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1.
A survey towards a selection of 35 methanol maser and/or ultracompact (UC) H  ii regions, reported in Papers I and II and by Norris et al., has been conducted in the near-infrared (NIR). Out of 25 methanol maser sites surveyed, 12 are associated with a NIR counterpart. Out of 18 UC H  ii regions (8 of which overlap with maser emission), 12 are associated with a NIR counterpart. Counterparts can be confidently identified not only by the positional agreements, but also by their unusually red colours. Spectral types for the embedded stars can be unambiguously determined for six sources, all of which imply massive, ionizing stars. One of these infrared sources has methanol maser emission, but no UC H  ii region. It is possible that the maser emission associated with this source arises from a pre-UC H  ii phase of massive stellar evolution or it could be that nearly all the ultraviolet photons are absorbed by dust within the UC H  ii region. We have modelled the spectral energy distributions (SEDs) for some sources and find that a single blackbody can be used to estimate the stellar luminosity, but cannot represent the whole infrared SED. A two-component blackbody model and a radiative transfer model were also used to derive essential parameters of the infrared sources. The radiative transfer model also indicates which infrared sources are relatively young and which are older. Both models show that silicate absorption at 9.7 μm must be a dominant feature of these SEDs.  相似文献   

2.
We report near-infrared molecular hydrogen and Brackett γ observations towards the massive star formation site G323.74−0.26. The region contains an H  ii region, ∼30 arcsec across, and two Class II methanol maser sites, which are separate from the H  ii region. We show that the spectral type of the star powering the H  ii region is B0. We also show that at least one of the maser sites is powered by an infrared source that appears to be at least as luminous as the star responsible for the H  ii region. However, neither of the two stars associated with the methanol maser sites shows any signs of radio continuum emission above 0.2 mJy. For at least one of these maser sites, this shows a real deficiency in the radio continuum flux, which we suggest is an indication that the star is in an early stage of development, before its H  ii region becomes visible, or it is a multiple intermediate mass star system. A shocked molecular hydrogen outflow is seen extending from one of the maser sites towards the west and possibly in a fan shape, suggesting that the stars associated with the maser sites are indeed at a very early stage of evolution.  相似文献   

3.
High spatial resolution radio continuum and 6.67-GHz methanol spectral line data are presented for methanol masers previously detected by Walsh et al. (1997). Methanol maser and/or radio continuum emission is found in 364 cases towards IRAS -selected regions. For those sources with methanol maser emission, relative positions have been obtained to an accuracy of typically 0.05 arcsec, with absolute positions accurate to around 1 arcsec. Maps of selected sources are provided. The intensity of the maser emission does not seem to depend on the presence of a continuum source. The coincidence of water and methanol maser positions in some regions suggests there is overlap in the requirements for methanol and water maser emission to be observable. However, there is a striking difference between the general proximity of methanol and water masers to both cometary and irregularly shaped ultracompact (UC) H  ii regions, indicating that, in other cases, there must be differing environments conducive to stimulating their emission. We show that the methanol maser is most likely present before an observable UC H  ii region is formed around a massive star and is quickly destroyed as the UC H  ii region evolves. There are 36 out of 97 maser sites that are linearly extended. The hypothesis that the maser emission is found in a circumstellar disc is not inconsistent with these 36 maser sites, but is unlikely. It cannot, however, account for all other maser sites. An alternative model which uses shocks to create the masing spots can more readily reproduce the maser spot distributions.  相似文献   

4.
K -band spectra of young stellar candidates in four Southern hemisphere clusters have been obtained with the Gemini Near-Infrared Spectrograph in Gemini South. The clusters are associated with IRAS sources that have colours characteristic of ultracompact H  ii regions. Spectral types were obtained by comparison of the observed spectra with those of a near-infrared (NIR) library; the results include the spectral classification of nine massive stars and seven objects confirmed as background late-type stars. Two of the studied sources have K -band spectra compatible with those characteristic of very hot stars, as inferred from the presence of C  iv , N  iii and N  v emission lines at 2.078, 2.116 and 2.100 μm, respectively. One of them, I16177_IRS1, has a K -band spectrum similar to that of Cyg OB2 7, an O3If* supergiant star. The nebular K -band spectrum of the associated Ultra-Compact (UC) H  ii region shows the s-process [Kr  iii ] and [Se  iv ] high excitation emission lines, previously identified only in planetary nebula. One young stellar object was found in each cluster, associated with either the main IRAS source or a nearby resolved Midecourse Space eXperiment ( MSX ) component, confirming the results obtained from previous NIR photometric surveys. The distances to the stars were derived from their spectral types and previously determined JHK magnitudes; they agree well with the values obtained from the kinematic method, except in the case of IRAS  15408−5356  , for which the spectroscopic distance is about a factor of 2 smaller than the kinematic value.  相似文献   

5.
Near-infrared (NIR) images at I , J , H and K bands were made of 12 southern 6.7-GHz methanol maser sources. Astrometry accurate to 0.5 arcsec was obtained. The positions of known H  ii regions, water masers, hydroxyl masers, and mid- and far-infrared objects in the region are examined in order to try to determine the nature of the methanol maser source. Deeply embedded NIR sources were found close to seven out of 14 maser sites. In three cases, no NIR source, H  ii region, water maser or hydroxyl maser could be found in likely association with the methanol masers.  相似文献   

6.
We present high resolution, mid-infrared (MIR) images towards three hot molecular cores signposted by methanol maser emission: G173.49+2.42 (S231, S233IR), G188.95+0.89 (S252, AFGL-5180) and G192.60−0.05 (S255IR). Each of the cores was targeted with Michelle on Gemini North using five filters from 7.9 to 18.5 μm. We find each contains both large regions of extended emission and multiple, luminous point sources which, from their extremely red colours ( F 18.5/ F 7.9≥ 3), appear to be embedded young stellar objects. The closest angular separations of the point sources in the three regions are 0.79, 1.00 and 3.33 arcsec corresponding to linear separations of 1700, 1800 and 6000 au, respectively. The methanol maser emission is found closest to the brightest MIR point source (within the assumed 1-arcsec pointing accuracy). Mass and luminosity estimates for the sources range from 3 to  22 M⊙  and from 50 to 40 000  L⊙  , respectively. Assuming the MIR sources are embedded objects and the observed gas mass provides the bulk of the reservoir from which the stars formed, it is difficult to generate the observed distributions for the most massive cluster members from the gas in the cores using a standard form of the initial mass function.  相似文献   

7.
We investigate the conditions of star formation in the Large Magellanic Cloud (LMC). We have conducted a survey for water maser emission arising from massive young stellar objects in the 30 Doradus region (N 157) and several other H  ii regions in the LMC (N 105A, N 113 and N 160A). We have identified a new maser source in 30 Dor at the systemic velocity of the LMC. We have obtained 3–4 μm spectra, with the European Southern Observatory (ESO)-Very Large Telescope (VLT), of two candidate young stellar objects. N 105A IRS1 shows H recombination line emission, and its Spectral Energy Distribution (SED) and mid-infrared colours are consistent with a massive young star ionizing the molecular cloud. N 157B IRS1 is identified as an embedded young object, based on its SED and a tentative detection of water ice. The data on these four H  ii regions are combined with mid-infrared archival images from the Spitzer Space Telescope to study the location and nature of the embedded massive young stellar objects and signatures of stellar feedback. Our analysis of 30 Dor, N 113 and N 160A confirms the picture that the feedback from the massive O- and B-type stars, which creates the H  ii regions, also triggers further star formation on the interfaces of the ionized gas and the surrounding molecular cloud. Although in the dense cloud N 105A star formation seems to occur without evidence of massive star feedback, the general conditions in the LMC seem favourable for sequential star formation as a result of feedback. In an Appendix , we present water maser observations of the galactic red giants R Doradus and W Hydrae.  相似文献   

8.
The southern maser site OH 300.969+1.147 has been studied using the Long Baseline Array of the Australia Telescope National Facility. The 1665- and 1667-MHz hydroxyl ground-state transitions were observed simultaneously. A series of maps with 0.1-arcsec spatial resolution, at velocity spacing  0.09 km s−1  , and in both senses of circular polarization reveals 59 small diameter maser spots. The spots are scattered over 2 arcsec, coincident with a strong ultracompact H  ii region, at a distance of 4.3 kpc. 17 Zeeman pairs of oppositely polarized spots were found, all yielding magnetic field estimates towards us (negative), ranging from −1.1 to −4.7 mG, with a median value of −3.5 mG. Excited state masers of OH at 6035 and 6030 MHz at this site also display Zeeman pairs revealing a magnetic field of −5.0 mG. Weak methanol maser emission is intermingled with the OH masers, but there is no detectable closely related water maser. The consistent magnetic field direction found within this site is a striking feature of several other maser sites associated with strong H  ii regions studied in comparable detail. We interpret the site as a mature region nearing the end of the brief evolutionary stage that can support maser emission.  相似文献   

9.
We have obtained wide-field thermal infrared (IR) images of the Carina nebula, using the SPIREX/Abu telescope at the South Pole. Emission from polycyclic aromatic hydrocarbons (PAHs) at 3.29 μm, a tracer of photodissociation regions (PDRs), reveals many interesting well-defined clumps and diffuse regions throughout the complex. Near-IR images  (1–2 μm)  , along with images from the Midcourse Space Experiment ( MSX ) satellite  (8–21 μm)  have been incorporated to study the interactions between the young stars and the surrounding molecular cloud in more detail. Two new PAH emission clumps have been identified in the Keyhole nebula, and have been mapped in  12CO(2–1)  and  (1–0)  using the Swedish–ESO Submillimetre Telescope (SEST). Analysis of their physical properties reveals that they are dense molecular clumps, externally heated with PDRs on their surfaces and supported by external pressure in a similar manner to the other clumps in the region. A previously identified externally heated globule containing IRAS 10430−5931 in the southern molecular cloud shows strong 3.29-, 8- and 21-μm emission, the spectral energy distribution (SED) revealing the location of an ultracompact (UC) H  ii region. The northern part of the nebula is complicated, with PAH emission intermixed with mid-IR dust continuum emission. Several point sources are located here, and through a two-component blackbody fit to their SEDs we have identified three possible UC H  ii regions as well as a young star surrounded by a circumstellar disc. This implies that star formation in this region is ongoing and not halted by the intense radiation from the surrounding young massive stars.  相似文献   

10.
We report the discovery of H2 line emission associated with 6.67-GHz methanol maser emission in massive star-forming regions. In our UNSWIRF/AAT observations, H2 1–0 S(1) line emission was found associated with an ultracompact H  ii region IRAS 14567–5846 and isolated methanol maser sites in G318.95–0.20 , IRAS 15278–5620 and IRAS 16076–5134 . Owing to the lack of radio continuum in the latter three sources, we argue that their H2 emission is shock excited, while it is UV-fluorescently excited in IRAS 14567–5846 . Within the positional uncertainties of 3 arcsec, the maser sites correspond to the location of infrared sources. We suggest that 6.67-GHz methanol maser emission is associated with hot molecular cores, and propose an evolutionary sequence of events for the process of massive star formation.  相似文献   

11.
A flare of OH maser emission was discovered in W75N in 2000. Its location was determined with the Very Long Baseline Array (VLBA) to be within 110 au from one of the ultracompact H  ii regions, Very Large Array 2 (VLA2). The flare consisted of several maser spots. Four of the spots were found to form Zeeman pairs, all of them with a magnetic field strength of about 40 mG. This is the highest ever magnetic field strength found in OH masers, an order of magnitude higher than in typical OH masers. Three possible sources for the enhanced magnetic field are discussed: (i) the magnetic field of the exciting star dragged out by the stellar wind; (ii) the general interstellar field in the gas compressed by the magnetohydrodynamic shock; and (iii) the magnetic field of planets which orbit the exciting star and produce maser emission in gaseous envelopes.  相似文献   

12.
We report the results of a submillimetre continuum emission survey targeted towards 78 star formation regions, 72 of which are devoid of methanol maser and UC H  ii  regions, identified in the Swedish ESO Submillimetre Telescope (SEST)/SEST IMaging Bolometer Array (SIMBA) millimetre continuum survey of Hill et al. At least 45 per cent of the latter sources, dubbed 'mm-only', detected in this survey are also devoid of the mid-infrared MSX  emission. The 450- and 850-μm continuum emission was mapped using the Submillimetre Common User Bolometer Array (SCUBA) instrument on the James Clerk Maxwell Telescope (JCMT). Emission is detected towards 97 per cent of the 78 sources targeted as well as towards 28 other SIMBA sources lying in the SCUBA fields.
In total, we have identified 212 cores in this submillimetre survey, including 106 previously known from the SIMBA survey. Of the remaining 106 sources, 53 result from resolving a SIMBA source into multiple submillimetre components, whilst the other 53 sources are submillimetre cores, not seen in the SIMBA. Additionally, we have identified two further mm-only sources in the SIMBA images. Of the total 405 sources identified in the SIMBA survey, 255 are only seen at millimetre wavelengths.
We concatenate the results from four (sub)millimetre continuum surveys of massive star formation, together with the Galactic plane map of Pierce-Price et al. in order to determine the dust grain emissivity index β for each of the sources in the SIMBA source list. We examine the value of β with respect to temperature, as well as for the source classes identified in the SIMBA survey, for variation of this index. Our results indicate that β is typically 2, which is consistent with previous determinations in the literature, but for a considerably larger sample than previous work.  相似文献   

13.
We present SCUBA 850-μm, JCMT  CO( J =2→1)  , B -band imaging and VLA H  i observations of the NGC 7465/4/3 group of galaxies. The 850-μm emission associated with NGC 7465 extends to at least ∼2 R 25 and is well correlated with the H  i . We investigate a range of possible mechanisms by which dust beyond R 25 may be heated to give the observed extended submillimetre emission. By modelling the dust heating by stars in two extreme geometries, we fail to find any reasonable star formation scenario that is consistent with both the 850-μm and optical data. Furthermore, we do not detect any  CO( J =2→1)  emission coincident with the extended dust and atomic gas as would be expected if significant star formation were occurring. We show that shock-heating of dust via cloud–cloud collisions in the stripped interstellar medium of NGC 7465 could be sufficient to explain the extended 850-μm emission and lack of optical emission in the stripped gas, and suggest that cloud–cloud collisions may be an important dust heating mechanism in gas-rich systems.  相似文献   

14.
New observations are reported of J -band spectra (1.04–1.4 μm) of three Seyfert 2 galaxies, Mkn 34, Mkn 78 and NGC 5929. In each case the spectral range includes the near-infrared lines of [Fe  ii ], [P  ii ], He  i and Paβ. Each Seyfert galaxy has a known radio jet, and we investigate the infrared line ratios of the nuclear and extended regions of each galaxy compared to the radio structure. In Mkn 34 there is a clear indication of an extranuclear region, probably coincident with a shock induced by the radio jet, in which [Fe  ii ] is considerably enhanced, although the nuclear emission is almost certainly the result of photoionization by the continuum of the active nucleus. Similar effects in extranuclear regions are seen in the other objects, in the case of Mkn 78 confirming recent studies by Ramos Almeida et al. A possible detection of extranuclear [P  ii ] emission suggests, if real, that photoionization by the active nucleus is the dominant line excitation mechanism over the whole source, including the regions coincident with the radio jet.  相似文献   

15.
We present near-infrared observations of a sample of mainly interacting ultraluminous infrared galaxies, comprising H - and K -band spectra. Our main aims are to investigate the power source of these extremely luminous objects and the various excitation mechanisms of the strong molecular hydrogen emission often seen in such objects. Broadened emission lines were only detected in one object, IRAS 23498, consistent with the previous results for this galaxy. The [Si  vi ] emission line was detected in IRAS 17179 and 20210, both classified as Sy2s. Two of the samples were unclassified, IRAS 00150 and 23420, which exhibit neither [Si  vi ] emission nor broadened H  i emission. However this does not rule out the presence of an obscured AGN. Analysis of the molecular hydrogen emission showed that the major excitation mechanism for most objects was thermal. Modelling of the more luminous objects indicates that for IRAS 20210 10 per cent, and for both IRAS 23365 and IRAS 23420 30 per cent, of the 1–0 S(1) line emission has a non-thermal origin.  相似文献   

16.
Class II methanol masers are believed to be associated with high-mass star formation. Recent observations by Walsh et al. and Phillips et al. reported a very low detection rate of radio continuum emission toward a large sample of 6.7-GHz methanol masers. These results raise questions about the evolutionary phase and/or the mass range of the exciting stars of the masers. Here we report the results of a VLA search for 8.4-GHz continuum emission from the area around five Class II methanol masers, four of which were not detected by Walsh et al. at 8.6 GHz. Radio continuum emission was detected in all five fields although only two of the nine maser spot groups in the five fields were found to be superimposed on radio continuum sources that appear to be ultra-compact H  ii (UCH  ii ) regions. This suggests that continuum counterparts for some masers might be found in further surveys for which the sensitivity level is lower than  1 mJy beam−1  . Considering our results as well as observations from other studies of methanol masers we conclude that masers without radio continuum counterparts are most likely associated with high-mass stars in a very early evolutionary stage, either prior to the formation of a UCH  ii region or when the H  ii region is still optically thick at centimetre wavelengths. With one exception all maser spot groups in the five fields were found to be associated with mid-infrared objects detected in the Midcourse Space Experiment survey.  相似文献   

17.
We present Australia Telescope Compact Array observations towards six massive star formation regions, which, from their strong 24 GHz continuum emission but no compact 8 GHz continuum emission, appeared good candidates for hypercompact H  ii regions. However, the properties of the ionized gas derived from the 19 to 93 GHz continuum emission and  H70α+ H57α  radio recombination line data show the majority of these sources are, in fact, regions of spatially extended, optically thin free–free emission. These extended sources were missed in the previous 8 GHz observations due to a combination of spatial filtering, poor surface brightness sensitivity and primary beam attenuation.
We consider the implications that a significant number of these extended H  ii regions may have been missed by previous surveys of massive star formation regions. If the original sample of 21 sources is representative of the population as a whole, the fact that six contain previously undetected extended free–free emission suggests a large number of regions have been mis-classified. Rather than being very young objects prior to UCH  ii region formation, they are, in fact, associated with extended H  ii regions and thus significantly older. In addition, inadvertently ignoring a potentially substantial flux contribution (up to ∼0.5 Jy) from free–free emission has implications for dust masses derived from sub-mm flux densities. The large spatial scales probed by single-dish telescopes, which do not suffer from spatial filtering, are particularly susceptible and dust masses may be overestimated by up to a factor of ∼2.  相似文献   

18.
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.  相似文献   

19.
We have used the Australia Telescope Compact Array (ATCA) to make observations of a sample of eight young ultra-compact H  ii regions, selected on the basis that they have associated class II methanol maser emission. We have made observations sensitive to both compact and extended structures and find both to be present in most sources. The scale of the extended emission in our sample is in general less than that observed towards samples based on IRAS properties, or large single-dish flux densities. Our observations are consistent with a scenario where extended and compact radio continuum emission co-exists within H  ii regions for a significant period of time.
We suggest that these observations are consistent with a model where H  ii evolution takes place within hierarchically structured molecular clouds. This model, which is the subject of an upcoming companion paper by Shabala et al., addresses both the association between compact and extended emission and the ultra-compact H  ii region lifetime problem.  相似文献   

20.
We present the K -band infrared spectrum of the intermediate polar XY Ari. The spectrum confirms the cataclysmic binary nature of XY Ari, showing emission lines of He  i ( λ 2.0587 μm) and the Brackett and Paschen series of H  i . The broad nature of these lines suggests an origin in an accretion disc. The spectrum is strongly reddened by absorption within the molecular cloud Lynds 1457 and shows prominent absorption features from the secondary star, from which we determine a spectral type for the secondary of M0V. The secondary contributes     per cent of the K -band light. We derive a visual extinction to XY Ari of     and a distance of     , placing XY Ari behind the molecular cloud.  相似文献   

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