arXiv subjects

E. R. Seaquist

Publications and source records attributed to E. R. Seaquist.

24 records · Page 2Linked to original sources

arXiv1998

HCO+ in the Starburst Galaxy M82

We report observations and an analysis of the distribution of HCO+(1-0) and HCO+(4-3) emission in the central 1 kpc star forming region of M82. Comparisons are made with other star formation indicators such as the mm continuum, the distribution of radio SNR's and the molecules CO and OH. In a broad sense, the HCO+ is distributed in a way similar to the CO, although there are noticeable differences in detail, including an inward displacement of spiral arm emission relative to CO. A comparison of the position-velocity plots for CO, HCO+(1-0), HCO+(4-3), and ionized gas, with orbits expected in the presence of the nuclear bar suggest an inward transfer of gas associated with star formation toward the nucleus. The HCO+(4-3)/(1-0) line ratios are comparatively uniform in the observed region, and according to an LVG analysis, reflect mean gas densities in the range 10^4 cm^{-3} - 10^5 cm^{-3} for kinetic temperatures in the range 20K - 60K. The comparative uniformity of these conditions and the low filling factor suggest that each sampled point comprises alarge number of clouds occupying a broad range of density, and possibly temperature. We briefly examine fractal type models in the context of the HCO+ data as an alternative way to analyze molecular line emission in M82.

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arXiv1998

Molecular Gas in Mrk 109: Constraining the O2/CO Ratio in Chemically Young Galaxies

We report on observations of CO(1-0) emission from the chemically young starburst galaxy Mrk 109. These observations were part of a study to constrain the O2/CO ratio in metal-deficient galaxies, which were motivated by theoretical work that suggests the possible enhancement of the O2/CO ratio in chemically young systems. Five low metallicity (Z < 0.5 Z(solar)) IRAS galaxies at redshifts z > 0.02 (required to shift the 118.75 GHz O2 line away from the atmospheric line) were searched for CO emission. We detected the CO line in only Mrk 109. From O2 observations of Mrk 109, we achieved an upper limit for the O2 column density abundance ratio of N(O2)/N(CO) < 0.31. These results provide useful constraints for the theoretical models of chemically young galaxies. We argue that either most of the molecular gas in Mrk 109 does not reside in dark clouds (Av >~ 5), or the standard equilibrium chemistry models are inadequate for metal-poor systems. The molecular gas mass implied by the CO observations of Mrk 109 is M(H2) ~= 4 x 10^{9} M(sun), and the CO data are consistent with a central starburst induced by the interaction with a nearby companion.

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arXiv1997

OH Satellite Line Masers in the Nucleus of NGC253

We report the detection of 1720 and 1612 MHz OH ground-state satellite line maser regions in the starburst galaxy NGC253. We find ten compact maser features all of which are located along the narrow ridge of bright radio continuum emission within the central 10 square arcseconds of the nucleus. The compact OH features appear physically associated with regions of dense molecular gas, supernova remnants, and HII regions. The data for several of the features are consistent with collisional inversion from the interaction of supernova remnant shocks with molecular gas, similar to that found for Galactic supernova remnants (Frail et al. 1996). For many other features near the nucleus of NGC253, we find conjugate behavior in the 1720 and 1612 MHz lines, with one line showing stimulated emission and the other showing stimulated absorption. These results suggest the importance of infrared pumping associated with the active star formation in NGC253 and is consistent with large column densities of molecular gas (N[H_2] >~ 10^{22}cm^{-2}).

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arXiv1997

OH Satellite Line Masers and an AGN Candidate in M82

We report the detection of OH satellite line masers at 1720 MHz and 1612 MHz from the nuclear region of the starburst galaxy M82. The observations were aimed at detecting 1720 MHz maser emission from the known radio emitting SNR in the nuclear region. At 1720 MHz we detect six features above the 5-sigma limit set by noise, four in emission and two in absorption. Three of the emission features appear closely associated with known discrete continuum radio sources, and one of the absorption features is precisely coincident with the discrete continuum source 44.01+59.6. The latter source also exhibits strong 1612 MHz emission at the same velocity. No other 1612 MHz features were detected. All of the 1720 MHz emission features are consistent with masers pumped by collisions with molecular hydrogen at densities between 10^3 cm^{-3} and 10^5 cm^{-3}, and T_k between 50 K and 250 K. The absorption and emission associated with the two satellite lines in 44.01+59.6, together with other evidence, points to the possibility that this source is the AGN in M82.

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arXiv1994

Discovery of hydroxyl and water masers in R Aquarii and H1-36 Arae

We present the first results from an all-sky maser-line survey of symbiotic Miras. Interferometric spectral-line observations of R Aqr and H1-36 Arae have revealed a 22-GHz water maser in the former and 1612-MHz hydroxyl and weak 22-GHz water maser emission from the latter. H1-36 has thus become the first known symbiotic OH/IR star. We have also detected weak OH line emission from the vicinity of R Aqr, but we note that there are small discrepencies between the OH- and H2O-line velocities and positions. These detections demonstrate unequivocally that dust can shield some circumstellar hydroxyl and water molecules from dissociation, even in systems which possess intense local sources of UV. Finally, we discuss some of the implications of these observations. The narrow profile of the water maser in R Aqr means that there may finally be an opportunity to determine the system's orbital parameters. We also point out that high resolution synthesis observations may trace the distribution of dust in H1-36 and R Aqr, possibly throwing light on the mass-loss process in symbiotic Miras and placing constraints on the amount of collimation experienced by UV radiation from their hot, compact companions.

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arXiv1994

A search for symbiotic behaviour amongst OH/IR colour mimics

Recent maser surveys have shown that many potential OH/IR stars have no OH masers in their circumstellar envelopes, despite the modest requirements which should be implicitly met by IRAS colour-selected candidates. It has been suggested that these OH/IR colour mimics must have a degenerate companion which dissociates OH molecules and disrupts the masing action, ie. that they are related to symbiotic Miras. Coincidentally, there is a paucity of long-period symbiotic Miras and symbiotic OH/IR stars. Phenomonologically, those that are known seem to cluster in the zone where field Miras transform into OH/IR stars. If it could be proven that OH/IR colour mimics contain a degenerate star, that observable evidence of this star is hidden from view by CS dust whilst it slowly accretes from the wind of its Mira companion, then we have an excellent explanation for not only the existence of OH/IR colour mimics, but also for the low observed frequency of symbiotic OH/IR stars and the common occurrence of very slow novae in long-period symbiotic Miras. Here, we employ radio continuum radiation (which should escape unhindered from within the dust shells) as a simple probe of the postulated hot degenerate companions which would inevitably ionize a region of their surrounding gas. We compare the radio and infrared properties of the colour mimics with those of normal symbiotic Miras, using the strong correlation between radio and mid-IR emission in symbiotic stars. We show that if a hot companion exists then, unlike their symbiotic counterparts, they must produce radiation-bounded nebulae. Our observations provide no support for the above scenario for the lack of observed masers, but neither do they permit a rejection of this scenario.

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