334 research outputs found
The Remarkable Mid-Infrared Jet of Massive Young Stellar Object G35.20-0.74
The young massive stellar object G35.20-0.74 was observed in the mid-infrared
using T-ReCS on Gemini South. Previous observations have shown that the near
infrared emission has a fan-like morphology that is consistent with emission
from the northern lobe of a bipolar radio jet known to be associated with this
source. Mid-infrared observations presented in this paper show a monopolar
jet-like morphology as well, and it is argued that the mid-infrared emission
observed is dominated by thermal continuum emission from dust. The mid-infrared
emission nearest the central stellar source is believed to be directly heated
dust on the walls of the outflow cavity. The hydroxyl, water, and methanol
masers associated with G35.20-0.74 are spatially located along these
mid-infrared cavity walls. Narrow jet or outflow cavities such as this may also
be the locations of the linear distribution of methanol masers that are found
associated with massive young stellar objects. The fact that G35.20-0.74 has
mid-infrared emission that is dominated by the outflow, rather than disk
emission, is a caution to those that consider mid-infrared emission from young
stellar objects as only coming from circumstellar disks.Comment: Accepted for publication in ApJ Letters; 4 pages; 2 figures; a
version with full resolution images is available here:
http://www.ctio.noao.edu/~debuizer
A sub-arcsecond study of the hot molecular core in G023.01-00.41
(Abridged) METHODS: We performed SMA observations at 1.3 mm with both the
most extended and compact array configurations, providing sub-arcsecond and
high sensitivity maps of various molecular lines, including both hot-core and
outflow tracers. We also reconstruct the spectral energy distribution of the
region from millimeter to near infrared wavelengths, using the Herschel/Hi-GAL
maps, as well as archival data. RESULTS: From the spectral energy distribution,
we derive a bolometric luminosity of about 4x10^4 Lsun. Our interferometric
observations reveal that the distribution of dense gas and dust in the HMC is
significantly flattened and extends up to a radius of 8000 AU from the center
of radio continuum and maser emission in the region. The equatorial plane of
this HMC is strictly perpendicular to the elongation of the collimated bipolar
outflow, as imaged on scales of about 0.1-0.5 pc in the main CO isotopomers as
well as in the SiO(5-4) line. In the innermost HMC regions (ca. 1000 AU), the
velocity field traced by the CH3CN(12_K-11_K) line emission shows that
molecular gas is both expanding along the outflow direction following a
Hubble-law, and rotating about the outflow axis, in agreement with the (3-D)
velocity field traced by methanol masers. The velocity field associated with
rotation indicates a dynamical mass of 19 Msun at the center of the core. The
latter is likely to be concentrated in a single O9.5 ZAMS star, consistent with
the estimated bolometric luminosity of G023.01-00.41. The physical properties
of the CO(2-1) outflow emission, such as its momentum rate 6x10^-3 Msun km/s
/yr and its outflow rate 2x10^-4 Msun/yr, support our estimates of the
luminosity (and mass) of the embedded young stellar object.Comment: 24 pages, 11 figures, 6 tables, accepted by Astronomy & Astrophysic
Trigonometric Parallaxes of Massive Star Forming Regions: IV. G35.20-0.74 and G35.20-1.74
We report trigonometric parallaxes for the high-mass star forming regions
G35.20-0.74 and G35.20-1.74, corresponding to distances of 2.19 (+0.24 -0.20)
kpc and 3.27 (+0.56 -0.42) kpc, respectively. The distances to both sources are
close to their near kinematic distances and place them in the
Carina-Sagittarius spiral arm. Combining the distances and proper motions with
observed radial velocities gives the locations and full space motions of the
star forming regions. Assuming a standard model of the Galaxy, G35.20-0.74 and
G35.20-1.74 have peculiar motions of ~13 km/s and ~16 km/s counter to Galactic
rotation and ~9 km/s toward the North Galactic Pole.Comment: 16 pages, 8 figure
VLA observations of water masers towards 6.7 GHz methanol maser sources
22 GHz water and 6.7 GHz methanol masers are usually thought as signposts of
early stages of high-mass star formation but little is known about their
associations and the physical environments they occur in.
The aim was to obtain accurate positions and morphologies of the water maser
emission and relate them to the methanol maser emission recently mapped with
Very Long Baseline Interferometry. A sample of 31 methanol maser sources was
searched for 22 GHz water masers using the VLA and observed in the 6.7 GHz
methanol maser line with the 32 m Torun dish simultaneously. Water maser
clusters were detected towards 27 sites finding 15 new sources. The detection
rate of water maser emission associated with methanol sources was as high as
71%. In a large number of objects (18/21) the structure of water maser is well
aligned with that of the extended emission at 4.5 m confirming the origin
of water emission from outflows. The sources with methanol emission with
ring-like morphologies, which likely trace a circumstellar disk/torus, either
do not show associated water masers or the distribution of water maser spots is
orthogonal to the major axis of the ring. The two maser species are generally
powered by the same high-mass young stellar object but probe different parts of
its environment. The morphology of water and methanol maser emission in a
minority of sources is consistent with a scenario that 6.7 GHz methanol masers
trace a disc/torus around a protostar while the associated 22 GHz water masers
arise in outflows. The majority of sources in which methanol maser emission is
associated with the water maser appears to trace outflows. The two types of
associations might be related to different evolutionary phases.Comment: accepted by Astronomy & Astrophysic
Trigonometric Parallaxes of Massive Star Forming Regions: II. Cep A & NGC 7538
We report trigonometric parallaxes for the sources NGC 7538 and Cep A,
corresponding to distances of 2.65 [+0.12/-0.11] kpc and 0.70 [+0.04/-0.04]
kpc, respectively. The distance to NGC 7538 is considerably smaller than its
kinematic distance and places it in the Perseus spiral arm. The distance to Cep
A is also smaller than its kinematic distance and places it in the Local arm or
spur. Combining the distance and proper motions with observed radial velocities
gives the location and full space motion of the star forming regions. We find
significant deviations from circular Galactic orbits for these sources: both
sources show large peculiar motions (> 10 km/s) counter to Galactic rotation
and NGC 7538 has a comparable peculiar motion toward the Galactic center.Comment: 21 pages, 8 figures; to appear in the Astrophysical Journa
VLBI study of maser kinematics in high-mass SFRs. I. G16.59-0.05
The present paper focuses on the high-mass star-forming region G16.59-0.05.
Methods: Using the VLBA and the EVN arrays, we conducted phase-referenced
observations of the three most powerful maser species in G16.59-0.05: H2O at
22.2 GHz (4 epochs), CH3OH at 6.7 GHz (3 epochs), and OH at 1.665 GHz (1
epoch). In addition, we performed high-resolution (> 0".1), high-sensitivity (<
0.1 mJy) VLA observations of the radio continuum emission from the star-forming
region at 1.3 and 3.6 cm. Results: This is the first work to report accurate
measurements of the "relative" proper motions of the 6.7 GHz CH3OH masers. The
different spatial and 3-D velocity distribution clearly indicate that the 22
GHz water and 6.7 GHz methanol masers are tracing different kinematic
environments. The bipolar distribution of 6.7 GHz maser l.o.s. velocities and
the regular pattern of observed proper motions suggest that these masers are
tracing rotation around a central mass of about 35 solar masses. The flattened
spatial distribution of the 6.7 GHz masers, oriented NW-SE, suggests that they
can originate in a disk/toroid rotating around the massive YSO which drives the
12CO(2-1) outflow, oriented NE-SW, observed on arcsec scale. The extended,
radio continuum source observed close to the 6.7 GHz masers could be excited by
a wide-angle wind emitted from the YSO associated with the methanol masers, and
such a wind is proven to be sufficiently energetic to drive the NE-SW 12CO(2-1)
outflow. The H2O masers distribute across a region offset about 0".5 to the NW
of the CH3OH masers, in the same area where emission of high-density molecular
tracers, typical of HMCs, was detected. We postulate that a distinct YSO,
possibly in an earlier evolutionary phase than that exciting the methanol
masers, is responsible for the excitation of the water masers and the HMC
molecular lines. (Abridged)Comment: 20 pages, 8 figures, 3 tables, accepted by Astronomy and Astrophysic
The RMS Survey: Mid-Infrared Observations of Candidate Massive YSOs in the Southern Hemisphere
Abridged abstract: The Red MSX Source (RMS) survey is an ongoing effort to
return a large, well-selected sample of massive young stellar objects (MYSOs)
within our Galaxy. A series of ground-based follow-up observations are being
undertaken in order to remove contaminant objects from our list of 2000
candidates, and to begin characterising these MYSOs. As a part of these
follow-up observations, high resolution (~1") mid-IR imaging aids the
identification of contaminant objects which are resolved (UCHII regions, PN) as
opposed to those which are unresolved (YSOs, evolved stars) as well as
identifying YSOs near UCHII regions and other multiple sources. We present 10.4
micron imaging observations for 346 candidate MYSOs in the RMS survey in the
Southern Hemisphere, primarily outside the region covered by the GLIMPSE
Spitzer Legacy Survey. These were obtained using TIMMI2 on the ESO 3.6m
telescope in La Silla, Chile. Our photometric accuracy is of order 0.05Jy, and
our astrometric accuracy is 0.8", which is an improvement over the nominal 2"
accuracy of the MSX PSC.Comment: 9 page paper accepted to A&A. Online data for table 2 and figure 1
will be available in the published online version of this paper via A&A. The
paper contains 7 figures and 3 table
SiO Outflow Signatures Toward Massive Young Stellar Objects with Linearly Distributed Methanol Masers
Methanol masers are often found in linear distributions, and it has been
hypothesized that these masers are tracing circumstellar accretion disks around
young massive stars. However, recent observations in H2 emission have shown
what appear to be outflows at similar angles to the maser distribution angles,
not perpendicular as expected in the maser-disk scenario. The main motivation
behind the observations presented here is to determine from the presence and
morphology of an independent outflow tracer, namely SiO, if there are indeed
outflows present in these regions and if they are consistent or inconsistent
with the maser-disk hypothesis. For ten sources with H2 emission we obtained
JCMT single dish SiO (6-5) observations to search for the presence of this
outflow indicator. We followed up those observations with ATCA interferometric
mapping of the SiO emission in the (2-1) line in six sources. The JCMT
observations yielded a detection in the SiO (6-5) line in nine of the ten
sources. All of the sources with bright SiO lines display broad line wings
indicative of outflow. A subset of the sources observed with the JCMT have
methanol maser velocities significantly offset from their parent cloud
velocities, supporting the idea that the masers in these sources are likely not
associated with circumstellar disks. The ATCA maps of the SiO emission show
five of the six sources do indeed have SiO outflows. The spatial orientations
of the outflows are not consistent with the methanol masers delineating disk
orientations. Overall, the observations presented here seem to provide further
evidence against the hypothesis that linearly distributed methanol masers
generally trace the orientations of circumstellar disks around massive young
stars.Comment: Accepted for publication in A&A; 36 pages, 8 figures; a version with
higher quality figures can be found at http://www.jim-debuizer.net/researc
First science results from SOFIA/FORCAST: The mid-infrared view of the compact HII region W3A
The massive star forming region W3 was observed with the faint object
infrared camera for the SOFIA telescope (FORCAST) as part of the Short Science
program. The 6.4, 6.6, 7.7, 19.7, 24.2, 31.5 and 37.1 \um bandpasses were used
to observe the emission of Polycyclic Aromatic Hydrocarbon (PAH) molecules,
Very Small Grains and Big Grains. Optical depth and color temperature maps of
W3A show that IRS2 has blown a bubble devoid of gas and dust of 0.05 pc
radius. It is embedded in a dusty shell of ionized gas that contributes 40% of
the total 24 \um emission of W3A. This dust component is mostly heated by far
ultraviolet, rather than trapped Ly photons. This shell is itself
surrounded by a thin (0.01 pc) photodissociation region where PAHs show
intense emission. The infrared spectral energy distribution (SED) of three
different zones located at 8, 20 and 25\arcsec from IRS2, show that the peak of
the SED shifts towards longer wavelengths, when moving away from the star.
Adopting the stellar radiation field for these three positions, DUSTEM model
fits to these SEDs yield a dust-to-gas mass ratio in the ionized gas similar to
that in the diffuse ISM. However, the ratio of the IR-to-UV opacity of the dust
in the ionized shell is increased by a factor 3 compared to the diffuse
ISM.Comment: Accepted for publication in ApJ letters; 13 pages, 3 figures 1 tabl
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