The Local Interstellar Medium

Summary. Substantial progress in the field of the Local Interstellar Medium has been largely due to recent launches of space missions, mostly in the UV and X–ray domains, but also to ground-based observations, mainly in high resolution spectroscopy. However, a clear gap seems to remain between the wealth of new data and the theoretical understanding. This paper gives an overview of some observational aspects, with no attempt of completeness or doing justice to all the people involved in the field. As progress rarely evolves in straight paths, we can expect that our present picture of the solar system surroundings is not definitive. Received 30 October 1998     

Evidence for Co-Propagation of 4765- and 1720-MHz OH Masers in Star-Forming Regions

Two star-forming regions Cepheus A and W75N, were searched for the 4765-MHz OH maser emission using the multi-element radio linked interferometer network (MERLIN). The excited OH emission has an arc-like structure of 40 mas in Cep A and a linear structure of size 45 mas in W75N. We also found the 1720-MHz line in Cep A and Hutawarakorn [MNRAS 330 (2002) 349] reported the 1720-MHz emission in W75N. The 1720- and 4765-MHz OH spots coincided in space within 60 mas and in velocity within 0.3 km s^–1 in both targets implying that both maser transitions arise from the same region. According to the modelling by Gray [MNRAS 252 (1991) 30] the 1720/4765-MHz co-propagation requires a low density, warm environment. The masers lie at the edges of H II regions where such conditions are expected.     

Interstellar Extinction and the Intrinsic Colors of Classical Cepheids in the Galaxy, the LMC, and the SMC

New methods are applied to samples of classical cepheids in the galaxy, the Large Magellanic Cloud, and the Small Magellanic Cloud to determine the interstellar extinction law for the classical cepheids, R _B:R _V:R _I:R _J:R _H:R _K= 4.190:3.190:1.884:0.851:0.501:0.303, the color excesses for classical cepheids in the galaxy, E(B-V)=-0.382-0.168logP+0.766(V-I), and the color excesses for classical cepheids in the LMC and SMC, E(B-V)=-0.374-0.166logP+0.766(V-I). The dependence of the intrinsic color (B-V)₀ on the metallicity of classical cepheids is discussed. The intrinsic color (V-I)₀ is found to be absolutely independent of the metallicity of classical cepheids. A high precision formula is obtained for calculating the intrinsic colors of classical cepheids in the galaxy: (<B>-<V>)₀=0.365(±0.011)+0.328(±0.012)logP.     

Triplet structure of the H<Subscript>2</Subscript>O spectra in S255

We analyzed the monitoring data for the maser S255 obtained in the H₂O line at λ=1.35 cm with the 22-m radio telescope at the Pushchino Radio Astronomy Observatory in 1981–2002. The maser was most active during 1998–2002. Since 2001, the H₂O spectra have been extended and complex; their triplet structure has been disrupted. The extent of the spectra was 24 km s^?1 (from ?6 to 18 km s^?1). We calculated orbital parameters for some of the components. We estimated the mass of the central star to be (6–7)M_⊙ and the outer Keplerian-disk radius to be ～160 AU.     

Spectroscopic research with the Hartebeesthoek radio telescope

Research into star-forming regions, evolved HII regions, late-type stars with circumstellar dust shells and comet Halley using spectroscopic observations made with the Hartebeesthoek radio telescope is reviewed.     

Testing the 'clump' model of SiO maser emission

Building on the detection of the J =7–6 SiO maser emission in both the v =1 and v =2 vibrational states towards the symbiotic Mira R Aquarii, we have used the James Clerk Maxwell Telescope to study the changes in the SiO maser features from R Aqr over a stellar pulsational period. The observations, complemented by contemporaneous data taken at 86 GHz, represent a test of the popular thermal-instability clump models of SiO masers. The 'clump' model of SiO maser emission considers the SiO masers to be discrete emitting regions which differ from their surroundings in the values of one or more physical variables (SiO abundance, for example). We find that our observational data are consistent with a clump model in which the appearance of maser emission in the J =7–6 transitions coincides with an outward-moving shock impinging on the inner edge of the maser zone.     

Co-propagation of maser emission at 1720 and 4765 MHz

MERLIN observations are presented of OH 4765-MHz and OH 1720-MHz masers in the massive star-forming region W3(OH). Two of the three intense spots of maser emission at 4765 MHz are spatially coincident with two similar spots at 1720 MHz in both left-hand circular (LHC) and right-hand circular (RHC) polarizations, to an accuracy of 15 mas. The spots also overlap in velocity when allowance is made for Zeeman splitting of the 1720-MHz line. We conclude that we have found two examples of masers in different rotational levels of OH which are co-propagating through the same column of gas and experiencing competitive gain effects. The third 4765-MHz maser spot was found to have no overlapping counterpart amongst the 1720-MHz masers.     

The structure and dynamics of M17SW

The M17SW molecular cloud core has been mapped at high resolution in the C¹⁷OJ = 3 2 transition and in 450, 600, 800, 1100 and 1300µm continuum emission, using the JCMT. The clumpy nature of the cloud core is clearly revealed and the northern condensation has been resolved into 3 main clumps, each of which lies close to an H₂O maser, suggesting that they may contain young embedded stellar objects.     

Measuring the Distance of VX Sagittarii with SiO Maser Proper Motions

We report on 43GHz v=1, J=1-0 SiO maser proper motions in the circumstellar envelope of the M-type semi-regular variable star VX Sgr, observed by Very Long Baseline Array (VLBA) at 3 epochs during 1999 April-May. Applying the statistical parallax analysis to these proper motions, we estimated a distance of VX Sgr of 1.57±0.27kpc, which is consistent with that based on the proper motions of H2O masers, or on the assumption that VX Sgr belongs to the Sgr OB1 association. At this distance, VX Sgr can be classified as a red supergiant. Comparing the statistical parallax method with those of model fitting and annual parallax, we think that the statistical parallax method may be a good way of estimating SiO maser distances at present.     

Discovery of large-scale methanol and hydroxyl maser filaments in W3(OH)

Images of the 6.7-GHz methanol maser emission from W3(OH) made at 50- and 100-mas angular resolution with the Multi-Element Radio-Linked Interferometer Network (MERLIN) are presented. The masers lie across the western face of the ultracompact H  ii region in extended filaments which may trace large-scale shocks. There is a complex interrelation between the 6.7-GHz methanol masers and hydroxyl (OH) masers at 1.7 and 4.7 GHz. Together the two species trace an extended filamentary structure that stretches at least 3100 au across the face of the ultracompact H  ii region. The dominant 6.7-GHz methanol emission coincides with the radio continuum peak and is populated by masers with broad spectral lines. The 6.7-GHz methanol emission is elongated at position angle 50° with a strong velocity gradient, and bears many similarities to the methanol maser disc structure reported in NGC 7538. It is surrounded by arcs of ground state OH masers at 1.7 GHz and highly excited OH masers at 13.44 GHz, some of which have the brightest methanol masers at their focus. We suggest that this region hosts the excitation centre for the ultracompact H  ii region.     

The disappearance of the 1667-MHz OH maser in IRAS 17436+5003 (HD 161796)

We report the diminution of the 1667-MHz OH maser in the post-asymptotic giant branch star IRAS 17436+5003, by a factor of ≳17 over a period of ≲12 yr, from observations with MERLIN. This circumstellar maser was detected by Likkel in 1987, at the 13σ level of her observations with the Green Bank Telescope. We discuss a number of possible reasons for this phenomenon and conclude that it is most likely due to turbulence arising from interacting stellar winds.     

The association of OH and methanol masers in W3(OH)

We present single-baseline Multi-Element Radio-Linked Interferometer Network (MERLIN) measurements of excited OH 6.0-GHz masers and methanol 6.7-GHz masers for the source W3(OH). These allow us to compare the positions of individual maser spots of these two species to ∼15 mas accuracy for the first time, and to compare these with previously published positions of ground-state OH masers near 1.7 GHz and excited-state OH masers near 4.7 GHz. There is a strong association between OH 6035-MHz and 1665-MHz masers. OH and methanol have very similar distributions, but associations of individual masers are relatively rare: most methanol 6.7-GHz masers are within 100 mas of OH 6.0-GHz masers, but only four methanol masers are within 15 mas of an OH 6.0-GHz maser. There are no correspondences of either species with excited OH 4.7-GHz masers. Zeeman splitting of the 6.0-GHz OH lines indicates an ordered magnetic field ranging from 3.2 to 14.4 mG. The magnetic fields estimated from co-propagating masers such as 6035 and 1665 MHz are generally in good agreement with each other.     

Evidence for a 2-h optical modulation in GS 1826 – 24

OH maser emission from the circumstellar envelope of the M-type supergiant VX Sagittarii has been mapped at 1612 MHz in both hands of circular polarization using MERLIN, with an angular resolution of 0.4 arcsec and a velocity resolution of 0.3 km s⁻¹. Four likely Zeeman pairs of maser components are identified, each with a similar Zeeman splitting. The inferred magnetic field strength is approximately −1 mG in each case, with the field directed towards us. The Zeeman components lie ∼ 1400 au from the star. The data lend support to the dipole magnetic field model which has recently been suggested for this circumstellar envelope.     

A GLIMPSE-based search for 6.7-GHz methanol masers and the lifetime of their spectral features

The University of Tasmania Mt Pleasant 26-m and Ceduna 30-m radio telescopes have been used to search for 6.7-GHz class II methanol masers towards 200 GLIMPSE ( The Galactic Legacy Infrared Mid-Plane Survey Extraordinaire ) sources. The target regions were selected on the basis of their mid-infrared colours as being likely to be young high-mass star formation regions and are either bright at 8.0 μm, or have extreme [3.6]–[4.5] colour. Methanol masers were detected towards 38 sites, nine of these being new detections. The prediction was that approximately 20 new 6.7-GHz methanol masers would be detected within 3.5 arcmin of the target GLIMPSE sources, but this is the case for only six of the new detections. A number of possible reasons for the discrepancy between the predicted and actual number of new detections have been investigated. It was not possible to draw any firm conclusions as to the cause, but it may be because many of the target sources are at an evolutionary phase prior to that associated with 6.7-GHz methanol masers. Through comparison of the spectra collected as part of this search with those in the literature, the average lifetime of individual 6.7-GHz methanol maser spectral features is estimated to be around 150 yr, much longer than is observed for 22-GHz water masers.     

脉冲星PSR2217＋47的折射闪烁和星际介质电子密度的涨落谱

本文探讨脉冲星ＰＳＲ２２１７＋４７分米波段流量变化的星际折射闪烁解释。利用衍射闪烁测量所获得的星际介质参数和连续介质的折射闪烁理论，计算了多频率（０．３１，０．４２，０．６１和０．７５ＧＨｚ）上的折射闪烁结构函数，并与观测结构函数加以比较。结果表明，如果星际介质电子密度不均匀性具有幂律谱，那么幂律指数β＝４要比β＝１１／３（即Ｋｏｌｍｏｇｏｒｏｖ谱）能更好地符合观测。     

星际闪烁和BLO 0716＋71的快速射电变化

利用星际介质的折射闪烁理论解释在ＢＬＬａｃ天体０７１６＋７１中观测到的射电快速变化的结构函数，确定了对射电源结构和散射介质特性所提出的条件。为了解释这个源内观测到的射电变化的“快模式”和“慢模式”的转换，必须考虑时电源内致密结构的角径或运动状态的快速变化。射电变化和光学变化的相关性仍然是析射闪烁解释的一个难于克服的困难。     

A search for 22-GHz water masers within the giant molecular cloud associated with RCW 106

We report the results of a blind search for 22-GHz water masers in two regions, covering approximately half a square degree, within the giant molecular cloud associated with RCW 106. The complete search of the two regions was carried out with the 26-m Mount Pleasant radio telescope and resulted in the detection of nine water masers, five of which are new detections. Australia Telescope Compact Array (ATCA) observations of these detections have allowed us to obtain positions with arcsecond accuracy, allowing meaningful comparison with infrared and molecular data for the region. We find that for the regions surveyed there are more water masers than either 6.7-GHz methanol, or main-line OH masers. The water masers are concentrated towards the central axis of the star formation region, in contrast to the 6.7-GHz methanol masers which tend to be located near the periphery. The colours of the GLIMPSE point sources associated with the water masers are similar to those of 6.7-GHz methanol masers, but slightly less red. We have made a statistical investigation of the properties of the ¹³CO and 1.2-mm dust clumps with and without associated water masers. We find that the water masers are associated with the more massive, denser and brighter ¹³CO and 1.2-mm dust clumps. We present statistical models that are able to predict those ¹³CO and 1.2-mm dust clumps that are likely to have associated water masers, with a low misclassification rate.     

Variation of Flux Density of the Radio Source Cassiopeia A

The character of the yearly average and secular variations of the intensity of the radio emission of Cassiopeia A is discussed on the basis of multiyear observations (1953-2001) made at Byurakan Astrophysical Observatory and the Institute of Radiophysics and Electronics. It is shown that in this period the intensity of the radio emission of Cassiopeia A has decreased by about 34%, at a yearly average rate of 0.7%.