A search for OH 6 GHz maser emission towards supernova remnants

Abstract

OH masers at 1720 MHz have proven to be excellent indicators of interactions between supernova remnants and molecular clouds. OH excitation calculations suggest that the 6049 MHz OH maser line is excited for higher column densities than for the 1720 MHz line. Previous observations and modelling of 1612, 1665 and 1667 MHz OH absorption and 1720 MHz OH masers indicated that the column densities in some supernova remnants, ~1e17 cm^-2, may be high enough for 6049 MHz OH masers to exist. It is therefore a potentially valuable indicator of remnant-cloud interaction. We present excitation calculations predicting the formation of 6049 MHz OH masers and results of a survey using the Parkes Methanol Multibeam receiver for 6049, 6035 and 6030 MHz OH masers towards 35 supernova remnants, a star-forming region and 4 fields in the Large and Small Magellanic Clouds. Two new sites of 6035 and 6030 MHz OH maser emission associated with star-forming regions have been discovered, but no 6049 MHz masers were detected to a brightness temperature limit of ~0.3-0.6 K, even though modelling of the OH excitation suggests that maser emission should have been detected. Our upper-limits indicate that the OH column density for a typical remnant is less than 1e16.4 cm^-2, which conflicts with observed and modelled column densities. One possible explanation is that 6049 MHz OH masers may be more sensitive to velocity coherence than 1720 MHz OH masers under some conditions.Comment: 11 pages, 5 figures, mn2e.cls. Submitted to MNRAS Apr 2008. Accepted for publication in MNRAS 2008 July 15. Minor changes in the accepted version. LaTex2

Multiwavelength observations of the supernova remnant G349.7+0.2 interacting with a molecular cloud

“…Our non‐detection of excited‐state OH masers is consistent with previous searches: Fish, Sjouwerman & Pihlström (2007) searched for 6‐GHz OH masers towards 10 SNRs with the Effelsberg 100‐m telescope; Pihlström et al (2008) searched for 4.7‐, 7.8‐, 8.2‐ and 23.8‐GHz OH transitions in four SNRs with the Very Large Telescope; and McDonnell et al (2008) searched for the 6‐GHz OH transitions with the Parkes 64‐m telescope in about 40 SNRs, including sources from the Magellanic clouds and G349.7+0.2. These results suggest that the shock conditions suitable for the production of 1720‐MHz OH transitions do not support the other OH transitions, even those at 6049 and 4765 MHz, which have the most compatible requirements.…”

“…Interestingly, Wardle (2007) found that for OH column densities larger than 10 17 cm −2 , the 6049 MHz masers switch on while the 1720‐MHz transition is quenched, even with suitable gas densities and temperatures for the production of 1720‐MHz masers, suggesting that the higher frequency line might serve as a complementary signpost of warm, shocked gas. Unfortunately, a dedicated search for the 6049‐MHz OH line by McDonnell et al (2008) was unsuccessful. However, they found indications that this transition is more sensitive than the 1720 MHz line to velocity coherence.…”

Progenitors and explosion properties of supernova remnants hosting central compact objects: I. RCW 103 associated with the peculiar source 1E 161348−5055

“…With this in mind we calculate the blast wave velocity using the soft component's temperature leading to an upper age limit of 4.4 D 3.1 kyr. This is consistent with the age estimates obtained from studies at other wavelengths between 2000-4000 yrs (McDonnell et al (2008); Paron et al (2006); Andersen et al (2011);Oliva et al (1999);Frail et al (1996)).…”

OH Masers and Supernova Remnants

“…The 1720 MHz inversion dies away for OH columns 10 17 cm −2 due to photon trapping, but under similar physical conditions the analogous 6049 MHz transition in the first rotationally-excited state of OH becomes inverted (Pavlakis & Kylafis 2000;McDonnell, Wardle & Vaughan 2008). This prompted searches for masers at 6049 MHz associated with supernova remnants using Parkes (McDonnell, Wardle & Vaughan 2008), Effelsberg (Fish, Sjouwerman & Philstrom 2007) and the ATCA (McDonnell 2011) (see Table 1).…”

“…This prompted searches for masers at 6049 MHz associated with supernova remnants using Parkes (McDonnell, Wardle & Vaughan 2008), Effelsberg (Fish, Sjouwerman & Philstrom 2007) and the ATCA (McDonnell 2011) (see Table 1). These searches targeted SNR with 1720 MHz masers, mixed-morphology, or other evidence of interaction (note that there is substantial overlap between the SNR in the Parkes & Effelsberg and Parkes & ATCA searches), but all were unsuccessful.…”