arXiv subjects

L. Prato

Publications and source records attributed to L. Prato.

At least 73 records · Page 4Linked to original sources

arXiv2006

The NIRSPEC Brown Dwarf Spectroscopic Survey II: High-Resolution J-Band Spectra of M, L and T Dwarfs

We present a sequence of high resolution (R~20,000 or 15 km/s) infrared spectra of stars and brown dwarfs spanning spectral types M2.5 to T6. Observations of 16 objects were obtained using eight echelle orders to cover part of the J-band from 1.165-1.323 micron with NIRSPEC on the Keck II telescope. By comparing opacity plots and line lists, over 200 weak features in the J-band are identified with either FeH or H2O transitions. Absorption by FeH attains maximum strength in the mid-L dwarfs, while H2O absorption becomes systematically stronger towards later spectral types. Narrow resolved features broaden markedly after the M to L transition. Our high resolution spectra also reveal that the disappearance of neutral Al lines at the boundary between M and L dwarfs is remarkably abrupt, presumably because of the formation of grains. Neutral Fe lines can be traced to mid-L dwarfs before Fe is removed by condensation. The neutral potassium (K I) doublets that dominate the J-band have pressure broadened wings that continue to broaden from ~50 km/s (FWHM) at mid-M to ~500 km/s at mid-T. In contrast however, the measured pseudo-equivalent widths of these same lines reach a maximum in the mid-L dwarfs. The young L2 dwarf, G196-3B, exhibits narrow potassium lines without extensive pressure-broadened wings, indicative of a lower gravity atmosphere. Kelu-1AB, another L2, has exceptionally broad infrared lines, including FeH and H2O features, confirming its status as a rapid rotator. In contrast to other late T objects, the peculiar T6 dwarf 2MASS 0937+29 displays a complete absence of potassium even at high resolution, which may be a metallicity effect or a result of a cooler, higher-gravity atmosphere.

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arXiv2006

A Survey for Young Spectroscopic Binary K7-M4 Stars in Ophiuchus

This paper describes a high-resolution, infrared spectroscopic survey of young, low-mass stars designed to identify and characterize pre-main-sequence spectroscopic binaries. This is the first large infrared radial velocity survey of very young stars to date. The frequency and mass ratio distribution of the closest, low-mass binaries bear directly on models of stellar, brown dwarf, and planetary mass companion formation. Furthermore, spectroscopic binaries can provide mass ratios and ultimately masses, independent of assumptions, needed to calibrate models of young star evolution. I present the initial results from observations of a uniform sample of 33 T Tauri M stars in the Ophiuchus molecular cloud. The average mass of this sample is less than that of other young star radial velocity surveys of similar scope by a factor of ~2. Almost every star was observed at 3-4 epochs over 3 years with the 10 meter Keck II telescope and the facility infrared spectrometer NIRSPEC. An internal precision of 0.43 km/s was obtained with standard cross-correlation calibration techniques. Four of the targets are newly discovered spectroscopic binaries, one of which is located in a sub-arcsecond, hierarchical quadruple system. Three other sub-arcsecond visual binaries were also serendipitously identified during target acquisition. The spectroscopic multiplicity of the sample is comparable to that of earlier type, pre-main-sequence objects. Therefore, there is no dearth of young, low-mass spectroscopic binary stars, at least in the Ophiuchus region.

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arXiv2006

Dynamical Mass Estimates for Incomplete Orbits: Young Multiple Stars in Taurus and Ophiuchus

We present recent measurements of the orbital motion in the binaries, DF Tau and ZZ Tau, and in the triples, Elias 12, T Tau, and V853 Oph. We observed these systems with the Fine Guidance Sensors on the Hubble Space Telescope and with adaptive optics imaging at the W. M. Keck and Gemini North Observatories. Based on our measurements and those presented in the literature, we perform preliminary orbital analyses for DF Tau, ZZ Tau, Elias 12 Na-Nb, and T Tau Sa-Sb. Because the orbital coverage in most of these systems does not yet span a sufficient portion of the orbit, we are not able to find definitive orbit solutions. By using a Monte Carlo search technique, we explored the orbital parameter space allowed by the current set of data available for each binary. We constructed weighted distributions for the total mass of the binaries derived from a large sample of possible orbits that fit the data. These mass distributions show that the total mass is already well-defined. We compute total mass estimates of 0.78 (+0.25/-0.15) Msun, 0.66 (+0.15/-0.11) Msun, 1.13 (+0.36/-0.09) Msun, and 4.13 (+1.58/-0.97) Msun for DF Tau, ZZ Tau, Elias 12 Na-Nb, and T Tau Sa-Sb respectively, using a distance of 140 pc. For Elias 12 Na-Nb, where the orbital coverage spans ~ 164 deg, we compute a preliminary orbit solution with a period of ~ 9-12 years. By including an earlier lunar occultation measurement, we also find a likely orbit solution for ZZ Tau, with a period of ~ 32 years. With additional measurements to continue mapping the orbits, the derived dynamical masses will be useful in constraining the theoretical tracks of pre-main sequence evolution.

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arXiv2006

An Association in the Aquila Star-Forming Region: High Resolution Infrared Spectroscopy of T Tauri Stars

We present the properties of a group of young stars associated with the well-studied T Tauri star system AS 353, located in the Aquila star-forming region. The association is identified using radial velocity measurements of sample objects selected from the Herbig and Bell Catalog based on their spatial proximity to AS 353. Radial velocities of nine objects were measured from multi-epoch high-resolution (R~30,000) H-band spectra obtained with NIRSPEC on Keck II. High-resolution K-band spectra were also obtained for most of the sample objects. Spectral types and rotational velocities are determined for all objects in the sample. The multi-epoch H-band spectra were examined for radial velocity variations in order to detect possible spectroscopic binaries. Eight of the nine objects have radial velocities that are consistent within the 1-sigma scatter of the sample. From their mean of -8.6 km/s these eight objects have a standard deviation of 2 km/s, which suggests that the sample stars are related. The ninth object shows significant radial velocity variations between epochs, characteristic of a spectroscopic binary. The overall multiplicity of the sample is high; we observed 13 stars in seven systems, identifying three new candidate binary components in this project. Many of the spectra reveal hydrogen emission lines typical of strong accretion processes, indicating that most of these objects harbor circumstellar disks and are less than a few million years old. We discuss possible interpretations of the enigmatic pure emission line spectrum of HBC 684. This work represents the highest spectral resolution infrared observations to date of these intriguing, nearby young stars.

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arXiv2006

The GL 569 Multiple System

We report the results of high spectral and angular resolution infrared observations of the multiple system GL 569 A and B that were intended to measure the dynamical masses of the brown dwarf binary believed to comprise GL 569 B. Our analysis did not yield this result but, instead, revealed two surprises. First, at age ~100 Myr, the system is younger than had been reported earlier. Second, our spectroscopic and photometric results provide support for earlier indications that GL 569 B is actually a hierarchical brown dwarf triple rather than a binary. Our results suggest that the three components of GL 569 B have roughly equal mass, ~0.04 Msun.

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arXiv2006

Disk Evolution in Young Binaries: from Observations to Theory

The formation of a binary system surrounded by disks is the most common outcome of stellar formation. Hence studying and understanding the formation and the evolution of binary systems and associated disks is a cornerstone of star formation science. Moreover, since the components within binary systems are coeval and the sizes of their disks are fixed by the tidal truncation of their companion, binary systems provide an ideal "laboratory" in which to study disk evolution under well defined boundary conditions. In this paper, we review observations of several inner disk diagnostics in multiple systems, including hydrogen emission lines (indicative of ongoing accretion), $K-L$ and $K-N$ color excesses (evidence of warm inner disks), and polarization (indicative of the relative orientations of the disks around each component). We examine to what degree these properties are correlated within binary systems and how this degree of correlation depends on parameters such as separation and binary mass ratio. These findings will be interpreted both in terms of models that treat each disk as an isolated reservoir and those in which the disks are subject to re-supply from some form of circumbinary reservoir, the observational evidence for which we will also critically review. The planet forming potential of multiple star systems is discussed in terms of the relative lifetimes of disks around single stars, binary primaries and binary secondaries. Finally, we summarize several potentially revealing observational problems and future projects that could provide further insight into disk evolution in the coming decade

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arXiv2005

Investigating disk evolution: A high spatial resolution mid-infrared survey of T Tauri stars

We present a high spatial resolution, 10-20 micron survey of 65 T Tauri binary stars in Taurus, Ophiuchus, and Corona Australis using the Keck 10 m telescopes. Designed to probe the inner ~1 AU region of the circumstellar disks around the individual stellar components in these binary systems, this study increases the number of binaries with spatially resolved measurements at 10 micron by a factor of ~5. Combined with resolved near-infrared photometry and spectroscopic accretion diagnostics, we find that ~10% of stars with a mid-infrared excess do not appear to be accreting. In contrast to an actively accreting disk system, these passive disks have significantly lower near-infrared colors that are, in most cases, consistent with photospheric emission, suggesting the presence of an inner disk hole. In addition, there appears to be a spectral type/mass dependence associated with the presence of a passive disk, with all passive disks occurring around M type stars. The possibility that the passive disks are caused by the presence of an as yet undetected companion at a small separation (0.3-3 AU) is possible for any individual system, however, it cannot account for the spectral type dependence of the passive disk sample as a whole. We propose that these passive disks represent a subset of T Tauri stars that are undergoing significant disk evolution. The fraction of observed passive disks and the observed spectral type dependence can both be explained by models of disk evolution that include disk photoevaporation from the central star. (abridged).

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arXiv2004

An Upper Bound on the Flux Ratio of rho CrB's Companion at 1.6um

We use high resolution infrared spectroscopy to investigate the 2001 report by Gatewood and colleagues that rho CrB's candidate extrasolar planet companion is really a low-mass star with mass 0.14+-0.05 Msun. We do not detect evidence of such a companion; the upper bounds on the (companion/primary) flux ratio at 1.6 microns are less than 0.0024 and 0.005 at the 90 and 99% confidence levels, respectively. Using the H-band mass-luminosity relationship calculated by Baraffe and colleagues, the corresponding upper limits on the companion mass are 0.11 and 0.15 Msun. Our results indicate that the infrared spectroscopic technique can detect companions in binaries with flux ratios as low as 0.01 to 0.02.

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arXiv2004

The Double-Lined Spectroscopic Binary Haro 1-14c

We report detection of the low-mass secondary in the spectroscopic binary Haro 1-14c in the Ophiuchus star forming region. The secondary/primary mass ratio is $0.310\pm 0.014$. With an estimated photometric primary mass of 1.2 $M_{\odot}$, the secondary mass is $\sim 0.4 M_{\odot}$ and the projected semi-major axis is $\sim 1.5$ AU. The system is well-suited for astrometric mapping of its orbit with the current generation of ground-based IR interferometers. This could yield precision values of the system's component masses and distance.

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arXiv2004

A High Spatial Resolution Infrared View of the T Tauri Multiple System

We present the results of our monitoring study of the IR photometric and spectroscopic variability of the T Tau multiple system. We also present data on the apparent position of T Tau S with respect to T Tau N, and two new spatially resolved observations of the T Tau Sa-Sb binary. T Tau N has not varied by more than 0.2 magnitudes in K and L' flux during the 8 years of our observations, though its Br gamma and Br alpha emission line fluxes have varied by nearly a factor of four during this time. For the unresolved T Tau S system, we have derived a 20 year light curve based on our images and on measurements available in the literature. T Tau S varies by 2-3 magnitudes in K and L'-band brightness in a ``redder when faint'' manner, consistent with changes along the line of sight in the amount of material that follows an ISM extinction law. Absorption in the 3.05um water-ice feature is seen only in the spectra of T Tau S and it displays variations in depth and profile. H_2 (2.12 um) emission is also detected only at the position of T Tau S; the H_2, Br gamma and Br alpha emission line fluxes also vary. We have found that the apparent positions of T Tau S with respect to T Tau N and T Tau Sb with respect to Sa are consistent with gravitationally bound orbital motion. The possible orbits of the T Tau S binary imply that Sa is likely the most massive component in this young triple. A reanalysis of the motion of the radio source associated with T Tau S provides no evidence for an ejection event in the T Tau system.

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arXiv2003

S Ori 70: Just a Foreground Field Brown Dwarf?

We examine recent claims that the T-type brown dwarf S Ori 053810.1-023626 (S Ori 70) is a spectroscopically verified low mass (3$^{+5}_{-1}$ M$_{Jup}$) member of the 1--8 Myr $σ$ Orionis cluster. Comparative arguments by Mart{í}n & Zapatero Osorio asserting that S Ori 70 exhibits low surface gravity spectral features indicative of youth and low mass are invalidated by the fact that their comparison object was not the field T dwarf 2MASS 0559$-$1404 but rather a nearby background star. Instead, we find that the 1--2.5 $\micron$ spectra of S Ori 70 are well-matched to older (age $\sim$ few Gyr) field T6--T7 dwarfs. Moreover, we find that spectral model fits to late-type field T dwarf spectra tend to yield low surface gravities ($\log{g}$ = 3.0--3.5), and thus young ages ($\lesssim$ 5 Myr) and low masses ($\lesssim$ 3 M$_{Jup}$), inconsistent with expected and/or empirical values. Finally, we show that the identification of one T dwarf in the field imaged by Zapatero Osorio et al. is statistically consistent with the expected foreground contamination. Based on the re-examined evidence, we conclude that S Ori 70 may simply be an old, massive (30--60 M$_{Jup}$) field brown dwarf lying in the foreground of the $σ$ Orionis cluster. This interpretation should be considered before presuming the existence of so-called ``cluster planets.''

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arXiv2003

Identifying Young Brown Dwarfs Using Gravity-Sensitive Spectral Features

We report the initial results of the Brown Dwarf Spectroscopic Survey Gravity Project, to study gravity sensitive features as indicators of youth in brown dwarfs. Low-resolution (R~2000) J-band and optical (R~1000) observations using NIRSPEC and LRIS at the W.M. Keck Observatory reveal transitions of TiO, VO, K I, Na I, Cs I, Rb I, CaH, and FeH. By comparing these features in late-type giants and in old field dwarfs we show that they are sensitive to the gravity (g = GM/R^2) of the object. Using low-gravity spectral signatures as age indicators, we observed and analyzed J-band and optical spectra of two young brown dwarfs, G 196-3B (20-300 Myr) and KPNO Tau-4 (1-2 Myr), and two possible low mass brown dwarfs in the sigma Orionis cluster (3-7 Myr). We report the identification of the phi bands of TiO near 1.24 microns and the A-X band of VO near 1.18 microns together with extremely weak J-band lines of K I in KPNO-Tau4. This is the first detection of TiO and VO in the J-band in a sub-stellar mass object. The optical spectrum of KPNO-Tau4 exhibits weak K I and Na I lines, weak absorption by CaH, and strong VO bands, also signatures of a lower gravity atmosphere. G 196-3B shows absorption features in both wavelength regions like those of KPNO-Tau4 suggesting that its age and mass are at the lower end of published estimates. Whereas sigma Ori 51 appears to be consistent with a young sub-stellar object, sigma Ori 47 shows signatures of high gravity most closely resembling an old L1.5/L0, and can not be a member of the sigma Orionis cluster.

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arXiv2003

The NIRSPEC Brown Dwarf Spectroscopic Survey. I. Low-Resolution Near-Infrared Spectra

We present the first results of a near-infrared (0.96-2.31 micron) spectroscopic survey of M, L, and T dwarfs obtained with NIRSPEC on the Keck II telescope. Our new survey has a resolving power of R = 2000 and is comprised of two major data sets: 53 J-band (1.14-1.36 micron) spectra covering all spectral types from M6 to T8 with at least two members in each spectral subclass (wherever possible), and 25 flux-calibrated spectra from 1.14 to 2.31 microns for most spectral classes between M6 and T8. Sixteen of these 25 objects have additional spectral coverage from 0.96-1.14 microns to provide overlap with optical spectra. Spectral flux ratio indexes for prominent molecular bands are derived and equivalent widths (EWs) for several atomic lines are measured. We find that a combination of four H2O and two CH4 band strengths can be used for spectral classification. Weak (EW~1-2 angstrom) atomic lines of Al I and Ca I disappear at the boundary between M and L types.

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arXiv2003

The Mass Ratio Distribution in Main-Sequence Spectroscopic Binaries Measured by IR Spectroscopy

We report infrared spectroscopic observations of a large, well-defined sample of main-sequence, single-lined spectroscopic binaries in order to detect the secondaries and derive the mass ratio distribution of short-period binaries. The sample consists of 51 Galactic disk spectroscopic binaries found in the Carney and Latham high-proper-motion survey, with primary masses in the range of 0.6--0.85 msun. Our infrared observations detect the secondaries in 32 systems, two of which have mass ratios, q=M_2/M_1, as low as ~0.20. Together with 11 systems previously identified as double-lined binaries by visible light spectroscopy, we have a complete sample of 62 binaries, out of which 43 are double-lined. The mass ratio distribution is approximately constant over the range q=1.0 to 0.3. The distribution appears to rise at lower q values, but the uncertainties are sufficiently large that we cannot rule out a distribution that remains constant. The mass distribution derived for the secondaries in our sample, and that of the extra-solar planets, apparently represent two distinct populations.

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arXiv2003

The First Substellar Subdwarf? Discovery of a Metal-poor L Dwarf with Halo Kinematics

We present the discovery of the first L-type subdwarf, 2MASS J05325346+8246465. This object exhibits enhanced collision-induced H$_2$ absorption, resulting in blue NIR colors ($J-K_s = 0.26{\pm}0.16$). In addition, strong hydride bands in the red optical and NIR, weak TiO absorption, and an optical/J-band spectral morphology similar to the L7 DENIS 0205$-$1159AB imply a cool, metal-deficient atmosphere. We find that 2MASS 0532+8246 has both a high proper motion, $μ$ = 2$\farcs60\pm0\farcs$15 yr$^{-1}$, and a substantial radial velocity, $v_{rad} = -195{\pm}11$ km s$^{-1}$, and its probable proximity to the Sun (d = 10--30 pc) is consistent with halo membership. Comparison to subsolar-metallicity evolutionary models strongly suggests that 2MASS 0532+8246 is substellar, with a mass of 0.077 $\lesssim$ M $\lesssim$ 0.085 M$_{\sun}$ for ages 10--15 Gyr and metallicities $Z = 0.1-0.01$ $Z_{\sun}$. The discovery of this object clearly indicates that star formation occurred below the Hydrogen burning mass limit at early times, consistent with prior results indicating a flat or slightly rising mass function for the lowest-mass stellar subdwarfs. Furthermore, 2MASS 0532+8246 serves as a prototype for a new spectral class of subdwarfs, additional examples of which could be found in NIR proper motion surveys.

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arXiv2002

Astrophysics of Young Star Binaries

This paper describes our study of the astrophysics of individual components in close pre-main-sequence binaries. We observed both stars in 17 systems, located in 4 nearby star forming regions, using low-resolution (R=760), infrared spectroscopy and photometry. For 29 components we detected photospheric absorption lines and were able to determine spectral type, extinction, K-band excess, and luminosity. The other 5 objects displayed featureless or pure emission line spectra. In ~50 % of the systems, the extinction and K-band excess of the primary stars dominate those of the secondaries. Masses and ages were determined for these 29 objects by placing them on the H-R diagram, overlaid with theoretical pre-main-sequence tracks. Most of the binaries appear to be coeval. The ages span 5x10^5 to 1x10^7 years. The derived masses range from the substellar, 0.06 M_sun, to 2.5 M_sun, and the mass ratios from M_2/M_1=0.04 to 1.0. Fourteen stars show evidence of circumstellar disks. The K-band excess is well correlated with the K-L color for stars with circumstellar material.

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arXiv2002

Component Masses of the Young Spectroscopic Binary UZ Tau E

We report estimates of the masses of the component stars in the pre-main-sequence spectroscopic binary UZ Tau E. These results come from the combination of our measurements of the mass ratio, M2/M1=0.28 +/-0.01, obtained using high resolution H-band spectroscopy, with the total mass of the system, (1.31 +/-0.08)(D/140pc) M_sun, derived from millimeter observations of the circumbinary disk (Simon et al. 2000). The masses of the primary and secondary are (1.016 +/-0.065)(D/140pc) M_sun and (0.294 +/-0.027)(D/140pc) M_sun, respectively. Using the orbital parameters determined from our six epochs of observation, we find that the inclination of the binary orbit, 59.8 +/-4.4 degrees, is consistent with that determined for the circumbinary disk from the millimeter observations, indicating that the disk and binary orbits are probably coplanar.

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arXiv2002

Stellar Companions to Stars with Planets

A combination of high-resolution and wide-field imaging reveals two binary stars and one triple star system among the sample of the first 11 stars with planets detected by radial velocity variations. High resolution speckle or adaptive optics (AO) data probe subarcsecond scales down to the diffraction limit of the Keck 10-m or Lick 3-m, and direct images or AO images are sensitive to a wider field, extending to 10" or 38", depending upon the camera. One of the binary systems -- HD 114762 -- was not previously known to be a spatially resolved multiple system; additional data taken with the combination of Keck adaptive optics and NIRSPEC are used to characterize the new companion. The second binary system -- Tau Boo -- was a known multiple with two conflicting orbital solutions; the current data will help constrain the discrepant estimates of periastron time and separation. Another target -- 16 Cyg B -- was a known common proper motion binary, but the current data resolve a new third component, close to the wide companion 16 Cyg A. Both the HD 114762 and 16 Cyg B systems harbor planets in eccentric orbits, while the Tau Boo binary contains an extremely close planet in a tidally-circularized orbit. Although the sample is currently small, the proportion of binary systems is comparable to that measured in the field over a similar separation range. Incorporating the null result from another companion search project lowers the overall fraction of planets in binary systems, but the detections in our survey reveal that planets can form in binaries separated by less than 50 AU.

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