398 research outputs found
Disk and Envelope Structure in Class 0 Protostars: I. The Resolved Massive Disk in Serpens FIRS 1
We present the first results of a program to characterize the disk and
envelope structure of typical Class 0 protostars in nearby low-mass star
forming regions. We use Spitzer IRS mid-infrared spectra, high resolution CARMA
230 GHz continuum imaging, and 2-D radiative transfer models to constrain the
envelope structure, as well as the size and mass of the circum-protostellar
disk in Serpens FIRS 1. The primary envelope parameters (centrifugal radius,
outer radius, outflow opening angle, and inclination) are well constrained by
the spectral energy distribution (SED), including Spitzer IRAC and MIPS
photometry, IRS spectra, and 1.1 mm Bolocam photometry. These together with the
excellent uv-coverage (4.5-500 klam) of multiple antenna configurations with
CARMA allow for a robust separation of the envelope and a resolved disk. The
SED of Serpens FIRS 1 is best fit by an envelope with the density profile of a
rotating, collapsing spheroid with an inner (centrifugal) radius of
approximately 600 AU, and the millimeter data by a large resolved disk with
Mdisk~1.0 Msun and Rdisk~300 AU. These results suggest that large, massive
disks can be present early in the main accretion phase. Results for the larger,
unbiased sample of Class~0 sources in the Perseus, Serpens, and Ophiuchus
molecular clouds are needed to determine if relatively massive disks are
typical in the Class 0 stage.Comment: Comments: 13 pages, 8 figures, 3 tables; accepted for publication in
the Ap
A Candidate Detection of the First Hydrostatic Core
The first hydrostatic core (FHSC) represents a very early phase in the
low-mass star formation process, after collapse of the parent core has begun
but before a true protostar has formed. This large (few AU), cool (100 K),
pressure supported core of molecular hydrogen is expected from theory, but has
yet to be observationally verified. Here we present observations of an
excellent candidate for the FHSC phase: Per-Bolo 58, a dense core in Perseus
that was previously believed to be starless. The 70 micron flux of 65 mJy, from
new deep Spitzer MIPS observations, is consistent with that expected for the
FHSC. A low signal-to-noise detection at 24 micron leaves open the possibility
that Per-Bolo 58 could be a very low luminosity protostar, however. We utilize
radiative transfer models to determine the best-fitting FHSC and protostar
models to the spectral energy distribution and 2.9 mm visibilities of Per-Bolo
58. The source is consistent with a FHSC with some source of lower opacity
through the envelope allowing 24 micron emission to escape; a small outflow
cavity or a cavity in the envelope are both possible. While we are unable to
rule out the presence of a protostar, if present it would be one of the lowest
luminosity protostellar objects yet observed, with an internal luminosity of
approximately 0.01 Lsun.Comment: 7 pages, 4 figures. Accepted for publication in the Astrophysical
Journal Letter
Coherent States of the q--Canonical Commutation Relations
For the -deformed canonical commutation relations for in some Hilbert
space we consider representations generated from a vector
satisfying , where .
We show that such a representation exists if and only if .
Moreover, for these representations are unitarily equivalent
to the Fock representation (obtained for ). On the other hand
representations obtained for different unit vectors are disjoint. We
show that the universal C*-algebra for the relations has a largest proper,
closed, two-sided ideal. The quotient by this ideal is a natural -analogue
of the Cuntz algebra (obtained for ). We discuss the Conjecture that, for
, this analogue should, in fact, be equal to the Cuntz algebra
itself. In the limiting cases we determine all irreducible
representations of the relations, and characterize those which can be obtained
via coherent states.Comment: 19 pages, Plain Te
c2d Spitzer IRS spectra of embedded low-mass young stars: gas-phase emission lines
A survey of mid-IR gas-phase emission lines of H2, H2O and various atoms
toward a sample of 43 embedded low-mass young stars in nearby star-forming
regions is presented. The sources are selected from the Spitzer "Cores to
Disks" (c2d) legacy program. The environment of embedded protostars is complex
both in its physical structure (envelopes, outflows, jets, protostellar disks)
and the physical processes (accretion, irradiation by UV and/or X-rays,
excitation through slow and fast shocks) which take place. A key point is to
spatially resolve the emission in the Spitzer-IRS spectra. An optimal
extraction method is used to separate both spatially unresolved (compact, up to
a few 100 AU) and spatially resolved (extended, 1000 AU or more) emission from
the IRS spectra. The results are compared with the c2d disk sample and
literature PDR and shock models to address the physical nature of the sources.
Both compact and extended emission features are observed. Warm (Tex few 100 K)
H2, observed through the pure rotational H2 S(0), S(1) and S(2) lines, and [S
I] 25 mu emission is observed primarily in the extended component. [S I] is
observed uniquely toward truly embedded sources and not toward disks. On the
other hand hot (Tex>700 K) H2, observed primarily through the S(4) line, and
[Ne II] emission is seen mostly in the compact component. [Fe II] and [Si II]
lines are observed in both spatial components. Hot H2O emission is found in the
compact component of some sources. The observed emission on >=1000 AU scales is
characteristic of PDR emission and likely originates in the outflow cavities in
the remnant envelope created by the stellar wind and jets from the embedded
young stars. Weak shocks along the outflow wall can also contribute. The
compact emission is likely of mixed origin, comprised of optically thick
circumstellar disk and/or jet/outflow emission from the protostellar object.Comment: 22 pages, 11 figures, accepted for publication in A&
The Spitzer c2d Survey of Nearby Dense Cores: VI. The Protostars of Lynds Dark Nebula 1221
Observations of Lynds Dark Nebula 1221 from the Spitzer Space Telescope are
presented. These data show three candidate protostars towards L1221, only two
of which were previously known. The infrared observations also show signatures
of outflowing material, an interpretation which is also supported by radio
observations with the Very Large Array. In addition, molecular line maps from
the Five College Radio Astronomy Observatory are shown.
One-dimensional dust continuum modelling of two of these protostars, IRS1 and
IRS3, is described. These models show two distinctly different protostars
forming in very similar environments. IRS1 shows a higher luminosity and larger
inner radius of the envelope than IRS3. The disparity could be caused by a
difference in age or mass, orientation of outflow cavities, or the impact of a
binary in the IRS1 core.Comment: accepted for publication in Ap
Flexibility within the middle ears of vertebrates
Introduction and aims: Tympanic middle ears have evolved multiple times independently among vertebrates, and share common features. We review flexibility within tympanic middle ears and consider its physiological and clinical implications.
Comparative anatomy: The chain of conducting elements is flexible: even the ‘single ossicle’ ears of most non-mammalian tetrapods are functionally ‘double ossicle’ ears due to mobile articulations between the stapes and extrastapes; there may also be bending within individual elements.
Simple models: Simple models suggest that flexibility will generally reduce the transmission of sound energy through the middle ear, although in certain theoretical situations flexibility within or between conducting elements might improve transmission. The most obvious role of middle-ear flexibility is to protect the inner ear from high-amplitude displacements.
Clinical implications: Inter-ossicular joint dysfunction is associated with a number of pathologies in humans. We examine attempts to improve prosthesis design by incorporating flexible components
Pre-Main Sequence variables in the VMR-D : identification of T Tauri-like accreting protostars through Spitzer-IRAC variability
We present a study of the infrared variability of young stellar objects by
means of two Spitzer-IRAC images of the Vela Molecular Cloud D (VMR-D) obtained
in observations separated in time by about six months. By using the same
space-born IR instrumentation, this study eliminates all the unwanted effects
usually unavoidable when comparing catalogs obtained from different
instruments. The VMR-D map covers about 1.5 square deg. of a site where star
formation is actively ongoing. We are interested in accreting pre-main sequence
variables whose luminosity variations are due to intermittent events of disk
accretion (i.e. active T Tauri stars and EXor type objects). The variable
objects have been selected from a catalog of more than 170,000 sources detected
at a S/N ratio > 5. We searched the sample of variables for ones whose
photometric properties are close to those of known EXor's. These latter are
monitored in a more systematic way than T Tauri stars and the mechanisms that
regulate the observed phenomenology are exactly the same. Hence the modalities
of the EXor behavior is adopted as driving criterium for selecting variables in
general. We selected 19 bona fide candidates that constitute a well-defined
sample of new variable targets for further investigation. Out of these, 10
sources present a Spitzer MIPS 24 micron counterpart, and have been classified
as 3 Class I, 5 flat spectrum and 2 Class II objects, while the other 9 sources
have spectral energy distribution compatible with phases older than Class I.
This is consistent with what is known about the small sample of known EXor's,
and suggests that the accretion flaring or EXor stage might come as a Class
I/II transition. We present also new prescriptions that can be useful in future
searches for accretion variables in large IR databases.Comment: 35 pages, 12 figures To appear in Ap
The North American and Pelican Nebulae I. IRAC Observations
We present a 9 deg^2 map of the North American and Pelican Nebulae regions
obtained in all four IRAC channels with the Spitzer Space Telescope. The
resulting photometry is merged with that at JHKs from 2MASS and a more
spatially limited survey from previous ground-based work. We use a
mixture of color- color diagrams to select a minimally contaminated set of more
than 1600 objects that we claim are young stellar objects (YSOs) associated
with the star forming region. Because our selection technique uses IR excess as
a requirement, our sample is strongly biased against inclusion of Class III
YSOs. The distribution of IRAC spectral slopes for our YSOs indicates that most
of these objects are Class II, with a peak towards steeper spectral slopes but
a substantial contribution from a tail of flat spectrum and Class I type
objects. By studying the small fraction of the sample that is optically
visible, we infer a typical age of a few Myr for the low mass population. The
young stars are clustered, with about a third of them located in eight clusters
that are located within or near the LDN 935 dark cloud. Half of the YSOs are
located in regions with surface densities higher than 1000 YSOs / deg^2. The
Class I objects are more clustered than the Class II stars.Comment: 16 pages, 12 figures, ApJ In pres
The European Registered Toxicologist (ERT) : Current status and prospects for advancement
Acknowledgements We would like to thank the participants of the five workshops in which the issues presented in this paper were discussed and the revised guidelines prepared, as well as the EUROTOX Executive Committee and the societies of toxicology of Sweden, the Netherlands, Switzerland, Austria and France for their support which allowed the workshops to take place.Peer reviewedPostprin
- …