366 research outputs found
The initial conditions of stellar protocluster formation. II. A catalogue of starless and protostellar clumps embedded in IRDCs in the Galactic longitude range 15<l<55
We present a catalogue of starless and protostellar clumps associated with
infrared dark clouds (IRDCs) in a 40 degrees wide region of the inner Galactic
Plane (b<1). We have extracted the far-infrared (FIR) counterparts of 3493
IRDCs with known distance in the Galactic longitude range 15<l<55 and searched
for the young clumps using Hi-GAL, the survey of the Galactic Plane carried out
with the Herschel satellite. Each clump is identified as a compact source
detected at 160, 250 and 350 mum. The clumps have been classified as
protostellar or starless, based on their emission (or lack of emission) at 70
mum. We identify 1723 clumps, 1056 (61%) of which are protostellar and 667
(39%) starless. These clumps are found within 764 different IRDCs, 375 (49%) of
which are only associated with protostellar clumps, 178 (23%) only with
starless clumps, and 211 (28%) with both categories of clumps. The clumps have
a median mass of 250 M_sun and range up to >10^4$ M_sun in mass and up to 10^5
L_sun in luminosity. The mass-radius distribution shows that almost 30% of the
starless clumps identified in this survey could form high-mass stars, however
these massive clumps are confined in only ~4% of the IRDCs. Assuming a minimum
mass surface density threshold for the formation of high-mass stars, the
comparison of the numbers of massive starless clumps and those already
containing embedded sources suggests an upper limit lifetime for the starless
phase of 10^5 years for clumps with a mass M>500 M_sun.Comment: accepted for publication in MNRAS. Online catalogues available soon,
please contact the authors if intereste
Physical and chemical differentiation of the luminous star-forming region W49A - Results from the JCMT Spectral Legacy Survey
The massive and luminous star-forming region W49A is a well known Galactic
candidate to probe the physical conditions and chemistry similar to those
expected in external starburst galaxies. We aim to probe the physical and
chemical structure of W49A on a spatial scale of ~0.8 pc based on the JCMT
Spectral Legacy Survey, which covers the frequency range between 330 and 373
GHz. The wide 2x2 arcminutes field and the high spectral resolution of the HARP
instrument on JCMT provides information on the spatial structure and kinematics
of the cloud. For species where multiple transitions are available, we estimate
excitation temperatures and column densities. We detected 255 transitions
corresponding to 60 species in the 330-373 GHz range at the center position of
W49A. Excitation conditions can be probed for 16 molecules. The chemical
composition suggests the importance of shock-, PDR-, and hot core chemistry.
Many molecular lines show a significant spatial extent across the maps
including high density tracers (e.g. HCN, HNC, CS, HCO+) and tracers of
UV-irradiation (e.g. CN and C2H). Large variations are seen between the
sub-regions with mostly blue-shifted emission toward the Eastern tail, mostly
red-shifted emission toward the Northern clump, and emission peaking around the
expected source velocity toward the South-west clump. A comparison of column
density ratios of characteristic species observed toward W49A to Galactic PDRs
suggests that while the chemistry toward the W49A center is driven by a
combination of UV-irradiation and shocks, UV-irradiation dominates for the
Northern Clump, Eastern tail, and South-west clump regions. A comparison to a
starburst galaxy and an AGN suggests similar C2H, CN, and H2CO abundances (with
respect to the dense gas tracer 34CS) between the ~0.8 pc scale probed for W49A
and the >1 kpc regions in external galaxies with global star-formation.Comment: Proposed for acceptance in A&A, abstract abridge
A Connection between Submillimeter Continuum Flux and Separation in Young Binaries
We have made sensitive 800-micron continuum observations of low-mass,
pre-main sequence (PMS) binary stars with projected separations less than 25 AU
in Taurus-Auriga to study disks in the young binary environment. We did not
detect any of the observed binaries, with typical 3-sigma upper limits of about
30 mJy. Combining our observations with previous 1300-micron observations of
PMS Taurus binaries by Beckwith et al. (1990) and others, we find that the
submillimeter fluxes from binaries with projected separations between 1 AU and
50 AU are significantly lower than fluxes from binaries with projected
separations > 50 AU. The submillimeter fluxes from the wider binaries are
consistent with those of PMS single stars. This may indicate lower disk surface
densities and masses in the close binaries. Alternatively, dynamical clearing
of gaps by close binaries is marginally sufficient to lower their submillimeter
fluxes to the observed levels, even without reduction of surface densities
elsewhere in the disks.Comment: 12 pages, uuencoded compressed postscript with figures; Wisconsin
Astrophysics 526; to appear in ApJ Letter
Deuteration in infrared dark clouds
Much of the dense gas in molecular clouds has a filamentary structure but the detailed structure and evolution of this gas is poorly known. We have observed 54 cores in infrared dark clouds (IRDCs) using N2H+ (1−0) and (3−2) to determine the kinematics of the densest material, where stars will form. We also observed N2D+ (3−2) towards 29 of the brightest peaks to analyse the level of deuteration which is an excellent probe of the quiescent of the early stages of star formation. There were 13 detections of N2D+ (3−2). This is one of the largest samples of IRDCs yet observed in these species. The deuteration ratio in these sources ranges between 0.003 and 0.14. For most of the sources the material traced by N2D+ and N2H+ (3−2) still has significant turbulent motions, however three objects show subthermal N2D+ velocity dispersion. Surprisingly the presence or absence of an embedded 70μm source shows no correlation with the detection of N2D+ (3−2), nor does it correlate with any change in velocity dispersion or excitation temperature. Comparison with recent models of deuteration suggest evolutionary time-scales of these regions of several free-fall times or less
125 - 211 GHz low noise MMIC amplifier design for radio astronomy
To achieve the low noise and wide bandwidth required for millimeter wavelength astronomy applications, superconductor-insulator-superconductor (SIS) mixer based receiver systems have typically been used. This paper investigates the performance of high electron mobility transistor (HEMT) based low noise amplifiers (LNAs) as an alternative approach for systems operating in the 125 — 211 GHz frequency range. A four-stage, common-source, unconditionally stable monolithic microwave integrated circuit (MMIC) design is presented using the state-of-the-art 35 nm indium phosphide HEMT process from Northrop Grumman Corporation. The simulated MMIC achieves noise temperature (T_e) lower than 58 K across the operational bandwidth, with average T_e of 38.8 K (corresponding to less than 5 times the quantum limit (hf/k) at 170 GHz) and forward transmission of 20.5 ± 0.85 dB. Input and output reflection coefficients are better than -6 and -12 dB, respectively, across the desired bandwidth. To the authors knowledge, no LNA currently operates across the entirety of this frequency range. Successful fabrication and implementation of this LNA would challenge the dominance SIS mixers have on sub-THz receivers
The low wind expansion velocity of metal-poor carbon stars in the Halo and the Sagittarius stream
We report the detection, from observations using the James Clerk Maxwell
Telescope, of CO J 3 2 transition lines in six carbon stars, selected
as members of the Galactic Halo and having similar infrared colors. Just one
Halo star had been detected in CO before this work. Infrared observations show
that these stars are red (J-K 3), due to the presence of large dusty
circumstellar envelopes. Radiative transfer models indicates that these stars
are losing mass with rather large dust mass-loss rates in the range 1--3.3
Myr, similar to what can be observed in the
Galactic disc. We show that two of these stars are effectively in the Halo, one
is likely linked to the stream of the Sagittarius Dwarf Spheroidal galaxy (Sgr
dSph), and the other three stars certainly belong to the thick disc. The wind
expansion velocities of the observed stars are low compared to carbon stars in
the thin disc and are lower for the stars in the Halo and the Sgr dSph stream
than in the thick disc. We discuss the possibility that the low expansion
velocities result from the low metallicity of the Halo carbon stars. This
implies that metal-poor carbon stars lose mass at a rate similar to metal-rich
carbon stars, but with lower expansion velocities, as predicted by recent
theoretical models. This result implies that the current estimates of mass-loss
rates from carbon stars in Local Group galaxies will have to be reconsidered.Comment: 10 pages, 7 figures, accepted for publication in MNRA
The Connection between Submillimeter Continuum Flux and Binary Separation in Young Binaries: Evidence of Interaction between Stars and Disks
We present 800 micron continuum photometry of pre-main-sequence binary stars
with projected separations a_p < 150 AU in the Sco-Oph star-forming region.
Combining our observations with published 1300 micron photometry, we find that
binaries in Sco-Oph with 1 < a_p < 50--100 AU have lower submillimeter fluxes
than wider binaries or single stars, as previously found for Taurus- Auriga
binaries. The wide binaries and single stars have indistinguishable
submillimeter flux distributions. Thus, binary companions with separations less
than 50--100 AU significantly influence the nature of associated disks.
We have explored the hypothesis that the reduction in submillimeter flux is
the result of gaps cleared in disks by companions. Gap clearing produces the
qualitative dependence of submillimeter flux on binary separation, and a simple
model suggests that large gaps in disks with surface densities typical of
wide-binary or single-star disks can reduce submillimeter fluxes to levels
consistent with the observed limits. This model shows that the present
submillimeter flux upper limits do not necessarily imply a large reduction in
disk surface densities.
Two-thirds of the young binaries were detected by IRAS, showing that most
binaries have circumstellar disks. These fluxes place lower limits of 10^{-5}
M_sun on circumstellar disk masses. The submillimeter fluxes place upper limits
of 0.005 M_sun on circumbinary disk masses. Thus massive circumbinary disks are
rare among binaries with separations between a few AU and 100 AU. Circumbinary
disks are found around some close binaries.Comment: ApJ in press (Feb. 10, 1996). LaTeX, 35 pages, uses AASTeX macros.
Complete PostScript version with figures available from
http://www.astro.wisc.edu/prints/prints.html or by e-mail request to
[email protected]
A Photometrically and Morphologically Variable Infrared Nebula in L483
We present narrow and broad K-band observations of the Class 0/I source IRAS
18148-0440 that span 17 years. The infrared nebula associated with this
protostar in the L483 dark cloud is both morphologically and photometrically
variable on a time scale of only a few months. This nebula appears to be an
infrared analogue to other well-known optically visible variable nebulae
associated with young stars, such as Hubble's Variable Nebula. Along with
Cepheus A, this is one of the first large variable nebulae to be found that is
only visible in the infrared. The variability of this nebula is most likely due
to changing illumination of the cloud rather than any motion of the structure
in the nebula. Both morphological and photometric changes are observed on a
time scale only a few times longer than the light crossing time of the nebula,
suggesting very rapid intrinsic changes in the illumination of the nebula. Our
narrow-band observations also found that H_2 knots are found nearly twice as
far to the east of the source as to its west, and that H_2 emission extends
farther east of the source than the previously known CO outflow.Comment: 19 pages, 6 figures, 1 tabl
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