396 research outputs found
AzTEC 1.1 mm Observations of the MBM12 Molecular Cloud
We present 1.1 mm observations of the dust continuum emission from the MBM12
high-latitude molecular cloud observed with the Astronomical Thermal Emission
Camera (AzTEC) mounted on the James Clerk Maxwell Telescope on Mauna Kea,
Hawaii. We surveyed a 6.34 deg centered on MBM12, making this the largest
area that has ever been surveyed in this region with submillimeter and
millimeter telescopes. Eight secure individual sources were detected with a
signal-to-noise ratio of over 4.4. These eight AzTEC sources can be considered
to be real astronomical objects compared to the other candidates based on
calculations of the false detection rate. The distribution of the detected 1.1
mm sources or compact 1.1 mm peaks is spatially anti-correlated with that of
the 100 micronm emission and the CO emission. We detected the 1.1 mm
dust continuum emitting sources associated with two classical T Tauri stars,
LkHalpha262 and LkHalpha264. Observations of spectral energy distributions
(SEDs) indicate that LkHalpha262 is likely to be Class II (pre-main-sequence
star), but there are also indications that it could be a late Class I
(protostar). A flared disk and a bipolar cavity in the models of Class I
sources lead to more complicated SEDs. From the present AzTEC observations of
the MBM12 region, it appears that other sources detected with AzTEC are likely
to be extragalactic and located behind MBM12. Some of these have radio
counterparts and their star formation rates are derived from a fit of the SEDs
to the photometric evolution of galaxies in which the effects of a dusty
interstellar medium have been included.Comment: 8 pages, 6 figures, The Astrophysical Journal, in pres
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The Spitzer Infrared Spectrograph Survey of T Tauri Stars in Taurus
We present 161 Spitzer Infrared Spectrograph (IRS) spectra of T Tauri stars and young brown dwarfs in the Taurus star-forming region. All of the targets were selected based on their infrared excess and are therefore surrounded by protoplanetary disks; they form the complete sample of all available IRS spectra of T Tauri stars with infrared excesses in Taurus. We also present the IRS spectra of seven Class 0/I objects in Taurus to complete the sample of available IRS spectra of protostars in Taurus. We use spectral indices that are not significantly affected by extinction to distinguish between envelope-and disk-dominated objects. Together with data from the literature, we construct spectral energy distributions for all objects in our sample. With spectral indices derived from the IRS spectra we infer disk properties such as dust settling and the presence of inner disk holes and gaps. We find a transitional disk frequency, which is based on objects with unusually large 13-31 mu m spectral indices indicative of a wall surrounding an inner disk hole, of about 3%, and a frequency of about 20% for objects with unusually large 10 mu m features, which could indicate disk gaps. The shape and strength of the 10 mu m silicate emission feature suggests weaker 10 mu m emission and more processed dust for very low mass objects and brown dwarfs (spectral types M6-M9). These objects also display weaker infrared excess emission from their disks, but do not appear to have more settled disks than their higher-mass counterparts. We find no difference for the spectral indices and properties of the dust between single and multiple systems.NASANASA through JPL/CaltechNASA through the Spitzer Space TelescopeNational Science Foundation AST-0544588, 0901947Pennsylvania State UniversityEberly College of SciencePennsylvania Space Grant ConsortiumNSFAstronom
V819 Tau: A Rare Weak-Lined T Tauri Star with a Weak Infrared Excess
We use Spitzer data to infer that the small infrared excess of V819 Tau, a
weak-lined T Tauri star in Taurus, is real and not attributable to a
"companion" 10 arcsec to the south. We do not confirm the mid-infrared excess
in HBC 427 and V410 X-ray 3, which are also non-accreting T Tauri stars in the
same region; instead, for the former object, the excess arises from a red
companion 9 arcsec to the east. A single-temperature blackbody fit to the
continuum excess of V819 Tau implies a dust temperature of 143 K; however, a
better fit is achieved when the weak 10 and 20 micron silicate emission
features are also included. We infer a disk of sub-micron silicate grains
between about 1 AU and several 100 AU with a constant surface density
distribution. The mid-infrared excess of V819 Tau can be successfully modeled
with dust composed mostly of small amorphous olivine grains at a temperature of
85 K, and most of the excess emission is optically thin. The disk could still
be primordial, but gas-poor and therefore short-lived, or already at the debris
disk stage, which would make it one of the youngest debris disk systems known.Comment: 7 pages, 7 figures; accepted for publication in Ap
MBM 12: young protoplanetary discs at high galactic latitude
(abridged) We present Spitzer infrared observations to constrain disc and
dust evolution in young T Tauri stars in MBM 12, a star-forming cloud at high
latitude with an age of 2 Myr and a distance of 275 pc. The region contains 12
T Tauri systems, with primary spectral types between K3 and M6; 5 are weak-line
and the rest classical T Tauri stars. We first use MIPS and literature
photometry to compile spectral energy distributions for each of the 12 members
in MBM 12, and derive their IR excesses. The IRS spectra are analysed with the
newly developed two-layer temperature distribution (TLTD) spectral
decomposition method. For the 7 T Tauri stars with a detected IR excess, we
analyse their solid-state features to derive dust properties such as
mass-averaged grain size, composition and crystallinity. We find a spatial
gradient in the forsterite to enstatite range, with more enstatite present in
the warmer regions. The fact that we see a radial dependence of the dust
properties indicates that radial mixing is not very efficient in the discs of
these young T Tauri stars. The SED analysis shows that the discs in MBM 12, in
general, undergo rapid inner disc clearing, while the binary sources have
faster discevolution. The dust grains seem to evolve independently from the
stellar properties, but are mildly related to disc properties such as flaring
and accretion rates.Comment: 14 pages, accepted by Astronomy and Astrophysic
Multiplicity of very low-mass objects in the Upper Scorpius OB association: a possible wide binary population
We report the initial results of a VLT/NACO high spatial resolution imaging
survey for multiple systems among 58 M-type members of the nearby Upper
Scorpius OB association. Nine pairs with separations below 100 have been
resolved. Their small angular separations and the similarity in the brightness
of the components (DMagK <1 for all of them), indicate there is a reasonable
likelihood several of them are true binaries rather than chance projections.
Follow-up imaging observations with WHT/LIRIS of the two widest binaries
confirm that their near-infrared colours are consistent with physical very low
mass binaries. For one of these two binaries, WHT/LIRIS spectra of each
component were obtained. We find that the two components have similar M6-M7
spectral types and signatures of low-gravity, as expected for a young brown
dwarf binary in this association. Our preliminary results indicate a possible
population of very low-mass binaries with semimajor axis in the range 100 AU
150 AU, which has not been seen in the Pleiades open cluster. If these
candidates are confirmed (one is confirmed by this work), these results would
indicate that the binary properties of very low-mass stars and brown dwarfs may
depend on the environment where they form.Comment: 11 pages, 1 table, 7 figures, request high resolution copies to
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Oph 1622-2405: Not a Planetary-Mass Binary
We present an analysis of the mass and age of the young low-mass binary Oph
1622-2405. Using resolved optical spectroscopy of the binary, we measure
spectral types of M7.25+/-0.25 and M8.75+/-0.25 for the A and B components,
respectively. We show that our spectra are inconsistent with the spectral types
of M9 and M9.5-L0 from Jayawardhana & Ivanov and M9+/-0.5 and M9.5+/-0.5 from
Close and coworkers. Based on our spectral types and the theoretical
evolutionary models of Chabrier and Baraffe, we estimate masses of 0.055 and
0.019 Msun for Oph 1622-2405A and B, which are significantly higher than the
values of 0.013 and 0.007 Msun derived by Jayawardhana & Ivanov and above the
range of masses observed for extrasolar planets (M<=0.015 Msun). Planet-like
mass estimates are further contradicted by our demonstration that Oph
1622-2405A is only slightly later (by 0.5 subclass) than the composite of the
young eclipsing binary brown dwarf 2M 0535-0546, whose components have
dynamical masses of 0.034 and 0.054 Msun. To constrain the age of Oph
1622-2405, we compare the strengths of gravity-sensitive absorption lines in
optical and near-infrared spectra of the primary to lines in field dwarfs (>1
Gyr) and members of Taurus (~1 Myr) and Upper Scorpius (~5 Myr). The line
strengths for Oph 1622-2405A are inconsistent with membership in Ophiuchus (<1
Myr) and instead indicate an age similar to that of Upper Sco, which is
agreement with a similar analysis performed by Close and coworkers. We conclude
that Oph 1622-2405 is part of an older population in Sco-Cen, perhaps Upper Sco
itself.Comment: The Astrophysical Journal, in pres
Far Infrared and Submillimeter Emission from Galactic and Extragalactic Photo-Dissociation Regions
Photodissociation Region (PDR) models are computed over a wide range of
physical conditions, from those appropriate to giant molecular clouds
illuminated by the interstellar radiation field to the conditions experienced
by circumstellar disks very close to hot massive stars. These models use the
most up-to-date values of atomic and molecular data, the most current chemical
rate coefficients, and the newest grain photoelectric heating rates which
include treatments of small grains and large molecules. In addition, we examine
the effects of metallicity and cloud extinction on the predicted line
intensities. Results are presented for PDR models with densities over the range
n=10^1-10^7 cm^-3 and for incident far-ultraviolet radiation fields over the
range G_0=10^-0.5-10^6.5, for metallicities Z=1 and 0.1 times the local
Galactic value, and for a range of PDR cloud sizes. We present line strength
and/or line ratio plots for a variety of useful PDR diagnostics: [C II] 158
micron, [O I] 63 and 145 micron, [C I] 370 and 609 micron, CO J=1-0, J=2-1,
J=3-2, J=6-5 and J=15-14, as well as the strength of the far-infrared
continuum. These plots will be useful for the interpretation of Galactic and
extragalactic far infrared and submillimeter spectra observable with ISO,
SOFIA, SWAS, FIRST and other orbital and suborbital platforms. As examples, we
apply our results to ISO and ground based observations of M82, NGC 278, and the
Large Magellenic Cloud.Comment: 54 pages, 20 figures, accepted for publication in The Astrophysical
Journa
A Medium-Resolution Near-Infrared Spectral Library of Late Type Stars: I
We present an empirical infrared spectral library of medium resolution
(R~2000-3000) H (1.6 micron) and K (2.2 micron) band spectra of 218 red stars,
spanning a range of [Fe/H] from ~-2.2 to ~+0.3. The sample includes Galactic
disk stars, bulge stars from Baade's window, and red giants from Galactic
globular clusters. We report the values of 19 indices covering 12 spectral
features measured from the spectra in the library. Finally, we derive
calibrations to estimate the effective temperature, and diagnostic
relationships to determine the luminosity classes of individual stars from
near-infrared spectra.
This paper is part of a larger effort aimed at building a near-IR spectral
library to be incorporated in population synthesis models, as well as, at
testing synthetic stellar spectra.Comment: 34 pages, 12 figures; accepted for publication at ApJS; the spectra
are available from the authors upon reques
Disk Evolution in the three Nearby Star-Forming Regions of Taurus, Chamaeleon, and Ophiuchus
We analyze samples of Spitzer Infrared Spectrograph (IRS) spectra of T Tauri
stars in the Ophiuchus, Taurus, and Chamaeleon I star-forming regions, whose
median ages lie in the <1 to 2 Myr range. The median mid-infrared spectra of
objects in these three regions are similar in shape, suggesting, on average,
similar disk structures. When normalized to the same stellar luminosity, the
medians follow each other closely, implying comparable mid-infrared excess
emission from the circumstellar disks. We use the spectral index between 13 and
31 micron and the equivalent width of the 10 micron silicate emission feature
to identify objects whose disk configuration departs from that of a continuous,
optically thick accretion disk. Transitional disks, whose steep 13-31 micron
spectral slope and near-IR flux deficit reveal inner disk clearing, occur with
about the same frequency of a few percent in all three regions. Objects with
unusually large 10 micron equivalent widths are more common (20-30%); they
could reveal the presence of disk gaps filled with optically thin dust. Based
on their medians and fraction of evolved disks, T Tauri stars in Taurus and
Chamaeleon I are very alike. Disk evolution sets in early, since already the
youngest region, the Ophiuchus core (L1688), has more settled disks with larger
grains. Our results indicate that protoplanetary disks show clear signs of dust
evolution at an age of a few Myr, even as early as ~1 Myr, but age is not the
only factor determining the degree of evolution during the first few million
years of a disk's lifetime.Comment: 23 pages, 20 figures; accepted for publication in Ap
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