19 research outputs found
The nature of the methanol maser ring G23.657-00.127
Methanol masers are associated with young high-mass stars and are an
important tool for investigating the process of massive star formation. The
recently discovered methanol maser ring in G23.657-00.127 provides an excellent
``laboratory'' for a detailed study of the nature and physical origin of
methanol maser emission, as well as parallax and proper motion measurements.
Multi-epoch observations of the 12.2 GHz methanol maser line from the ring were
conducted using the Very Long Baseline Array. Interferometric observations with
milliarcsecond resolution enabled us to track single maser spots in great
detail over a period of 2 years. We have determined the trigonometric parallax
of G23.657-00.127 to be 0.313+/-0.039 mas, giving a distance of
3.19{+0.46}{-0.35} kpc. The proper motion of the source indicates that it is
moving with the same circular velocity as the LSR, but it shows a large
peculiar motion of about 35 km/s toward the Galactic center.Comment: 6 pages, 3 figures, accepted for publication in A&
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
The VLBA Calibrator Search for the BeSSeL Survey
We present the results of a survey of radio continuum sources near the
Galactic plane using the Very Long Baseline Array (VLBA). Our observations are
designed to identify compact extragalactic sources of milliarcsecond size that
can be used for parallax measurements in the Bar and Spiral Structure Legacy
Survey. We selected point sources from the NVSS and CORNISH catalogs with flux
densities above 30 mJy and within 1.5\degr of known maser targets. Of the
1529 sources observed, 199 were detected. For sources detected on 3 or more
baselines, we determined accurate positions and evaluated their quality as
potential calibrators. Most of the 1330 sources that were not detected with the
VLBA are probably of extragalactic origin.Comment: 27 pages, 3 figures, 3 tables. Table 3 is available on the homepage
of the BeSSeL survey:
http://www.mpifr-bonn.mpg.de/staff/abrunthaler/BeSSeL/index.shtm
The Bar and Spiral Structure Legacy (BeSSeL) Survey: Mapping the Milky Way with VLBI Astrometry
Astrometric Very Long Baseline Interferometry (VLBI) observations of maser
sources in the Milky Way are used to map the spiral structure of our Galaxy and
to determine fundamental parameters such as the rotation velocity ()
and curve and the distance to the Galactic center (R). Here, we present an
update on our first results, implementing a recent change in the knowledge
about the Solar motion. It seems unavoidable that the IAU recommended values
for R and need a substantial revision. In particular the
combination of 8.5 kpc and 220 \kms\, can be ruled out with high confidence.
Combining the maser data with the distance to the Galactic center from stellar
orbits and the proper motion of Sgr\,A* gives best values of R = 8.3
0.23 kpc and = 239 or 246 7 \kms, for Solar motions of V = 12.23 and 5.25 \kms, respectively. Finally, we give an outlook to
future observations in the Bar and Spiral Structure Legacy (BeSSeL) Survey.Comment: 6 pages, 3 figures. 'Highlight talk' held at the Meeting of the
Astronomische Gesellschaft (2010). To be published in Reviews in Modern
Astronomy, Volume 2
Techniques for Accurate Parallax Measurements for 6.7-GHz Methanol Masers
The BeSSeL Survey is mapping the spiral structure of the Milky Way by
measuring trigonometric parallaxes of hundreds of maser sources associated with
high-mass star formation. While parallax techniques for water masers at high
frequency (22 GHz) have been well documented, recent observations of methanol
masers at lower frequency (6.7 GHz) have revealed astrometric issues associated
with signal propagation through the ionosphere that could significantly limit
parallax accuracy. These problems displayed as a "parallax gradient" on the sky
when measured against different background quasars. We present an analysis
method in which we generate position data relative to an "artificial quasar" at
the target maser position at each epoch. Fitting parallax to these data can
significantly mitigate the problems and improve parallax accuracy
Trigonometric Parallaxes of High Mass Star Forming Regions: the Structure and Kinematics of the Milky Way
Over 100 trigonometric parallaxes and proper motions for masers associated
with young, high-mass stars have been measured with the BeSSeL Survey, a VLBA
key science project, the EVN, and the Japanese VERA project. These measurements
provide strong evidence for the existence of spiral arms in the Milky Way,
accurately locating many arm segments and yielding spiral pitch angles ranging
from 7 to 20 degrees. The widths of spiral arms increase with distance from the
Galactic center. Fitting axially symmetric models of the Milky Way with the 3-D
position and velocity information and conservative priors for the solar and
average source peculiar motions, we estimate the distance to the Galactic
center, Ro, to be 8.34 +/- 0.16 kpc, a circular rotation speed at the Sun, To,
to be 240 +/- 8 km/s, and a rotation curve that is nearly flat (a slope of -0.2
+/- 0.4 km/s/kpc) between Galactocentric radii of 5 and 16 kpc. Assuming a
"universal" spiral galaxy form for the rotation curve, we estimate the thin
disk scale length to be 2.44 +/- 0.16 kpc. The parameters Ro and To are not
highly correlated and are relatively insensitive to different forms of the
rotation curve. Adopting a theoretically motivated prior that high-mass star
forming regions are in nearly circular Galactic orbits, we estimate a global
solar motion component in the direction of Galactic rotation, Vsun = 14.6 +/-
5.0 km/s. While To and Vsun are significantly correlated, the sum of these
parameters is well constrained, To + Vsun = 255.2 +/- 5.1 km/s, as is the
angular speed of the Sun in its orbit about the Galactic center, (To + Vsun)/Ro
= 30.57 +/- 0.43 km/s/kpc. These parameters improve the accuracy of estimates
of the accelerations of the Sun and the Hulse-Taylor binary pulsar in their
Galactic orbits, significantly reducing the uncertainty in tests of
gravitational radiation predicted by general relativity.Comment: 38 pages, 6 tables, 6 figures; v2 fixed typos and updated pulsar
section; v3 replaced fig 2 (wrong file
European VLBI Network observations of 6.7 GHz methanol masers in clusters of massive young stellar objects
Context. Methanol masers at 6.7 GHz are associated with high-mass star-forming regions (HMSFRs) and often have mid-infrared (MIR) counterparts characterized by extended emission at 4.5 μm, which likely traces outflows from massive young stellar objects (MYSOs).
Aims. Our objectives are to determine the milliarcsecond (mas) morphology of the maser emission and to examine if it comes from one or several candidate MIR counterparts in the clusters of MYSOs.
Methods. The European VLBI Network (EVN) was used to image the 6.7 GHz maser line with ∼2.1' field of view toward 14 maser sites from the Torun catalog. Quasi-simultaneous observations were carried out with the Torun 32 m telescope.
Results. We obtained maps with mas angular resolution that showed diversity of methanol emission morphology: a linear distribution (e.g., G37.753−00.189), a ring-like (G40.425+00.700), and a complex one (e.g., G45.467+00.053). The maser emission is usually associated with the strongest MIR counterpart in the clusters; no maser emission was detected from other MIR sources in the fields of
view of 2.1' in diameter. The maser source luminosity seems to correlate with the total luminosity of the central MYSO. Although the Very Long Baseline Interferometry (VLBI) technique resolves a significant part of the maser emission, the morphology is still well determined. This indicates that the majority of maser components have compact cores
VLBI study of maser kinematics in high-mass SFRs. II. G23.01-0.41
The present paper focuses on the high-mass star-forming region G23.01-0.41.
Methods: Using the VLBA and the EVN arrays, we conducted phase-referenced
observations of the three most powerful maser species in G23.01-0.41: 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 HMC at 1.3
and 3.6 cm. Results: We have detected H2O, CH3OH, and OH maser emission
clustered within 2000 AU from the center of a flattened HMC, oriented SE-NW,
from which emerges a massive 12CO outflow, elongated NE-SW, extended up to the
pc-scale. Although the three maser species show a clearly different spatial and
velocity distribution and sample distinct environments around the massive YSO,
the spatial symmetry and velocity field of each maser specie can be explained
in terms of expansion from a common center, which possibly denotes the position
of the YSO driving the maser motion. Water masers trace both a fast shock (up
to 50 km/s) closer to the YSO, powered by a wide-angle wind, and a slower (20
km/s) bipolar jet, at the base of the large-scale outflow. Since the compact
free-free emission is found offset from the putative location of the YSO along
a direction consistent with that of the maser jet axis, we interpret the radio
continuum in terms of a thermal jet. The velocity field of methanol masers can
be explained in terms of a composition of slow (4 km/s in amplitude) motions of
radial expansion and rotation about an axis approximately parallel to the maser
jet. Finally, the distribution of line of sight velocities of the hydroxyl
masers suggests that they can trace gas less dense (n(H2) < 10^6 cm^-3) and
more distant from the YSO than that traced by the water and methanol masers,
which is expanding toward the observer. (Abridged)Comment: 23 pages, 8 figures, 4 tables, accepted by Astronomy and Astrophysic
European VLBI Network imaging of 6.7 GHz methanol masers
Context. Methanol masers at 6.7 GHz are well known tracers of high-mass star-forming regions. However, their origin is still not clearly understood. Aims. We aimed to determine the morphology and velocity structure for a large sample of the maser emission with generally lower peak flux densities than those in previous surveys. Methods. Using the European VLBI Network (EVN) we imaged the remaining sources from a sample of sources that were selected from the unbiased survey using the Torun 32 m dish. In this paper we report the results for 17 targets. Together they form a database of a total of 63 source images with high sensitivity (3σ rms = 15−30 mJy/beam ), milliarcsecond angular resolution (6−10 mas) and very good spectral resolution (0.09 km/s or 0.18 km/s ) for detailed studies. Results. We studied in detail the properties of the maser clouds and calculated the mean and median values of the projected size (17.4 ± 1.2 au and 5.5 au, respectively) as well as the FWHM of the line (0.373 ± 0.011 km/s and 0.315 km/s for the mean and median values, respectively), testing whether it was consistent with Gaussian profile. We also found maser clouds with velocity gradients (71%) that ranged from 0.005 km/s/au to 0.210 km/s/au. We tested the kinematic models to explain the observed structures of the 6.7 GHz emission. There were targets where the morphology supported the scenario of a rotating and expanding disk or a bipolar outflow. Comparing the interferometric and single-dish spectra we found that, typically, 50–70% of the flux was missing. This phenomena is not strongly related to the distance of the source. Conclusions. The EVN imaging reveals that in the complete sample of 63 sources the ring-like morphology appeared in 17% of sources, arcs were seen in a further 8%, and the structures were complex in 46% cases. The ultra-compact (UC) H II regions coincide in position in the sky for 13% of the sources. They are related both to extremely high and low luminosity masers from the sample