103 research outputs found
Profiles of Strong Permitted Lines in Classical T Tauri Stars
We present a spectral analysis of 30 T Tauri stars observed with the Hamilton
echelle spectrograph over more than a decade. One goal is to test
magnetospheric accretion model predictions. Observational evidence previously
published supporting the model, such as emission line asymmetry and a high
frequency of redshifted absorption components, are considered. We also discuss
the relation between different line forming regions and search for good
accretion rate indicators.
In this work we confirm several important points of the models, such as the
correlation between accretion and outflow, broad emission components that are
mostly central or slightly blueshifted and only the occasional presence of
redshifted absorption. We also show, however, that the broad emission
components supposedly formed in the magnetospheric accretion flow only
partially support the models. Unlike the predictions, they are sometimes
redshifted, and are mostly found to be symmetric. The published theoretical
profiles do not have a strong resemblance to our observed ones. We emphasize
the need for accretion models to include a strong turbulent component before
their profiles will match the observations. The effects of rotation, and the
outflow components, will also be needed to complete the picture.Comment: 25 pages including 9 figures, 3 tables, accepted for publication in
the Astronomical Journa
Measuring Fundamental Parameters of Substellar Objects. II: Masses and Radii
We present mass and radius derivations for a sample of very young, mid- to
late M, low-mass stellar and substellar objects in Upper Sco and Taurus. In a
previous paper, we determined effective temperatures and surface gravities for
these targets, from an analysis of their high-resolution optical spectra and
comparisons to the latest synthetic spectra. We now derive extinctions, radii,
masses and luminosities by combining our previous results with observed
photometry, surface fluxes from the synthetic spectra and the known cluster
distances. These are the first mass and radius estimates for young, very low
mass bodies that are independent of theoretical evolutionary models (though our
estimates do depend on spectral modeling). We find that for most of our sample,
our derived mass-radius and mass-luminosity relationships are in very good
agreement with the theoretical predictions. However, our results diverge from
the evolutionary model values for the coolest, lowest-mass targets: our
inferred radii and luminosities are significantly larger than predicted for
these objects at the likely cluster ages, causing them to appear much younger
than expected. We suggest that uncertainties in the evolutionary models - e.g.,
in the choice of initial conditions and/or treatment of interior convection -
may be responsible for this discrepancy. Finally, two of our late-M objects
(USco 128 and 130) appear to have masses close to the deuterium-fusion boundary
(9--14 Jupiters, within a factor of 2). This conclusion is primarily a
consequence of their considerable faintness compared to other targets with
similar extinction, spectral type and temperature (difference of 1 mag). Our
result suggests that the faintest young late-M or cooler objects may be
significantly lower in mass than the current theoretical tracks indicate.Comment: 54 pages, incl. 5 figs, accepted Ap
X-ray Properties of Pre--Main-Sequence Stars in the Orion Nebula Cluster with Known Rotation Periods
We re-analyze all archival Chandra/ACIS observations of the Orion Nebula
Cluster (ONC) to study the X-ray properties of a large sample of
pre--main-sequence (PMS) stars with optically determined rotation periods. Our
goal is to elucidate the origins of X-rays in PMS stars by seeking out
connections between the X-rays and the mechanisms most likely driving their
production--rotation and accretion. In our sample X-ray luminosity is
significantly correlated with stellar rotation, in the sense of decreasing
Lx/Lbol with more rapid rotation, suggesting that these stars are in the
"super-saturated" regime of the rotation-activity relationship. However, we
also find that stars with optical rotation periods are significantly biased to
high Lx. This is not the result of magnitude bias in the optical
rotation-period sample but rather to the diminishingly small amplitude of
optical variations in stars with low Lx. Evidently, there exists in the ONC a
population of stars whose rotation periods are unknown and that possess lower
average X-ray luminosities than those of stars with known rotation periods.
These stars may sample the linear regime of the rotation-activity relationship.
Accretion also manifests itself in X-rays, though in a somewhat
counterintuitive fashion: While stars with spectroscopic signatures of
accretion show harder X-ray spectra than non-accretors, they show lower X-ray
luminosities and no enhancement of X-ray variability. We interpret these
findings in terms of a common origin for the X-ray emission observed from both
accreting and non-accreting stars, with the X-rays from accreting stars simply
being attenuated by magnetospheric accretion columns. This suggests that X-rays
from PMS stars have their origins primarily in chromospheres, not accretion.Comment: Accepted by the Astronomical Journal. 43 pages, 16 figure
Very Low Mass Stars and Brown Dwarfs in Taurus-Auriga
We present high resolution optical spectra obtained with the HIRES
spectrograph on the Keck I telescope of low mass T Tauri stars and brown dwarfs
(LMTTs) in Taurus-Auriga. Of particular interest is the previously classified
"continuum T Tauri star" GM Tau, which has a spectral type of M6.5 and a mass
just below the stellar/substellar boundary. None of the LMTTs in Taurus are
rapidly rotating (vsini < 30 km/s), unlike low mass objects in Orion. Many of
the slowly rotating, non-accreting stars and brown dwarfs exhibit prominent
H-alpha emission (EWs of 3 - 36 A), indicative of active chromospheres. We
demonstrate empirically that the full-width at 10% of the H-alpha emission
profile peak is a more practical and possibly more accurate indicator of
accretion than either the equivalent width of H-alpha or optical veiling:
10%-widths > 270 km/s are classical T Tauri stars (i.e. accreting), independent
of stellar spectral type. Although LMTTs can have accretion rates comparable to
that of more typical, higher-mass T Tauri stars (e.g. K7-M0), the average mass
accretion rate appears to decrease with decreasing mass. The diminished
frequency of accretion disks for LMTTs, in conjunction with their lower, on
average, mass accretion rates, implies that they are formed with less massive
disks than higher-mass T Tauri stars. The radial velocities, circumstellar
properties and known binaries do not support the suggestion that many of the
lowest mass members of Taurus have been ejected from higher stellar density
regions within the cloud. Instead, LMTTs appear to have formed and are evolving
in the same way as higher-mass T Tauri stars, but with smaller disks and
shorter disk lifetimes.Comment: 27 pages, plus 8 figures, accepted for publication in Ap
An Effective Temperature Scale for Late M and L Dwarfs, from Resonance Absorption Lines of CsI and RbI
We present Keck HIRES spectra of 6 late-M dwarfs and 11 L dwarfs. Our goal is
to assign effective temperatures to the objects using detailed atmospheric
models and fine analysis of the alkali resonance absorption lines of CsI and
RbI. These yield mutually consistent results (+-150 K) when we use
``cleared-dust'' models, which account for the removal of refractory species
from the molecular states but do not include dust opacities. We find a tendency
for the RbI line to imply a slightly higher temperature, which we ascribe to an
incomplete treatment of the overlying molecular opacities. The final effective
temperatures we adopt are based on the CsI fits alone, though the RbI fits
support the CsI temperature sequence. This work, in combination with results
from the infrared, hints that dust in these atmospheres has settled out of the
high atmosphere but is present in the deep photosphere. We also derive radial
and rotational velocities for all the objects, finding that the previously
discovered trend of rapid rotation for very low mass objects is quite
pervasive. To improve on our analysis, there is a clear need for better
molecular line lists and a more detailed understanding of dust formation and
dynamics.Comment: 53 pages, including 20 figures and 2 Tables; accepted in Ap
Recommended from our members
Kepler-4B: A Hot Neptune-Like Planet of A G0 Star Near Main-Sequence Turnoff
Early time-series photometry from NASA's Kepler spacecraft has revealed a planet transiting the star we term Kepler-4, at R.A. = 19(h)02(m)27.(s)68, delta = +50 degrees 08'08 '' 7. The planet has an orbital period of 3.213 days and shows transits with a relative depth of 0.87 x 10(-3) and a duration of about 3.95 hr. Radial velocity (RV) measurements from the Keck High Resolution Echelle Spectrometer show a reflex Doppler signal of 9.3(-1.9)(+1.1) m s(-1), consistent with a low-eccentricity orbit with the phase expected from the transits. Various tests show no evidence for any companion star near enough to affect the light curve or the RVs for this system. From a transit-based estimate of the host star's mean density, combined with analysis of high-resolution spectra, we infer that the host star is near turnoff from the main sequence, with estimated mass and radius of 1.223(-0.091)(+0.053) M(circle dot) and 1.487(-0.084)(+0.071) R(circle dot).We estimate the planet mass and radius to be {M(P), R(P)} = {24.5 +/- 3.8 M(circle plus), 3.99 +/- 0.21 R(circle plus)}. The planet's density is near 1.9 g cm(-3); it is thus slightly denser and more massive than Neptune, but about the same size.W. M. Keck FoundationNASA's Science Mission DirectorateAstronom
Observations of T-Tauri Stars using HST-GHRS: I. Far Ultraviolet Emission Lines
We have analyzed GHRS data of eight CTTS and one WTTS. The GHRS data consists
of spectral ranges 40 A wide centered on 1345, 1400, 1497, 1550, and 1900 A.
These UV spectra show strong SiIV, and CIV emission, and large quantities of
sharp (~40 km/s) H2 lines. All the H2 lines belong to the Lyman band and all
the observed lines are single peaked and optically thin. The averages of all
the H2 lines centroids for each star are negative which may indicate that they
come from an outflow. We interpret the emission in H2 as being due to
fluorescence, mostly by Ly_alpha, and identify seven excitation routes within 4
A of that line. We obtain column densities (10^12 to 10^15 cm^-2) and optical
depths (~1 or less) for each exciting transition. We conclude that the
populations are far from being in thermal equilibrium. We do not observe any
lines excited from the far blue wing of Ly_alpha, which implies that the
molecular features are excited by an absorbed profile. SiIV and CIV (corrected
for H2 emission) have widths of ~200 km/s, and an array of centroids
(blueshifted lines, centered, redshifted). These characteristics are difficult
to understand in the context of current models of the accretion shock. For DR
Tau we observe transient strong blueshifted emission, perhaps the a result of
reconnection events in the magnetosphere. We also see evidence of multiple
emission regions for the hot lines. While CIV is optically thin in most stars
in our sample, SiIV is not. However, CIV is a good predictor of SiIV and H2
emission. We conclude that most of the flux in the hot lines may be due to
accretion processes, but the line profiles can have multiple and variable
components.Comment: 67 pages, 19 figures, Accepted in Ap
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