion is highly dependent on details of the models. Such a dynamic evolutionary state makes the 12 Boo system a unique and important test for stellar models.

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arXiv2005

The Challenge of Wide-Field Transit Surveys: The Case of GSC 01944-02289

Wide-field searches for transiting extra-solar giant planets face the difficult challenge of separating true transit events from the numerous false positives caused by isolated or blended eclipsing binary systems. We describe here the investigation of GSC 01944-02289, a very promising candidate for a transiting brown dwarf detected by the Transatlantic Exoplanet Survey (TrES) network. The photometry and radial velocity observations suggested that the candidate was an object of substellar mass in orbit around an F star. However, careful analysis of the spectral line shapes revealed a pattern of variations consistent with the presence of another star whose motion produced the asymmetries observed in the spectral lines of the brightest star. Detailed simulations of blend models composed of an eclipsing binary plus a third star diluting the eclipses were compared with the observed light curve and used to derive the properties of the three components. Our photometric and spectroscopic observations are fully consistent with a blend model of a hierarchical triple system composed of an eclipsing binary with G0V and M3V components in orbit around a slightly evolved F5 dwarf. We believe that this investigation will be helpful to other groups pursuing wide-field transit searches as this type of false detection could be more common than true transiting planets, and difficult to identify.

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arXiv2005

A transiting extrasolar giant planet around the star OGLE-TR-10

We report a transiting extrasolar giant planet around the star OGLE-TR-10 (orbital period = 3.1 days), which was uncovered as a candidate by the OGLE team in their photometric survey towards the Galactic center (Udalski et al, 2002a). We observed OGLE-TR-10 spectroscopically over a period of two years (2002-2004) using the HIRES instrument with an iodine cell on the Keck I telescope, and measured small radial velocity variations that are consistent with the presence of a planetary companion. This confirms the earlier identification of OGLE-TR-10b by our team and also recently by Bouchy et al (2004a) as a possible planet. Additionally, in this paper we are able to rule out a blend scenario as an alternative explanation. From an analysis combining all available radial velocity measurements with the OGLE light curve we find that OGLE-TR-10b has a mass of 0.57+/-0.12 MJup and a radius of 1.24+/-0.09 RJup. These parameters bear close resemblance to those of the first known transiting extrasolar planet, HD 209458b.

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arXiv2004

Testing blend scenarios for extrasolar transiting planet candidates. II. -- OGLE-TR-56

We re-examine the photometric and spectroscopic evidence available for the star OGLE-TR-56, recently discovered to harbor a giant planet presenting transits and orbiting with a period of 1.21 days. We investigate the possibility that the observational signatures reported might be the result of "blending" with the light of an eclipsing binary along the same line of sight. Using techniques developed earlier we perform fits to the light curve under a variety of blend configurations, subject to all observational constraints, and we infer further properties of these possible blends. We then carry out realistic end-to-end simulations based on those properties in order to quantify the spectral line asymmetries and radial velocity variations expected from such scenarios. We confront these calculations with the observations. The results from these models are clearly inconsistent with the measured radial velocity and bisector span variations, ruling out blends and confirming the planetary nature of the companion. The example of OGLE-TR-56 serves to illustrate the sort of tests that can and should be performed on transiting planet candidates to eliminate the possibility of "false positives", and in particular, line-of-sight contamination from eclipsing binaries.

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arXiv2004

TrES-1: The Transiting Planet of a Bright K0V Star

We report the detection of a transiting Jupiter-sized planet orbiting a relatively bright (V=11.79) K0V star. We detected the transit light-curve signature in the course of the TrES multi-site transiting planet survey, and confirmed the planetary nature of the companion via multicolor photometry and precise radial velocity measurements. We designate the planet TrES-1; its inferred mass is 0.75 +/- 0.07 Jupiter masses, its radius is 1.08 (+0.18/-0.04) Jupiter radii, and its orbital period is 3.030065 +/- 0.000008 days. This planet has an orbital period similar to that of HD 209458b, but about twice as long as those of the OGLE transiting planets. Its mass is indistinguishable from that of HD 209458b, but its radius is significantly smaller and fits the theoretical models without the need for an additional source of heat deep in the atmosphere, as has been invoked by some investigators for HD 209458b.

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arXiv2004

Testing blend scenarios for extrasolar transiting planet candidates. I. -- OGLE-TR-33: a false positive

We report high-resolution spectroscopic follow-up observations of the faint transiting planet candidate OGLE-TR-33 (V = 14.7), located in the direction of the Galactic center. Small changes in the radial velocity of the star were detected that suggested initially the presence of a large planet or brown dwarf in orbit. However, further analysis revealed spectral line asymmetries that change in phase with the 1.95-day period, casting doubt on those measurements. These asymmetries make it more likely that the transit-like events in the light curve are the result of contamination from the light of an eclipsing binary along the same line of sight (referred to as a "blend"). We performed detailed simulations in which we generated synthetic light curves resulting from such blend scenarios and fitted them to the measured light curve. Guided by these fits and the inferred properties of the stars, we uncovered a second set of lines in our spectra that correspond to the primary of the eclipsing binary and explain the asymmetries. Using all the constraints from spectroscopy we were then able to construct a model that satisfies all the observations, and to characterize the three stars based on model isochrones. OGLE-TR-33 is fully consistent with being a hierarchical triple system composed of a slightly evolved F6 star (the brighter object) near the end of its main-sequence phase, and an eclipsing binary with a K7-M0 star orbiting an F4 star. The application to OGLE-TR-33 of the formalism developed to fit light curves of transit candidates illustrates the power of such simulations for predicting additional properties of the blend and for guiding further observations that may serve to confirm that scenario, thereby ruling out a planet. Tests such as this can be very important for validating faint candidates.

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arXiv2004

A transiting extrasolar giant planet around the star OGLE-TR-113

We report the independent discovery of a new extrasolar transiting planet around OGLE-TR-113, a candidate star from the Optical Gravitational Lensing Experiment. Small radial-velocity variations have been detected based on observations conducted with the MIKE spectrograph on the Magellan I (Baade) telescope at the Las Campanas Observatory (Chile) during 2003. We have also carried out a light-curve analysis incorporating new photometry and realistic physical parameters for the star. OGLE-TR-113b has an orbital period of only 1.43 days, a mass of 1.08 +/- 0.28 M_Jup, and a radius of 1.09 +/- 0.10 R_Jup. Similar parameters have been obtained very recently in an independent study by Bouchy et al., from observations taken a year later. The orbital period of OGLE-TR-113b, and also that of the previously announced planet OGLE-TR-56b (P_orb = 1.21 days) -- the first two found photometrically -- are much shorter than the apparent cutoff of close-in giant planets at 3-4-day periods found from high-precision radial velocities surveys. Along with a third case reported by Bouchy et al. (OGLE-TR-132b, P_orb = 1.69 days), these objects appear to form a new class of ``very hot Jupiters'' that pose very interesting questions for theoretical study.

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arXiv2003

Combining Astrometry and Spectroscopy

Orbital solutions for binary or multiple stellar systems that combine astrometry (e.g., position angles and angular separations) with spectroscopy (radial velocities) have important advantages over astrometric-only or spectroscopic-only solutions. In many cases they allow the determination of the absolute masses of the components, as well as the distance. Yet, these kinds of combined solutions that use different types of observations in a global least-squares fit are still not very common in the literature. An outline of the procedure is presented, along with examples to illustrate the sort of results that can be obtained. The same method can easily be extended to include other types of measurements (times of eclipse, Hipparcos observations, interferometric visibilities and closure phases, parallaxes, lunar occultations, etc.), which often complement each other and strengthen the solution.

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arXiv2003

A double-lined spectroscopic orbit for the young star HD 34700

We report high-resolution spectroscopic observations of the young star HD 34700, which confirm it to be a double-lined spectroscopic binary. We derive an accurate orbital solution with a period of 23.4877 +/- 0.0013 days and an eccentricity of e = 0.2501 +/- 0.0068. The stars are found to be of similar mass (M2/M1 = 0.987 +/- 0.014) and luminosity. We derive also the effective temperatures (5900 K and 5800 K) and projected rotational velocities (28 km/s and 22 km/s) of the components. These values of v sin i are much higher than expected for main-sequence stars of similar spectral type (G0), and are not due to tidal synchronization. We discuss also the indicators of youth available for the object. Although there is considerable evidence that the system is young --strong infrared excess, X-ray emission, Li I 6708 absorption (0.17 Angstroms equivalent width), H alpha emission (0.6 Angstroms), rapid rotation-- the precise age cannot yet be established because the distance is unknown.

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arXiv2003

New Data and Improved Parameters for the Transiting Planet OGLE-TR-56b

We report new spectroscopic observations of the recently discovered transiting planet OGLE-TR-56b with the Keck/HIRES instrument. Our radial velocity measurements with errors of ~100 m/s show clear variations that are in excellent agreement with the phasing (period and epoch) derived from the OGLE transit photometry, confirming the planetary nature of the companion. The new data combined with measurements from the previous season allow an improved determination of the mass of the planet, M_p = 1.45 +/- 0.23 M_Jup. All available OGLE photometry, including new measurements made this season, have also been analyzed to derive an improved value for the planetary radius of R_p = 1.23 +/- 0.16 R_Jup. We discuss the implications of these results for the theory of extrasolar planets.

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arXiv2003

High-resolution spectroscopic follow-up of OGLE planetary transit candidates in the Galactic bulge: two possible Jupiter-mass planets and two blends

We report the results of our campaign to follow-up spectroscopically several candidate extrasolar transiting planets from the OGLE-III survey in the direction of the Galactic center, announced in 2001 (Udalski et al 2002a,b). All of these objects present shallow and periodic dips in brightness that may be due to planetary companions. Our high-resolution Keck I/HIRES observations have revealed two interesting cases (OGLE-TR-10 with a period of 3.1 days, and OGLE-TR-58 with a period of 4.3 days) that show no radial velocity variations at the level of 100--200 m/s. If orbited by companions, their masses would be similar to Jupiter. With the information in hand (including the light curves) we are not able to rule out that these candidates are instead the result of contamination from an eclipsing binary in the same line of sight (a "blend"). We discuss also the case of OGLE-TR-56 that was recently reported by Konacki et al (2003) to have a Jupiter-size companion, based on an earlier analysis of our data, and we present supporting information. Two other candidates, OGLE-TR-3 and OGLE-TR-33, show clear evidence that they are blends.

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arXiv2003

Optical photometry and X-ray monitoring of the "Cool Algol" BD+05 706: Determination of the physical properties

We present new photometric observations in the BVRI bands of the double-lined eclipsing binary BD+05 706 conducted over three observing seasons, as well as new X-ray observations obtained with the ROSAT satellite covering a full orbital cycle (P = 18.9 days). A detailed light-curve analysis of the optical data shows the system to be semidetached, confirming indications from an earlier analysis by Torres et al. (1998), with the less massive and cooler star filling its Roche lobe. The system is a member of the rare class of cool Algol systems, which are different from the "classical" Algol systems in that the mass-gaining component is also a late-type star rather than a B- or A-type star. By combining the new photometry with a reanalysis of the spectroscopic observations reported by Torres et al. (1998) we derive accurate absolute masses for the components of M1 = 2.633 +/- 0.028 Msun and M2 = 0.5412 +/- 0.0093 Msun, radii of R1 = 7.55 +/- 0.20 Rsun and R2 = 11.02 +/- 0.21 Rsun, as well as effective temperatures of 5000 +/- 100 K and 4640 +/- 150 K for the primary and secondary, respectively. There are obvious signs of activity (spottedness) in the optical light curve of the binary. Our X-ray light curve clearly shows the primary eclipse but not the secondary eclipse, suggesting that the primary star is the dominant source of the activity in the system. The depth and duration of the eclipse allow us to infer some of the properties of the X-ray emitting region around that star.

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arXiv2002

Radial-velocity survey of members and candidate members of the TW Hydrae association

We report our spectroscopic observations of stars belonging to the young nearby group known as the TW Hydrae association, as well as of a number of potential members of the association identified in kinematic and X-ray surveys. Multiple radial velocity measurements were obtained for each object, several of which turn out to be multiple systems. Orbital solutions are presented for 3 double-lined binaries, one single-lined binary, and a double-lined triple system, all with short periods. Effective temperatures and projected rotational velocities are presented for each visible object. None of the candidate members of the association in our sample is confirmed as a true member. The large fraction of close binaries among the candidate members has to do with their selection based on X-ray emission from ROSAT, which tends to favor the inclusion of tidally-locked systems that are active but not necessarily young.

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arXiv2002

An interferometric-spectroscopic orbit for the binary HD 195987: Testing models of stellar evolution for metal-poor stars

We report spectroscopic and interferometric observations of the moderately metal-poor double-lined binary system HD 195987, with an orbital period of 57.3 days. By combining our radial-velocity and visibility measurements we determine the orbital elements and derive absolute masses for the components of M(A) = 0.844 +/- 0.018 Msun and M(B) = 0.6650 +/- 0.0079 Msun, with relative errors of 2% and 1%, respectively. We also determine the orbital parallax, pi(orb) = 46.08 +/- 0.27 mas, corresponding to a distance of 21.70 +/- 0.13 pc. The parallax and the measured brightness difference between the stars in V, H, and K yield the component absolute magnitudes in those bands. We also estimate the effective temperatures of the stars as Teff(A) = 5200 +/- 100 K and Teff(B) = 4200 +/- 200 K. Together with detailed chemical abundance analyses from the literature giving [Fe/H] approximately -0.5 (corrected for binarity) and [alpha/Fe] = +0.36, we use these physical properties to test current models of stellar evolution for metal-poor stars. Among the four that we considered, we find that no single model fits all observed properties at the measured composition, although we identify the assumptions in each one that account for the discrepancy and we conclude that a model with the proper combination of assumptions should be able to reproduce all the radiative properties. The indications from the isochrone fits and the pattern of enhancement of the metals in HD 195987 are consistent with this being a thick disk object, with an age of 10-12 Gyr.

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