794 research outputs found
Empirical Isochrones for Low Mass Stars in Nearby Young Associations
Absolute ages of young stars are important for many issues in pre-main
sequence stellar and circumstellar evolution but are long recognized as
difficult to derive and calibrate. In this paper, we use literature spectral
types and photometry to construct empirical isochrones in HR diagrams for
low-mass stars and brown dwarfs in the eta Cha, epsilon Cha, and TW Hya
Associations and the beta Pic and Tuc-Hor Moving Groups. A successful theory of
pre-main sequence evolution should match the shapes of the stellar loci for
these groups of young stars. However, when comparing the combined empirical
isochrones to isochrones predicted from evolutionary models, discrepancies lead
to a spectral type (mass) dependence in stellar age estimates. Improved
prescriptions for convection and boundary conditions in the latest models of
pre-main sequence models lead to a significantly improved correspondence
between empirical and model isochrones, with small offsets at low temperatures
that may be explained by observational uncertainties or by model limitations.
Independent of model predictions, linear fits to combined stellar loci of these
regions provide a simple empirical method to order clusters by luminosity with
a reduced dependence on spectral type. Age estimates calculated from various
sets of modern models that reproduce Li depletion boundary ages of the beta Pic
Moving Group also imply a ~4 Myr age for the low mass members of the Upper Sco
OB Association, which is younger than the 11 Myr age that has been recently
estimated for intermediate mass members.Comment: Accepted by ApJ, 18 page
Sea Turtle Observations at Explosive Removals of Energy Structures
Observers were placed at offshore sites to monitor and protect sea turtles during explosive removals of oil and gas structures in the Gulf of Mexico off Louisiana and Texas. Data collected during more than 6,500 hours of monitoring at 106 structure removals in 1992 provided information on sea turtle distribution. Eighteen individuals were observed including 10 loggerheads, 2 leatherbacks, 1 hawksbill, and 5 unidentified sea turtles. The observation rate (individuals per monitoring hour) of sea turtles was about 30 times higher during aerial surveys than during day or night suiface surveys
An Optical Spectroscopic Study of T Tauri Stars. I. Photospheric Properties
Measurements of masses and ages of young stars from their location in the HR
diagram are limited by not only the typical observational uncertainties that
apply to field stars, but also by large systematic uncertainties related to
circumstellar phenomena. In this paper, we analyze flux calibrated optical
spectra to measure accurate spectral types and extinctions of 283 nearby T
Tauri stars. The primary advances in this paper are (1) the incorporation of a
simplistic accretion continuum in optical spectral type and extinction
measurements calculated over the full optical wavelength range and (2) the
uniform analysis of a large sample of stars. Comparisons between the
non-accreting TTS photospheric templates and stellar photosphere models are
used to derive conversions from spectral type to temperature. Differences
between spectral types can be subtle and difficult to discern, especially when
accounting for accretion and extinction. The spectral types measured here are
mostly consistent with spectral types measured over the past decade. However,
our new spectral types are 1-2 subclasses later than literature spectral types
for the original members of the TWA and are discrepant with literature values
for some well known Taurus CTTSs. Our extinction measurements are consistent
with other optical extinction measurements but are typically 1 mag lower than
nIR measurements, likely the result of methodological differences and the
presence of nIR excesses in most CTTSs. As an illustration of the impact of
accretion, SpT, and extinction uncertainties on the HR diagrams of young
clusters, we find that the resulting luminosity spread of stars in the TWA is
15-30%. The luminosity spread in the TWA and previously measured for binary
stars in Taurus suggests that for a majority of stars, protostellar accretion
rates are not large enough to significantly alter the subsequent evolution.Comment: Accepted by ApJ. 30 pages plus 12 pages of Tables and Reference
Collisional excitation of far-infrared line emissions from warm interstellar carbon monoxide (CO)
Motivated by recent observations with Herschel/PACS, and the availability of
new rate coefficients for the collisional excitation of CO (Yang et al. 2010),
the excitation of warm astrophysical CO is revisited with the use of numerical
and analytic methods. For the case of an isothermal medium, results have been
obtained for a wide range of gas temperatures (100 to 5000 K) and H2 densities
(1E+3 to 1E+9 cm-3), and presented in the form of rotational diagrams, in which
the logarithm of the column density per magnetic substate, log (N[J]/g[J]), is
plotted for each state, as a function of its energy, E[J]. For rotational
transitions in the wavelength range accessible to Herschel/PACS, such diagrams
are nearly linear when n(H2) > 1E+8 cm-3. When log10(n[H2]) = 6.8 to 8, they
exhibit significant negative curvature, whereas when log10(n[H2]) < 4.8 the
curvature is uniformly positive throughout the PACS-accessible range. Thus, the
observation of a positively-curved CO rotational diagram does not NECESSARILY
require the presence of multiple temperature components. Indeed, for some
sources observed with Herschel/PACS, the CO rotational diagrams show a modest
positive curvature that can be explained by a single isothermal component.
Typically, the required physical parameters are H2 densities in the 1E+4 to
1E+5 cm-3 range and temperatures, T, close to the maximum at which CO can
survive. Other sources exhibit rotational diagrams with more curvature than can
be accounted for by a single temperature component. For the case of a medium
with a power-law distribution of gas temperatures, with dN/dT proportional to T
to the power -b, results have been obtained for H2 densities 1E+3 to 1E+9 cm-3
and power-law indices, b, in the range 1 to 5; such a medium can account for a
CO rotational diagram that is more positively curved than any resulting from an
isothermal medium.Comment: Accepted for publication in the Astrophysical Journa
High-resolution Spectroscopy of [Ne II] Emission from TW Hya
We present high-resolution echelle spectra of [Ne II] 12.81 micron emission
from the classical T Tauri star (CTTS) TW Hya obtained with MICHELLE on Gemini
North. The line is centered at the stellar radial velocity and has an intrinsic
FWHM of 21\pm 4 km/s. The line width is broader than other narrow emission
lines typically associated with the disk around TW Hya. If formed in a disk,
the line broadening could result from turbulence in a warm disk atmosphere,
Keplerian rotation at an average distance of 0.1 AU from the star, or a
photoevaporative flow from the optically-thin region of the disk. We place
upper limits on the [Ne II] emission flux from the CTTSs DP Tau and BP Tau.Comment: Accepted by ApJ. 18 pages, including 2 figures and 2 table
Far infrared CO and HO emission in intermediate-mass protostars
Intermediate-mass young stellar objects (YSOs) provide a link to understand
how feedback from shocks and UV radiation scales from low to high-mass star
forming regions. Aims: Our aim is to analyze excitation of CO and HO in
deeply-embedded intermediate-mass YSOs and compare with low-mass and high-mass
YSOs. Methods: Herschel/PACS spectral maps are analyzed for 6 YSOs with
bolometric luminosities of . The maps
cover spatial scales of AU in several CO and HO lines located
in the m range. Results: Rotational diagrams of CO show two
temperature components at K and
K, comparable to low- and high-mass protostars
probed at similar spatial scales. The diagrams for HO show a single
component at K, as seen in low-mass protostars, and
about K lower than in high-mass protostars. Since the uncertainties in
are of the same order as the difference between the
intermediate and high-mass protostars, we cannot conclude whether the change in
rotational temperature occurs at a specific luminosity, or whether the change
is more gradual from low- to high-mass YSOs. Conclusions: Molecular excitation
in intermediate-mass protostars is comparable to the central AU of
low-mass protostars and consistent within the uncertainties with the high-mass
protostars probed at AU scales, suggesting similar shock
conditions in all those sources.Comment: Accepted to Astronomy & Astrophysics. 4 pages, 5 figures, 3 table
Magnetic fields of intermediate mass T Tauri stars
Aims. In this paper, we aim to measure the strength of the surface magnetic
fields for a sample of five intermediate mass T Tauri stars and one low mass T
Tauri star from late-F to mid-K spectral types. While magnetic fields of T
Tauri stars at the low mass range have been extensively characterized, our work
complements previous studies towards the intermediate mass range; this
complementary study is key to evaluate how magnetic fields evolve during the
transition from a convective to a radiative core.
Methods. We studied the Zeeman broadening of magnetically sensitive spectral
lines in the H-band spectra obtained with the CRIRES high-resolution
near-infrared spectrometer. These data are modelled using magnetic spectral
synthesis and model atmospheres. Additional constraints on non-magnetic line
broadening mechanisms are obtained from modelling molecular lines in the K band
or atomic lines in the optical wavelength region.
Results. We detect and measure mean surface magnetic fields for five of the
six stars in our sample: CHXR 28, COUP 107, V2062 Oph, V1149 Sco, and Par 2441.
Magnetic field strengths inferred from the most magnetically sensitive
diagnostic line range from 0.8 to 1.8 kG. We also estimate a magnetic field
strength of 1.9 kG for COUP 107 from an alternative diagnostic. The magnetic
field on YLW 19 is the weakest in our sample and is marginally detected, with a
strength of 0.8 kG.
Conclusions. We populate an uncharted area of the pre-main-sequence HR
diagram with mean magnetic field measurements from high-resolution
near-infrared spectra. Our sample of intermediate mass T Tauri stars in general
exhibits weaker magnetic fields than their lower mass counterparts. Our
measurements will be used in combination with other spectropolarimetric studies
of intermediate mass and lower mass T Tauri stars to provide input into
pre-main-sequence stellar evolutionary models.Comment: 8 pages, 8 figures, accepted for publication in Astronomy and
Astrophysic
The Eruption of the Candidate Young Star ASASSN-15QI
Outbursts on young stars are usually interpreted as accretion bursts caused by instabilities in the disk or the star–disk connection. However, some protostellar outbursts may not fit into this framework. In this paper, we analyze optical and near-infrared spectra and photometry to characterize the 2015 outburst of the probable young star ASASSN-15qi. The ~3.5 mag brightening in the V band was sudden, with an unresolved rise time of less than one day. The outburst decayed exponentially by 1 mag for 6 days and then gradually back to the pre-outburst level after 200 days. The outburst is dominated by emission from ~10,000 K gas. An explosive release of energy accelerated matter from the star in all directions, seen in a spectacular cool, spherical wind with a maximum velocity of 1000 km s^(−1). The wind and hot gas both disappeared as the outburst faded and the source returned to its quiescent F-star spectrum. Nebulosity near the star brightened with a delay of 10–20 days. Fluorescent excitation of H2 is detected in emission from vibrational levels as high as v = 11, also with a possible time delay in flux increase. The mid-infrared spectral energy distribution does not indicate the presence of warm dust emission, though the optical photospheric absorption and CO overtone emission could be related to a gaseous disk. Archival photometry reveals a prior outburst in 1976. Although we speculate about possible causes for this outburst, none of the explanations are compelling
How Do Stars Gain Their Mass? A JCMT/SCUBA-2 Transient Survey of Protostars in Nearby Star-forming Regions
Most protostars have luminosities that are fainter than expected from steady accretion over the protostellar lifetime. The solution to this problem may lie in episodic mass accretion—prolonged periods of very low accretion punctuated by short bursts of rapid accretion. However, the timescale and amplitude for variability at the protostellar phase is almost entirely unconstrained. In A James Clerk Maxwell Telescope/SCUBA-2 Transient Survey of Protostars in Nearby Star-forming Regions, we are monitoring monthly with SCUBA-2 the submillimeter emission in eight fields within nearby (<500 pc) star-forming regions to measure the accretion variability of protostars. The total survey area of ~1.6 deg^2 includes ~105 peaks with peaks brighter than 0.5 Jy/beam (43 associated with embedded protostars or disks) and 237 peaks of 0.125–0.5 Jy/beam (50 with embedded protostars or disks). Each field has enough bright peaks for flux calibration relative to other peaks in the same field, which improves upon the nominal flux calibration uncertainties of submillimeter observations to reach a precision of ~2%–3% rms, and also provides quantified confidence in any measured variability. The timescales and amplitudes of any submillimeter variation will then be converted into variations in accretion rate and subsequently used to infer the physical causes of the variability. This survey is the first dedicated survey for submillimeter variability and complements other transient surveys at optical and near-IR wavelengths, which are not sensitive to accretion variability of deeply embedded protostars
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