2,005 research outputs found
A multi-transition submillimeter water maser study of evolved stars - detection of a new line near 475 GHz
Context: Maser emission from the H2O molecule probes the warm, inner
circumstellar envelopes of oxygen-rich red giant and supergiant stars.
Multi-maser transition studies can be used to put constraints on the density
and temperature of the emission regions.
Aims: A number of known H2O maser lines were observed toward the long period
variables R Leo and W Hya and the red supergiant VY CMa. A search for a new,
not yet detected line near 475 GHz was conducted toward these stars.
Methods: The Atacama Pathfinder Experiment telescope was used for a
multi-transition observational study of submillimeter H2O lines.
Results: The 5_33-4_40 transition near 475 GHz was clearly detected toward VY
CMa and W Hya. Many other H2O lines were detected toward all three target
stars. Relative line intensity ratios and velocity widths were found to vary
significantly from star to star.
Conclusions: Maser action is observed in all but one line for which it was
theoretically predicted. In contrast, one of the strongest maser lines, in R
Leo by far the strongest, the 437 GHz 7_53-6_60 transition, is not predicted to
be inverted. Some other qualitative predictions of the model calculations are
at variance with our observations. Plausible reasons for this are discussed.
Based on our findings for W Hya and VY CMa, we find evidence that the H2O
masers in the AGB star W Hya arise from the regular circumstellar outflow,
while shock excitation in a high velocity flow seems to be required to excite
masers far from the red supergiant VY CMa.Comment: 9 pages, 4 figures, Astronomy and Astrophyics (in press
New Measurements of the Radio Photosphere of Mira based on Data from the JVLA and ALMA
We present new measurements of the millimeter wavelength continuum emission
from the long period variable Mira ( Ceti) at frequencies of 46 GHz, 96 GHz,
and 229 GHz (~7 mm, 3 mm, and 1 mm) based on observations obtained
with the Jansky Very Large Array (JVLA) and the Atacama Large
Millimeter/submillimeter Array (ALMA). The measured millimeter flux densities
are consistent with a radio photosphere model derived from previous
observations, where flux density, . The stellar disk
is resolved, and the measurements indicate a decrease in the size of the radio
photosphere at higher frequencies, as expected if the opacity decreases at
shorter wavelengths. The shape of the radio photosphere is found to be slightly
elongated, with a flattening of ~10-20%. The data also reveal evidence for
brightness non-uniformities on the surface of Mira at radio wavelengths. Mira's
hot companion, Mira B was detected at all three observed wavelengths, and we
measure a radius for its radio-emitting surface of
cm. The data presented here highlight the power of the JVLA and ALMA for the
study of the atmospheres of evolved stars.Comment: Accepted to ApJ; 27 pages, 7 figure
Outflow 20--2000 AU from a High-Mass Protostar in W51-IRS2
We present the results of the first high angular resolution observations of
SiO maser emission towards the star forming region W51-IRS2 made with the Very
Large Array (VLA) and Very Long Baseline Array (VLBA). Our images of the water
maser emission in W51-IRS2 reveal two maser complexes bracketing the SiO maser
source. One of these water maser complexes appears to trace a bow shock whose
opening angle is consistent with the opening angle observed in the distribution
of SiO maser emission. A comparison of our water maser image with an image
constructed from data acquired 19 years earlier clearly shows the persistence
and motion of this bow shock. The proper motions correspond to an outflow
velocity of 80 km/s, which is consistent with the data of 19 years ago (that
spanned 2 years). We have discovered a two-armed linear structure in the SiO
maser emission on scales of ~25 AU, and we find a velocity gradient on the
order of 0.1 km/s/AU along the arms. We propose that the SiO maser source
traces the limbs of an accelerating bipolar outflow close to an obscured
protostar. We estimate that the outflow makes an angle of <20 degrees with
respect to the plane of the sky. Our measurement of the acceleration is
consistent with a reported drift in the line-of-sight velocity of the W51 SiO
maser source.Comment: 19 pages, 5 figures (including 3 color). Accepted for publication in
ApJ (April 1, 2001 issue
A 1.3 cm wavelength radio flare from a deeply embedded source in the Orion BN/KL region
Aims: Our aim was to measure and characterize the short-wavelength radio
emission from young stellar objects (YSOs) in the Orion Nebula Cluster and the
BN/KL star-forming region. Methods: We used the NRAO Very Large Array at a
wavelength of 1.3 cm and we studied archival X-ray, infrared, and radio data.
Results: During our observation, a strong outburst (flux increasing >10 fold)
occurred in one of the 16 sources detected at a wavelength of 1.3cm, while the
others remained (nearly) constant. This source does not have an infrared
counterpart, but has subsequently been observed to flare in X-rays. Curiously,
a very weak variable double radio source was found at other epochs near this
position, one of whose components is coincident with it. A very high extinction
derived from modeling the X-ray emission and the absence of an infrared
counterpart both suggest that this source is very deeply embedded.Comment: 7 pages, 5 figures, accepted for publication in A&
A Parallax-based Distance Estimator for Spiral Arm Sources
The spiral arms of the Milky Way are being accurately located for the first
time via trigonometric parallaxes of massive star forming regions with the
BeSSeL Survey, using the Very Long Baseline Array and the European VLBI
Network, and with the Japanese VERA project. Here we describe a computer
program that leverages these results to significantly improve the accuracy and
reliability of distance estimates to other sources that are known to follow
spiral structure. Using a Bayesian approach, sources are assigned to arms based
on their (l,b,v) coordinates with respect to arm signatures seen in CO and HI
surveys. A source's kinematic distance, displacement from the plane, and
proximity to individual parallax sources are also considered in generating a
full distance probability density function. Using this program to estimate
distances to large numbers of star forming regions, we generate a realistic
visualization of the Milky Way's spiral structure as seen from the northern
hemisphere.Comment: 25 pages with 16 figures; to appear in Ap
Initial phases of massive star formation in high infrared extinction clouds. II. Infall and onset of star formation
The onset of massive star formation is not well understood because of
observational and theoretical difficulties. To find the dense and cold clumps
where massive star formation can take place, we compiled a sample of high
infrared extinction clouds, which were observed previously by us in the 1.2 mm
continuum emission and ammonia. We try to understand the star-formation stages
of the clumps in these high extinction clouds by studying the infall and
outflow properties, the presence of a young stellar object (YSO), and the level
of the CO depletion through a molecular line survey with the IRAM 30m and APEX
12m telescopes. Moreover, we want to know if the cloud morphology, quantified
through the column density contrast between the clump and the clouds, has an
impact on the star formation occurring inside it. We find that the HCO+(1-0)
line is the most sensitive for detecting infalling motions. SiO, an outflow
tracer, was mostly detected toward sources with infall, indicating that infall
is accompanied by collimated outflows. The presence of YSOs within a clump
depends mostly on its column density; no signs of YSOs were found below 4E22
cm-2. Star formation is on the verge of beginning in clouds that have a low
column density contrast; infall is not yet present in the majority of the
clumps. The first signs of ongoing star formation are broadly observed in
clouds where the column density contrast between the clump and the cloud is
higher than two; most clumps show infall and outflow. Finally, the most evolved
clumps are in clouds that have a column density contrast higher than three;
almost all clumps have a YSO, and in many clumps, the infall has already
halted. Hence, the cloud morphology, based on the column density contrast
between the cloud and the clumps, seems to have a direct connection with the
evolutionary stage of the objects forming inside
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