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1.
We present the results of direct mid-IR images of the luminous young stellar objects GL 2591, S140-IRS1, and W51-IRS2. The sources show an extended mid-IR emission at the limit of our spatial resolution indicating the presence of dense circumstellar dust disks.  相似文献   

2.
We present the results of an unbiased radio search for gravitational lensing events with image separations between 15 and 60 arcsec, which would be associated with clusters of galaxies with masses >1013–14 M⊙. A parent population of 1023 extended radio sources stronger than 35 mJy with stellar optical identifications was selected using the FIRST radio catalogue at 1.4 GHz and the APM optical catalogue. The FIRST catalogue was then searched for companions to the parent sources stronger than 7 mJy and with separation in the range 15 to 60 arcsec. Higher-resolution observations of the resulting 38 lens candidates were made with the VLA at 1.4 and 5 GHz, and with MERLIN at 5 GHz in order to test the lens hypothesis in each case. None of our targets was found to be a gravitational lens system. These results provide the best current constraint on the lensing rate for this angular scale, but improved calculations of lensing rates from realistic simulations of the clustering of matter on the relevant scales are required before cosmologically significant constraints can be derived from this null result. We now have an efficient, tested observational strategy with which it will be possible to make an order-of-magnitude larger unbiased search in the near future.  相似文献   

3.
We present maps and astrometric results from first epoch, simultaneous MERLIN and Global VLBI observations of the very weak core of a double-lobed radio source, using as reference the bright, compact, flat-spectrum quasar 1636+473 A, 20 arcsec away. The observations were made in May 1995 at 5 GHz. The phase-referenced VLBI map of the weak core, of resolution 1.5 mas, contains a high percentage of the core flux density seen in the MERLIN map. Further hybrid mapping iterations reveal a faint, one-sided core extension on the same side as the MERLIN jet. We explore the effect of both temporal and other coherence losses on the phase-reference map. We relate our results to the structural asymmetry in weak AGN cores within the context of unification models.  相似文献   

4.
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.  相似文献   

5.
We have made a deep search for radio emission from all the northern hemisphere supersoft X-ray sources using the Very Large Array (VLA) and multi-element radio-linked interferometer network (MERLIN) telescopes, at 5 and 8.4 GHz. Three previously undetected sources, T Pyx, V1974 Cygni and RX J0019.8+2156, were imaged in quiescence using the VLA in order to search for any persistent emission. No radio emission was detected in any of the VLA fields down to a typical 1 σ rms noise of 20 μJy beam−1, however, 17 new point sources were detected in the fields with 5-GHz fluxes between 100 and 1500 μJy, giving an average 100-μJy source density of ∼200 deg−2, comparable to what was found in the MERLIN Hubble Deep Field survey. The persistent source AG Draconis was observed by MERLIN to provide a confirmation of previous VLA observations and to investigate the source at a higher resolution. The core is resolved at the milliarcsec scale into two components that have a combined flux of ∼1 mJy. It is possible that we are detecting nebulosity, which is becoming resolved out by the higher MERLIN resolution. We have investigated possible causes of radio emission from a wind environment, both directly from the secondary star, and also consequently, of the high X-ray luminosity from the white dwarf. There is an order of magnitude discrepancy between observed and modelled values that can be explained by the uncertainty in fundamental quantities within these systems.  相似文献   

6.
We present new 1.6-GHz (18-cm) MERLIN maps of 15 Seyfert galaxies, with angular resolutions typically 0.1 to 0.3 arcsec. These and previous observations are used to investigate the properties of 19 of the 24 CfA Seyfert galaxies brighter than 2 mJy at 8.4 GHz. This is the first time a significant fraction of the CfA sample has been mapped at this frequency with subarcsecond resolution, and our observations provide the highest resolution radio maps available for several sources. We use our observations to measure the two-point spectral indices of compact radio components, and we investigate the correlation between infrared and radio emission shown by Seyfert galaxies.
Our results can be summarized as follows. Resolved structures as small as 20 pc are found in three previously unresolved radio sources, and only four sources show single, unresolved radio components. The mean 1.6 to 8.4 GHz spectral index of 31 radio components is         , and approximately 25 per cent of the components have a spectral index flatter than     . The spectral index distributions of type 1 and type 2 Seyferts are statistically indistinguishable. The cores of multiple-component sources tend to have flatter radio spectra than secondary components. The low-resolution infrared ( IRAS ) emission from Seyfert galaxies is usually dominated by kiloparsec-scale, extranuclear emission regions.  相似文献   

7.
We present MERLIN observations of Galactic 21-cm H  i absorption at an angular resolution of  ∼0.1–0.2  arcsec and a velocity resolution of 0.5 km s−1, in the direction of three moderately low latitude  (−8° < b < −12°)  extragalactic radio sources, 3C 111, 3C 123 and 3C 161, all of which are heavily reddened. H  i absorption is observed against resolved background emission sources up to ∼2 arcsec in extent and we distinguish details of the opacity distribution within 1–1.5 arcsec regions towards 3C 123 and 3C 161. This study is the second MERLIN investigation of small-scale structure in interstellar H  i (earlier work probed Galactic H  i in the directions of the compact sources 3C 138 and 3C 147). The 0.1-arcsec scale is intermediate between H  i absorption studies made with other fixed element interferometers with resolution of 1–10 arcsec and very long baseline interferometry studies with resolutions of 10–20 mas. At a scale of 1 arcsec (about 500 au), prominent changes in Galactic H  i opacity in excess of 1–1.5 are determined in the direction of 3C 161 with a signal-to-noise ratio of at least 10σ. Possible fluctuations in the H  i opacity at the level of about 1 are detected at the  2.5–3σ  level in the direction of 3C 123.  相似文献   

8.
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.  相似文献   

9.
A narrow-band Hα image and high-resolution spectroscopy have been obtained in order to investigate the nature of S 266 and its central star MWC 137. The analysis of the stellar and nebular spectra suggests that MWC 137 is a B[e] supergiant located ≥ 6 kpc away from the Sun and not a Herbig Ae/Be star, as it has been traditionally classified. Moreover, the morphology and other properties of the nebula suggest that S 266 is a ring nebula, probably produced by the interaction of stellar winds with the ambient interstellar medium or unprocessed ejected matter. This result would imply that S 266 is the first ring nebula around a B[e] supergiant known in the Galaxy.  相似文献   

10.
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11.
Photometric observations over three seasons show HD 288313 to be a light variable with a 2.2636-d period. The observed V amplitudes lie in the range of 0.06–0.15 mag. The star showed appreciable changes in the brightness at maximum and minimum of the light curve from season to season. The (   b − y   ) colour did not show any significant variation during the photometric cycle. The light variation appears to be caused by the rotational modulation of stellar flux by cool starspots distributed asymmetrically across the stellar longitudes. The Hα line strength in HD 288313 varied drastically from completely filled-in emission to almost full absorption, that is typical of a normal star of similar spectral type. The Hα equivalent width is found to show a clear rotational modulation only occasionally. Most of the time, chromospheric active regions are distributed well across the stellar longitudes, thereby suppressing obvious rotational modulations. Broad-band linear polarization measurements show HD 288313 to be a short period, low-amplitude polarization variable. The polarization variation is, apparently, rotationally modulated. Dust grain scattering in a non-spherical circumstellar envelope of a star with inhomogeneities in the surface brightness distribution seems to be the mechanism operating in producing the observed polarization.  相似文献   

12.
The distribution of galaxy properties in groups and clusters holds important information on galaxy evolution and growth of structure in the Universe. While clusters have received appreciable attention in this regard, the role of groups as fundamental to formation of the present-day galaxy population has remained relatively unaddressed. Here, we present stellar ages, metallicities and α-element abundances derived using Lick indices for 67 spectroscopically confirmed members of the NGC 5044 galaxy group with the aim of shedding light on galaxy evolution in the context of the group environment.
We find that galaxies in the NGC 5044 group show evidence for a strong relationship between stellar mass and metallicity, consistent with their counterparts in both higher and lower mass groups and clusters. Galaxies show no clear trend of age or α-element abundance with mass, but these data form a tight sequence when fitted simultaneously in age, metallicity and stellar mass. In the context of the group environment, our data support the tidal disruption of low-mass galaxies at small group-centric radii, as evident from an apparent lack of galaxies below  ∼109 M⊙  within ∼100 kpc of the brightest group galaxy. Using a joint analysis of absorption- and emission-line metallicities, we are able to show that the star-forming galaxy population in the NGC 5044 group appears to require gas removal to explain the ∼1.5 dex offset between absorption- and emission-line metallicities observed in some cases. A comparison with other stellar population properties suggests that this gas removal is dominated by galaxy interactions with the hot intragroup medium.  相似文献   

13.
We present a submillimetre continuum survey for accretion discs around seven embedded protostars in the Perseus and Serpens molecular clouds. Observations were made at frequencies between 339 and 357 GHz using the James Clerk Maxwell Telescope–Caltech Submillimeter Observatory single-baseline interferometer on Mauna Kea, Hawaii. All the objects in our survey show compact dust emission on scales ≲1 arcsec, assumed to arise in a circumstellar accretion disc. We compare the properties of this compact component with evolutionary indicators, such as the ratio of compact to extended emission, and bolometric temperature. We find that discs of mass ∼0.01 M⊙ have formed by the Class 0 stage, and that similar mass discs are observed in Class I and Class II sources. A trend is observed whereby the ratio of compact to extended emission in our sources increases from Class 0 to Class II sources. For three of the objects in the survey, NGC 1333 IRAS2:CR1 and SVS13 in Perseus, and FIRS1 in Serpens, the signal-to-noise ratio is sufficient to allow us to model the brightness distributions with elliptical Gaussian and power-law disc models. The Gaussian fits give semimajor half-power radii of approximately 90 to 140 au, at the assumed distance of 350 pc to the Perseus and Serpens clouds.  相似文献   

14.
We present images of the jets in the nearby radio galaxy NGC 315 made with the Very Large Array at five frequencies between 1.365 and 5 GHz with resolutions between 1.5 and 45 arcsec. Within 15 arcsec of the nucleus, the spectral index of the jets is  α= 0.61  . Further from the nucleus, the spectrum is flatter, with significant transverse structure. Between 15 and 70 arcsec from the nucleus, the spectral index varies from ≈0.55 on-axis to ≈0.44 at the edge. This spectral structure suggests a change of dominant particle acceleration mechanism with distance from the nucleus and the transverse gradient may be associated with shear in the jet velocity field. Further from the nucleus, the spectral index has a constant value of 0.47. We derive the distribution of Faraday rotation over the inner ±400 arcsec of the radio source and show that it has three components: a constant term, a linear gradient (both probably due to our Galaxy) and residual fluctuations at the level of 1–2 rad m−2. These residual fluctuations are smaller in the brighter (approaching) jet, consistent with the idea that they are produced by magnetic fields in a halo of hot plasma that surrounds the radio source. We model this halo, deriving a core radius of ≈225 arcsec and constraining its central density and magnetic field strength. We also image the apparent magnetic field structure over the first ±200 arcsec from the nucleus.  相似文献   

15.
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   

16.
We present results from a survey of the Rosette Molecular Cloud (RMC) using both the Infrared Array Camera (IRAC) and Multiband Imaging Photometer for Spitzer (MIPS) onboard the Spitzer Space Telescope . We have mapped a region of active star formation covering an area approximately 1° by 1.5° including several previously known clusters. Spectral energy distributions (SEDs) fitted to our data combined with that from Two Micron All Sky Survey (2MASS) are used to identify young stellar objects (YSOs) with infrared (IR) excesses. We find that roughly 50 per cent of the sources are forming in clustered environments and identify seven clusters of IR excess sources including four that were previously unknown. We investigate evidence for triggering of star formation due to the ionization front, identified in Brackett-α emission, associated with the young open cluster NGC 2244. Although the position of several of the clusters of IR excess sources are coincident with the ionization front, the bulk of the youngest YSOs are located far from the ionization front, in clusters located along the mid-plane of the cloud. We conclude that although triggering from the H  ii nebula is a possible origin for some of the recent star formation, the majority of the active star formation is occurring in already dense regions of the cloud not compressed by the expansion of the H  ii region.  相似文献   

17.
New high-quality high spectral resolution observations of the HI line emission from massive young stellar objects are described and discussed. It is proposed that two distinct physical components contribute to the observed emission. One of these is an optically-thick high-velocity stellar wind, the other a more slowly moving optically-thin volume of gas that may, in the case of S106IR at least, be caused by mass loading of the stellar wind. This decomposition is shown to resolve a long-standing problem regarding the relative widths of high and low opacity lines.  相似文献   

18.
We present a time-series analysis of quasi-simultaneous high-resolution profiles of the Hα, He i (λ5876 Å) and Na i D lines for the highly active T Tauri star LkHα 264.
Our data include repeated observations on time-scales ranging from approximately 1 d down to less than 1 h, at two different epochs. For the three lines we have analysed the time variability of the equivalent width and of the profile, using in this case both the normalized variance and correlation matrix analysis.
The behaviours of the equivalent widths of the He i and Na i lines on time-scales of the order of 1 d were found to correlate well. The correlation is more conspicuous when the level of variability of the lines is more dramatic. We conclude that there is a common region of formation for these lines, very close to the star. However, the Hα equivalent width follows such behaviour only when the He i and Na i D lines are less variable.
In terms of profile variations, He i and Na i differ from Hα. The He i and Na i D variations occur coherently over the entire profile, independently of the time-scales, although there are indications that such behaviour may break down for time-scales shorter than ≈20 min. In contrast, the variability of Hα occurs mainly on the blue side of the line. Variations on short time-scales seem to be associated with a secondary blue emission peak. Variations on time-scales of 1 d are linked to two weak blueshifted absorptions. These absorptions occur in an outer region and their behaviour can be understood in terms of an optical depth effect.
We conclude that the Hα line is formed in two distinct regions. The inner region, which is not too distinct from the one producing the He i and Na i D lines, is highly sensitive to the level of activity. The outer and more extended region dominates the contribution to the flux in Hα when the general level of stellar activity is high.  相似文献   

19.
In this paper I will review some recent developments in the field of circumstellar shocks, particularly as they relate to colliding stellar winds. I shall review the basic physics of colliding winds and shocks, and discuss recent developments in hydrodynamic modelling of colliding winds. I shall also report on recent X-ray observations of shock emission in Wolf-Rayet binary systems where high resolution X-ray spectra of colliding wind shock emission is being seen. I will discuss the occurrence of colliding winds to such diverse systems as Wolf-Rayet binaries, pre-main sequence binaries, symbiotic stars as well as the Galactic center object IRS 7, where recent results on interacting winds are yielded insight into the structure of winds in general.  相似文献   

20.
We present neutral hydrogen absorption observations of the luminous infrared merger NGC 6240 using MERLIN with a resolution of 0.2 arcsec. Broad absorption (a few hundred km s−1) has been found against two compact radio sources within the central kpc providing dynamical information about the neutral gas components in front of these sources. A narrow absorption component is also detected superimposed upon this broad absorption and additionally against some of the extended L -band continuum. From these results we deduce that the broad component is a result of absorption by a highly disturbed disc-like structure of neutral gas aligned along the position angle of the two compact radio sources, similar to the model previously proposed by Tacconi et al. at the end of the last century based on spectral CO emission data. The narrow component is likely to arise from absorption by less disturbed neutral gas at much larger scales within the system.
Continuum observations presented here at 1.4 and 5 GHz support the view that NGC 6240 contains a double nucleus resulting from a galactic merger event and show these as two compact radio sources separated by 1.52 arcsec. We have also applied luminosity and morphological considerations to the continuum results to determine the most feasible source of radio emission for this luminous merger galaxy. We conclude that the most likely source of the radio flux found in NGC 6240 is a combination of starburst emission from radio luminous supernova remnants, similar to those found in Arp 220, and emission from a weak AGN probably triggered by a merger event.  相似文献   

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