10 research outputs found
Astrometry of circumstellar masers
The circumstellar masers around evolved stars offer an interesting
possibility to measure stellar parameters through VLBI astrometry. In this
paper the application of this technique is discussed, including the accuracy
and the uncertainties of the method. The different maser species (OH, H_2O,
SiO) have slightly different characteristics and applications. This paper does
not concern astrometry of maser spots to study the kinematics of the envelope,
but concentrates on attempting to measure the motion of the underlying star.Comment: 8 pages, 2 figures, to appear in "Mass-losing Stars and their
Circumstellar Matter", eds Y. Nakada & M. Honma, Kluwer ASSL serie
Mapping photodissociation and shocks in the vicinity of Sgr A*
We have obtained maps of the molecular emission within the central five
arcminutes (12 pc) of the Galactic center (GC) in selected molecular tracers:
SiO(2-1), HNCO(5_{0,5}-4_{0,4}), and the J=1-->0 transition of H^{13}CO+,
HN^{13}C, and C^{18}O at an angular resolution of 30" (1.2 pc). The mapped
region includes the circumnuclear disk (CND) and the two surrounding giant
molecular clouds (GMCs) of the Sgr A complex, known as the 20 and 50 km s^{-1}
molecular clouds.Additionally, we simultaneously observed the J=2-1 and 3-2
transitions of SiO toward selected positions to estimate the physical
conditions of the molecular gas. The SiO(2-1) and H^{13}CO+(1-0) emission
covers the same velocity range and presents a similar distribution. In
contrast, HNCO(5-4) emission appears in a narrow velocity range mostly
concentrated in the 20 and 50 km s^{-1} GMCs. The HNCO column densities and
fractional abundances present the highest contrast, with difference factors of
60 and 28, respectively. Their highest values are found toward the cores
of the GMCs, whereas the lowest ones are measured at the CND. SiO abundances do
not follow this trend, with high values found toward the CND, as well as the
GMCs. By comparing our abundances with those of prototypical Galactic sources
we conclude that HNCO, similar to SiO, is ejected from grain mantles into
gas-phase by nondissociative C-shocks. This results in the high abundances
measured toward the CND and the GMCs. However, the strong UV radiation from the
Central cluster utterly photodissociates HNCO as we get closer to the center,
whereas SiO seems to be more resistant against UV-photons or it is produced
more efficiently by the strong shocks in the CND. Finally, we discuss the
possible connections between the molecular gas at the CND and the GMCs using
the HNCO/SiO, SiO/CS, and HNCO/CS intensity ratios as probes of distance to the
Central cluster.Comment: 26 pages plus 2 appendixes with additional figures. 17 figures in
total. Accepted for publication in A&
Interstellar gas within pc of Sgr A
We seek to obtain a coherent and realistic three-dimensional picture of the
interstellar gas out to about 10 pc of the dynamical center of our Galaxy,
which is supposed to be at Sgr A. We review the existing observational
studies on the different gaseous components that have been identified near Sgr
A, and retain all the information relating to their spatial configuration
and/or physical state. Based on the collected information, we propose a
three-dimensional representation of the interstellar gas, which describes each
component in terms of both its precise location and morphology and its
thermodynamic properties. The interstellar gas near Sgr A can represented
by five basic components, which are, by order of increasing size: (1) a central
cavity with roughly equal amounts of warm ionized and atomic gases, (2) a ring
of mainly molecular gas, (3) a supernova remnant filled with hot ionized gas,
(4) a radio halo of warm ionized gas and relativistic particles, and (5) a belt
of massive molecular clouds. While the halo gas fills of the
studied volume, the molecular components enclose of the
interstellar mass.Comment: 21 pages, 7 figure
GASKAP -- The Galactic ASKAP Survey
A survey of the Milky Way disk and the Magellanic System at the wavelengths
of the 21-cm atomic hydrogen (HI) line and three 18-cm lines of the OH molecule
will be carried out with the Australian Square Kilometre Array Pathfinder
telescope. The survey will study the distribution of HI emission and absorption
with unprecedented angular and velocity resolution, as well as molecular line
thermal emission, absorption, and maser lines. The area to be covered includes
the Galactic plane (|b|< 10deg) at all declinations south of delta = +40deg,
spanning longitudes 167deg through 360deg to 79deg at b=0deg, plus the entire
area of the Magellanic Stream and Clouds, a total of 13,020 square degrees. The
brightness temperature sensitivity will be very good, typically sigma_T ~ 1 K
at resolution 30arcsec and 1 km/s. The survey has a wide spectrum of scientific
goals, from studies of galaxy evolution to star formation, with particular
contributions to understanding stellar wind kinematics, the thermal phases of
the interstellar medium, the interaction between gas in the disk and halo, and
the dynamical and thermal states of gas at various positions along the
Magellanic Stream.Comment: 45 pages, 8 figures, Pub. Astron. Soc. Australia (in press
MALT-45: a 7 mm survey of the southern Galaxy - I. Techniques and spectral line data
We present the first results from the MALT-45 (Millimetre Astronomer's Legacy Team-45 GHz) Galactic Plane survey. We have observed 5 square degrees (l = 330°–335°, b = ±0 ∘ . 5) for spectral lines in the 7 mm band (42–44 and 48–49 GHz), including CS (1–0), class I CH3OH masers in the 7(0,7)–6(1,6) A+ transition and SiO (1–0) v = 0, 1, 2, 3. MALT-45 is the first unbiased, large-scale, sensitive spectral line survey in this frequency range. In this paper, we present data from the survey as well as a few intriguing results; rigorous analyses of these science cases are reserved for future publications. Across the survey region, we detected 77 class I CH3OH masers, of which 58 are new detections, along with many sites of thermal and maser SiO emission and thermal CS. We found that 35 class I CH3OH masers were associated with the published locations of class II CH3OH, H2O and OH masers but 42 have no known masers within 60 arcsec. We compared the MALT-45 CS with NH3 (1,1) to reveal regions of CS depletion and high opacity, as well as evolved star-forming regions with a high ratio of CS to NH3. All SiO masers are new detections, and appear to be associated with evolved stars from the Spitzer Galactic Legacy Infrared Mid-Plane Survey Extraordinaire (GLIMPSE). Generally, within SiO regions of multiple vibrational modes, the intensity decreases as v = 1, 2, 3, but there are a few exceptions where v = 2 is stronger than v = 1
Approaching hell's kitchen: Molecular daredevil clouds in the vicinity of Sagittarius A*
We report serendipitous detections of line emission with the Atacama Large Millimeter/submillimeter Array (ALMA) in bands 3, 6, and 7 in the central parsec down to within 100 around Sgr A* at an up to now highest resolution (< 0.5) view of the Galactic center (GC) in the submillimeter (sub-mm) domain. From the 100 GHz continuum and the H39 alpha emission we obtain a uniform electron temperature around T-e similar to 6000 K for the minispiral. The spectral index (S alpha nu(alpha)) of Sagittarius A* (Sgr A*) is similar to 0.5 at 100-250 GHz and similar to 0.0 at 230-340 GHz. The bright sources in the center show spectral indices around -0.1 implying Bremsstrahlung emission, while dust emission is emerging in the minispiral exterior. Apart from CS, which is most widespread in the center, H13CO+, HC3N, SiO, SO, C2H, CH3OH, (CS)-C-13 and N2H+ are also detected. The bulk of the clumpy emission regions is at positive velocities and in a region confined by the minispiral northern arm (NA), bar, and the sources IRS 3 and 7. Although partly spatially overlapping with the radio recombination line (RRL) emission at same negative velocities, the relation to the minispiral remains unclear. A likely explanation is an infalling clump consisting of denser cloud cores embedded in diffuse gas. This central association (CA) of clouds shows three times higher CS/X (X: any other observed molecule) ratios than the circumnuclear disk (CND) suggesting a combination of higher excitation, by a temperature gradient and/or infrared (IR) pumping, and abundance enhancement due to UV and/or X-ray emission. Hence, we conclude that this CA is closer to the center than the CND is to the center. Moreover, we find molecular line emission at velocities up to 200 km s(-1). Apart from the CA, we identified two intriguing regions in the CND. One region shows emission in all molecular species and higher energy levels tested in this and previous observations and contains a methanol class I maser. The other region shows similar behavior of the line ratios such as the CA. Outside the CND, we find the traditionally quiescent gas tracer N2H+ coinciding with the largest IR dark clouds in the field. Methanol emission is found at and around previously detected methanol class I masers in the same region. We propose to make these particular regions subject to further studies in the scope of hot core, cold core, and extreme photon and/or X-ray dominated region (PDR/XDR) chemistry and consequent star formation in the central few parsecs