22 research outputs found
High-Mass Proto-Stellar Candidates - I : The Sample and Initial Results
We describe a systematic program aimed at identifying and characterizing
candidate high-mass proto-stellar objects (HMPOs). Our candidate sample
consists of 69 objects selected by criteria based on those established by
Ramesh & Sridharan (1997) using far-infrared, radio-continuum and molecular
line data. Infrared-Astronomical-Satellite (IRAS) and
Midcourse-Space-Experiment (MSX) data were used to study the larger scale
environments of the candidate sources and to determine their total luminosities
and dust temperatures. To derive the physical and chemical properties of our
target regions, we observed continuum and spectral line radiation at millimeter
and radio wavelengths. We imaged the free-free and dust continuum emission at
wavelengths of 3.6 cm and 1.2 mm, respectively, searched for H2O and CH3OH
maser emission and observed the CO 2-1 and several NH3 lines toward all sources
in our sample. Other molecular tracers were observed in a subsample. The
presented results indicate that a substantial fraction of our sample harbors
HMPOs in a pre-UCHII region phase, the earliest known stage in the high-mass
star formation process.Comment: 16 pages, 11 eps-figures. Astrophysical Journal, in pres
Three intermediate-mass YSOs with different properties emerging from the same natal cloud in IRAS 00117+6412
We observed with the VLA, PdBI, and SMA the centimeter and millimeter
continuum, N2H+(1-0), and CO(2-1) emission associated with a dusty cloud
harboring a nascent cluster with intermediate-mass protostars. At centimeter
wavelengths we found a strong source, tracing a UCHII region, at the eastern
edge of the dusty cloud, with a shell-like structure, and with the
near-infrared counterpart falling in the center of the shell. This is
presumably the most massive source of the forming cluster. About 15'' to the
west of the UCHII region and well embedded in the dusty cloud, we detected a
strong millimeter source, MM1, associated with centimeter and near-infrared
emission. MM1 seems to be driving a prominent high-velocity CO bipolar outflow,
and is embedded in a ridge of dense gas traced by N2H+. We estimated that MM1
is an intermediate-mass source in the Class 0/I phase. About 15'' to the south
of MM1, and still more deeply embedded in the dusty cloud, we detected a
compact millimeter source, MM2, with neither centimeter nor near-infrared
emission, but with water maser emission. MM2 is associated with a clump of
N2H+, whose kinematics reveal a clear velocity gradient and additionally we
found signposts of infall motions. MM2, being deeply embedded within the dusty
cloud, with an associated water maser but no hints of CO outflow emission, is
an intriguing object, presumably of intermediate mass. In conclusion, the UCHII
region is found at the border of a dusty cloud which is currently undergoing
active star formation. Two intermediate-mass protostars in the dusty cloud seem
to have formed after the UCHII region and have different properties related to
the outflow phenomenon.Comment: accepted to Astronomy and Astrophysic
Massive star formation in 100,000 years from turbulent and pressurized molecular clouds
Massive stars (with mass m_* > 8 solar masses) are fundamental to the
evolution of galaxies, because they produce heavy elements, inject energy into
the interstellar medium, and possibly regulate the star formation rate. The
individual star formation time, t_*f, determines the accretion rate of the
star; the value of the former quantity is currently uncertain by many orders of
magnitude, leading to other astrophysical questions. For example, the variation
of t_*f with stellar mass dictates whether massive stars can form
simultaneously with low-mass stars in clusters. Here we show that t_*f is
determined by conditions in the star's natal cloud, and is typically ~10^5 yr.
The corresponding mass accretion rate depends on the pressure within the cloud
- which we relate to the gas surface density - and on both the instantaneous
and final stellar masses. Characteristic accretion rates are sufficient to
overcome radiation pressure from ~100 solar mass protostars, while
simultaneously driving intense bipolar gas outflows. The weak dependence of
t_*f on the final mass of the star allows high- and low-mass star formation to
occur nearly simultaneously in clusters.Comment: 9 pages plus 2 figures, Nature, 416, 59 (7th March 2002
Survey of intermediate/high-mass star-forming regions at centimeter and millimeter wavelengths
We present the results of millimeter and centimeter continuum observations,
made with the IRAM 30m telescope and the VLA, toward a sample of 11 luminous
IRAS sources classified as high-mass protostellar object candidates. We find
1.2 mm emission for all (but one) regions likely tracing the dust core in which
the massive young stellar object is forming, for which we estimate masses
ranging from 10 to 140 Msun. For all the sources, but one, we detect centimeter
emission associated with the IRAS source, being compact or ultracompact HII
region candidates, with early B-type stars as ionizing stars. The 7 mm emission
is partially resolved for the four sources observed at this wavelength, with
contribution of dust emission at 7 mm ranging from negligible to 44%. By
combining our data with infrared surveys we fitted the spectral energy
distribution of the sources. Finally, we find a correlation between the degree
of disruption of the natal cloud, estimated from the fraction of dust emission
associtaed with the centimeter source relative to the total amount of dust in
its surroundings, and the size of the centimeter source. From this correlation,
we establish an evolutionary sequence which is consistent with the evolutionary
stage expected from maser/outflow/dense gas emission and with the infrared
excess.Comment: 20 pages, 12 figures, accepted for publication in A&
Bypass of glycan-dependent glycoprotein delivery to ERAD by up-regulated EDEM1
Extensive trimming of mannose residues targets a misfolded glycoprotein for endoplasmic reticulum–associated degradation (ERAD). Surprisingly, overexpression of EDEM1 or its up-regulation by the unfolded protein response bypasses this requirement. Delivery to OS9 in the ER-derived quality control compartment and ERAD becomes mannose trimming–independent, accelerating glycoprotein disposal