CO Masers Research Articles

Recent mass ejection from AGB star W Hya

We analyse archival ALMA (Atacama Large Millimeter/submillimeter Array) observations of two molecular line emissions, 12CO(3-2) and 29SiO(8-7), from oxygen-rich AGB (Asymptotic Giant Branch) star W Hya. Together with results of earlier VLT observations at visible and infrared wavelengths, our results suggest a two-component picture of the morpho-kinematics of the circumstellar envelope (CSE), one stable over time, at the scale of centuries, and the other variable, at the scale of years. The stable component consists of an approximately spherical shell of gas and dust expanding radially to a terminal velocity of ~5 km s−1at a distance of ~30 au from the star. It is found to display comparable features as seen in the CSE of R Dor, a star similar to W Hya. The variable component projects on the plane of the sky over a region confined to the neighbourhood of the star and elongated toward the north. Its very high density and sudden acceleration suggest an interpretation in terms of mass ejection initiated a few years ago. We discuss its properties in relation with earlier observations of dust formation in the same region. Our results offer a picture of the wind of W Hya that differs significantly from the picture that could be suggested by earlier analyses, giving evidence for a mass ejection that had been previously overlooked and underscoring new relations between the dust and gas emissions. They have an impact on the evidence published earlier for the presence of CO masers. They favour an interpretation in terms of convective cell ejections playing the main role in the generation of the nascent wind, the stable component of the CSE being seen as the result of many successive such events occurring in different directions at short time intervals.

Maser emission from the CO envelope of the asymptotic giant branch star W Hydrae

Context. Observation of CO emission around asymptotic giant branch (AGB) stars is the primary method to determine gas mass-loss rates. While radiative transfer models have shown that molecular levels of CO can become mildly inverted, causing maser emission, CO maser emission has yet to be confirmed observationally. Aims. High-resolution observations of the CO emission around AGB stars now have the brightness temperature sensitivity to detect possible weak CO maser emission. Methods. We used high angular resolution observations taken with the Atacama Large Millimeter/submillimeter Array (ALMA) to observe the small-scale structure of CO J = 3−2 emission around the oxygen-rich AGB star W Hya. Results. We find CO maser emission amplifying the stellar continuum with an optical depth τ ≈−0.55. The maser predominantly amplifies the limb of the star because CO J = 3−2 absorption from the extended stellar atmosphere is strongest towards the centre of the star. Conclusions. The CO maser velocity corresponds to a previously observed variable component of high-frequency H2O masers and with the OH maser that was identified as the amplified stellar image. This implies that the maser originates beyond the acceleration region and constrains the velocity profile since we find the population inversion primarily in the inner circumstellar envelope. We find that inversion can be explained by the radiation field at 4.6 μm and that the existence of CO maser emission is consistent with the estimated mass-loss rates for W Hya. However, the pumping mechanism requires a complex interplay between absorption and emission lines in the extended atmosphere. Excess from dust in the circumstellar envelope of W Hya is not sufficient to contribute significantly to the required radiation field at 4.6 μm. The interplay between molecular lines that cause the pumping can be constrained by future multi-level CO observations.

Long-term Variability of H2CO Masers in Star-forming Regions

Abstract We present results of a multi-epoch monitoring program on variability of 6 cm formaldehyde (H2CO) masers in the massive star-forming region NGC 7538 IRS 1 from 2008 to 2015, conducted with the Green Bank Telescope, the Westerbork Radio Telescope , and the Very Large Array. We found that the similar variability behaviors of the two formaldehyde maser velocity components in NGC 7538 IRS 1 (which was pointed out by Araya and collaborators in 2007) have continued. The possibility that the variability is caused by changes in the maser amplification path in regions with similar morphology and kinematics is discussed. We also observed 12.2 GHz methanol and 22.2 GHz water masers toward NGC 7538 IRS 1. The brightest maser components of CH3OH and H2O species show a decrease in flux density as a function of time. The brightest H2CO maser component also shows a decrease in flux density and has a similar LSR velocity to the brightest H2O and 12.2 GHz CH3OH masers. The line parameters of radio recombination lines and the 20.17 and 20.97 GHz CH3OH transitions in NGC 7538 IRS 1 are also reported. In addition, we observed five other 6 cm formaldehyde maser regions. We found no evidence of significant variability of the 6 cm masers in these regions with respect to previous observations, the only possible exception being the maser in G29.96−0.02. All six sources were also observed in the isotopologue transition of the 6 cm H2CO line; absorption was detected in five of the sources. Estimated column density ratios [ ]/[ ] are reported.

High-mass star-forming cloud G0.38+0.04 in the Galactic center dust ridge contains H2CO and SiO masers

We have discovered a new H$_2$CO (formaldehyde) $1_{1,0}-1_{1,1}$ 4.82966 GHz maser in Galactic Center Cloud C, G0.38+0.04. At the time of acceptance, this is the eighth region containing an H$_2$CO maser detected in the Galaxy. Cloud C is one of only two sites of confirmed high-mass star formation along the Galactic Center Ridge, affirming that H$_2$CO masers are exclusively associated with high-mass star formation. This discovery led us to search for other masers, among which we found new SiO vibrationally excited masers, making this the fourth star-forming region in the Galaxy to exhibit SiO maser emission. Cloud C is also a known source of CH$_3$OH Class-II and OH maser emission. There are now two known SiO and H$_2$CO maser containing regions in the CMZ, compared to two and six respectively in the Galactic disk, while there is a relative dearth of H$_2$O and CH$_3$OH Class-II masers in the CMZ. SiO and H$_2$CO masers may be preferentially excited in the CMZ, perhaps due to higher gas-phase abundances from grain destruction and heating, or alternatively H$_2$O and CH$_3$OH maser formation may be suppressed in the CMZ. In any case, Cloud C is a new testing ground for understanding maser excitation conditions.

Pumping of the 4.8 GHz H2CO masers and its implications for the periodic masers in G37.55+0.20

Periodic or regular flaring of class II methanol masers in nine high mass star forming regions is now a well established phenomenon. Amongst the nine star forming regions, G37.55+0.20 is the only case at present where apart from the presence of a periodic class II methanol maser, correlated flaring of another masing species, formaldehyde in this case, has been detected. We perform numerical calculations to investigate under which conditions the 4.8 GHz transition of ortho-formaldehyde is inverted in order to address the question of the correlated flaring of the 6.7 GHz methanol and 4.8 GHz formaldehyde masers in G37.55+0.20. We developed a numerical code to study the population inversion of o-formaldehyde. Equilibrium solutions for the level populations are found by integrating the rate equations using Heun's method. It is found that collisional excitation with H$_2$ as well as radiative excitation by the free-free radio continuum radiation from a nearby ultra- or hyper-compact HII region can invert the 4.8 GHz transition. It is also found that radiative excitation by the dust infrared radiation field does not lead to an inversion of the 4.8 GHz transition. The 14.5 GHz and 28.9 GHz transitions are inverted only in the presence of the free-free continuum radiation field of a very compact HII region. Due to the different pumping mechanisms of the formaldehyde and methanol masers it is unlikely that the near simultaneous flaring of the methanol and formaldehyde masers in G37.55+0.20 is due to changes in the pumping of the masers.

Search for 6 Centimeter Formaldehyde Masers in 22 Galactic Star-forming Regions

A search for 6 cm H<SUB>2</SUB>CO (formaldehyde) masers in 22 Galactic star-forming regions has been carried out using the B configuration of the VLA. These observations have 2" angular resolution. The sensitivity ranges between 2 and 4 mJy beam<SUP>-1</SUP>. No new H<SUB>2</SUB>CO masers are detected. The paucity of these masers suggests that (1) their lifetimes are quite short, and/or (2) a narrow range of physical parameters is necessary for their formation. The known H<SUB>2</SUB>CO maser source NGC 7538 was also observed. The line flux of one of its two components has tripled over a 15 yr period.

Are the NGC 7538 formaldehyde masers really unusual?

The molecular cloud around NGC 7358 IRS l has been studied in order to understand the excitation conditions for the unusual H 2 CO masers. Maps in the J=1→0 transition of 13 CO indicate densities with a lower bound of ∼2 × 10 3 cm -3 or more in the cloud around the H II region. The results from the study of various molecular transitions indicate that the conditions required by the Boland & de Jong (1981, A&A, 98) model for the excitation of the H 2 CO maser are not very stringent or unique. The maser can be excited by a compact H II region which is optically thin at millimeter wavelengths

CO cooling rates - Revised calculations

view Abstract Citations (21) References (18) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS CO Cooling Rates: Revised Calculations Viscuso, Paul J. ; Chernoff, David F. Abstract The steady state cooling rates for collisionally excited CO have been recalculated using recent cross sections of Schinke et al. (1985) and Flower and Launay (1985). Despite large differences in particular rate coefficients, it is found that the original calculations of McKee et al. (1982) for hot gas (T equal to or greater than 100 K) are nearly correct, so that the level populations and cooling rates are not grossly affected by the changes. CO masers are slightly less prevalent than previously thought. In the lower temperature limit (T equal to or greater than 100 K), the cooling rates are insensitive to the discrepancies between different theoretical cross sections. Publication: The Astrophysical Journal Pub Date: April 1988 DOI: 10.1086/166199 Bibcode: 1988ApJ...327..364V Keywords: Abundance; Carbon Monoxide; Cooling; Gas Temperature; Interstellar Gas; Molecular Gases; Maser Outputs; Molecular Rotation; Potential Energy; Steady State; Atomic and Molecular Physics; INTERSTELLAR: ABUNDANCES; INTERSTELLAR: MOLECULES; MOLECULAR PROCESSES full text sources ADS |

Dust grains and gas in the circumstellar envelopes around luminous red giant stars

view Abstract Citations (203) References (27) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Dust Grains and Gas in the Circumstellar Envelopes around Luminous Red-Giant Stars Zuckerman, B. ; Dyck, H. M. Abstract With 12, 25, 60, and 100 μm fluxes given in the IRAS Point Source Catalog, the authors have constructed color-color plots that include more than 100 infrared bright red-giant stars. Carbon-rich and oxygen-rich red giants can be distinguished from each other based on the ratio of their 25 to 60 μm fluxes. Previously, the authors reported CO rotational emission from 64 cool red giants with large fluxes in the IRAS Point Source Catalog. Here they report CO emission from an additional 15 stars as well as HCN emission toward 11 stars. The HCN data and the color-color plots are used to classify and reclassify various stars as oxygen- or carbon-rich. In 1985 June the carbon star V Hya displayed a narrow CO emission feature superposed on a standard broad stellar CO profile. The narrow emission is probably the first example of a CO maser ever seen in either a circumstellar or interstellar environment. Publication: The Astrophysical Journal Pub Date: December 1986 DOI: 10.1086/164776 Bibcode: 1986ApJ...311..345Z Keywords: Carbon Stars; Cosmic Dust; Infrared Sources (Astronomy); Infrared Stars; Red Giant Stars; Stellar Envelopes; Color-Color Diagram; Hydrocyanic Acid; Infrared Astronomy Satellite; Molecular Clouds; Pre-Main Sequence Stars; Radial Velocity; Stellar Spectra; Astrophysics; INFRARED: SOURCES; INTERSTELLAR: GRAINS; STARS: CARBON; STARS: CIRCUMSTELLAR SHELLS; STARS: LATE-TYPE full text sources ADS | data products SIMBAD (163)

RECENT PROGRESS ON MOLECULAR HYDROGEN IN ORION*

The H2 emission from the Orion Molecular Cloud is, by an order of magnitude, the brightest known example of H2 emission, offering a means by which to estimate quantities connected with the mass flow. The size, temperature, velocity, and intensity of the lines can be used to infer the mass loss rate, the rate at which momentum is yielded to the cloud through masss loss, and the energy dissipation rate as the cloud is disrupted internally. Attention is accordingly given to the most recent observational results, with respect to morphology, extinction, level populations, and excitation temperature. The estimates concerning the mass loss phenomenon derived from H2 observations yield approximately the same numbers of those inferred from CO and H2O maser observations.

Far-infrared rotational emission by carbon monoxide

Accurate theoretical collisional excitation rates are used to determine the emissivities of CO rotational lines 10 to the 4th power/cu cm n(H2), 100 K T 2000 K, and J 50. An approximate analytic expression for the emissitivities which is valid over most of this region is obtained. Population inversions in the lower rotational levels occur for densities n(H2) approximately 10 (to the 3rd to 5th power)/cu cm and temperatures T approximately 50 K. Interstellar shocks observed edge on are a potential source of millimeter wave CO maser emission. The CO rotational cooling function suggested by Hollenbach and McKee (1979) is verified, and accurate numerical values given. Application of these results to other linear molecules should be straightforward.