Home     Getting Started     To Survive in the Universe    
Inhabited Sky
    News@Sky     Astro Photo     The Collection     Forum     Blog New!     FAQ     Press     Login  

NGC 7538



Upload your image

DSS Images   Other Images

Related articles

3He in the Milky Way Interstellar Medium: Ionization Structure
The cosmic abundance of the 3He isotope has importantimplications for many fields of astrophysics. We are using the 8.665 GHzhyperfine transition of 3He+ to determine the3He/H abundance in Milky Way H II regions and planetarynebulae. This is one in a series of papers in which we discuss issuesinvolved in deriving accurate 3He/H abundance ratios from theavailable measurements. Here we describe the ionization correction weuse to convert the 3He+/H+ abundance,y+3, to the 3He/H abundance,y3. In principle the nebular ionization structure cansignificantly influence the y3 derived for individualsources. We find that in general there is insufficient informationavailable to make a detailed ionization correction. Here we make asimple correction and assess its validity. The correction is based onradio recombination line measurements of H+ and4He+, together with simple core-halo sourcemodels. We use these models to establish criteria that allow us toidentify sources that can be accurately corrected for ionization andthose that cannot. We argue that this effect cannot be very large formost of the sources in our observational sample. For a wide range ofmodels of nebular ionization structure we find that the ionizationcorrection factor varies from 1 to 1.8. Although larger corrections arepossible, there would have to be a conspiracy between the density andionization structure for us to underestimate the ionization correctionby a substantial amount.

Changes in the Radio Appearance of MWC 349A
We present new sensitive, high angular resolution 1.3, 2, and 6 cmobservations of the continuum emission from the peculiar emission-linestar MWC 349A made with the Very Large Array. This radio emission isbelieved to originate in an ionized flow produced by thephotoevaporation of a disk that surrounds the star. We determine for thefirst time the proper motion of this source, which is consistent withthat expected for the location of the source in the Galaxy. Our analysisof the images, that include the new observations as well as archive datacovering a time interval of more than 20 years, indicates that theappearance of MWC 349A has been systematically changing over time. Thewell-defined ``hourglass'' shape that characterized the 2 and 1.3 cmappearance of the source in the early 1980s has disappeared, to bereplaced by a more ``square'' shape. We discuss if these changes can beaccounted for by precession of the MWC 349A disk or by intrinsic changesin the parameters of the disk, but could not reach a satisfactoryexplanation.

Infrared Spectroscopy of Diamondoid Molecules: New Insights into the Presence of Nanodiamonds in the Interstellar Medium
Although they are relatively different in band shape, infrared featuresaround 3.4-3.5 μm in the emission spectra of HD 97048 and Elias 1 andin the absorption spectra of various dense clouds have both beenattributed to diamondoid molecules/particles. This assignment is basedmainly on infrared spectra of hydrogenated diamond thin films and ofdiamond nanocrystals of known average size. Here we present an analysisof the astrophysical implications of recently reported solid-state2.5-12.5 μm spectra of individual diamondoid molecules, up to thesize of hexamantane (C26H30). These spectraprovide the first experimental measurements of the infrared frequenciesof this class of molecules. In addition, laboratory gas-phase infraredemission spectra of the three smallest members of the diamondoid familyare reported, as well as theoretical spectra for some larger species.The present data set allows us to relate spectral signatures to themolecular size and structure. The spectra of tetrahedral diamondoids arefound to be qualitatively different from those of lower symmetryspecies, which possibly explains the differences between theastrophysical emission and absorption spectra. Interestingly, the 3.53μm band is clearly observed in the spectra of these small moleculardiamondoids, whereas previous studies on nanodiamond particles foundthis band only for species larger than ~50 nm. Our results support theassignment of the 3.43 and 3.53 μm emission features in HD 97048 andElias 1 to diamondoids of a few nanometers in size as well as thesuggestion that smaller diamondoid molecules contribute to the 3.47μm interstellar absorption band.

High-Resolution Molecular Gas Maps of M33
New observations of CO (J=1-->0) line emission from M33, using the 25element BEARS focal plane array at the Nobeyama Radio Observatory 45 mtelescope, in conjunction with existing maps from the BIMAinterferometer and the FCRAO 14 m telescope, give the highest resolution(13") and most sensitive (σrms~60 mK) maps to date ofthe distribution of molecular gas in the central 5.5 kpc of the galaxy.A new catalog of giant molecular clouds (GMCs) has a completeness limitof 1.3×105 Msolar. The fraction of moleculargas found in GMCs is a strong function of radius in the galaxy,declining from 60% in the center to 20% at galactocentric radiusRgal~4 kpc. Beyond that radius, GMCs are nearly absent,although molecular gas exists. Most (90%) of the emission from low-massclouds is found within 100 pc projected separation of a GMC. In anannulus 2.1 kpc

A Search for Formaldehyde 6 cm Emission toward Young Stellar Objects. II. H2CO and H110α Observations
We report the results of our second survey for Galactic H2COmaser emission toward young stellar objects. Using the GBT and the VLAin the A configuration we observed 58 star-forming regions anddiscovered the fifth H2CO 6 cm maser region in the Galaxy(G23.71-0.20). We have discussed the detection toward G23.71-0.20 in aprevious paper. Here we present all the other results from our survey,including detection of H2CO absorption features toward 48sources, detection of the H110α recombination line toward 29sources, detection (including tentative detections) of the carbonrecombination line C110α toward 14 sources, subarcsecond angularresolution images of 6 cm continuum emission toward five sources, andobservations of the H2CO masers in IRAS 18566+0408 and NGC7538. In the case of NGC 7538, we detected the two main H2COmaser components, and our observations confirm variability of theblueshifted component recently reported by Hoffman et al. Thevariability of both maser components in NGC 7538 could be caused by ashock wave that reached the redshifted component approximately 14 yrbefore reaching the blueshifted component. If that were the case, wewould expect to detect an increase in the flux density rate of change ofthe blueshifted component sometime after the year 2009. Our data alsosupport the use of H2CO/H110α observations as a tool toresolve the kinematic distance ambiguity of massive star-forming regionsin the Galaxy.

Star formation in the Vela molecular ridge. Large scale mapping of cloud D in the mm continuum
Context: The Vela molecular ridge is one of the nearestintermediate-mass star forming regions, located within the galacticplane and outside the solar circle. Cloud D, in particular, hosts anumber of small embedded young clusters. Aims: We present the resultsof a large-scale map in the dust continuum at 1.2 mm of a ~ 1°× 1° area within cloud D. The main aim of the observations wasto obtain a complete census of cluster-forming cores and isolated (bothhigh- and low-mass) young stellar objects in early evolutionary phases. Methods: The bolometer array SIMBA at SEST was used to map the dustemission in the region with a typical sensitivity of ~ 20 mJy/beam. Thisallows a mass sensitivity of ~ 0.2 Mȯ. The resolution is24 arcsec, corresponding to ~ 0.08 pc, roughly the radius of a typicalyoung embedded cluster in the region. The continuum map is also comparedto a large scale map of CO(1-0) integrated emission. Results: Using theCLUMPFIND algorithm, a robust sample of 29 cores has been obtained,spanning the size range 0.03-0.25 pc and the mass range 0.4-88Mȯ. The most massive cores are associated both with redIRAS sources and with embedded young clusters, and coincide with CO(1-0)integrated emission peaks. The cores are distributed according to a massspectrum ~ M-α and a mass-versus-size relation ~Dx, with α ˜ 1.45-1.9 and x ˜ 1.1-1.7. Theyappear to originate in the fragmentation of gas filaments seen inCO(1-0) emission and their formation is probably induced by expandingshells of gas. The core mass spectrum is flatter than the Initial MassFunction of the associated clusters in the same mass range, suggestingfurther fragmentation within the most massive cores. A thresholdAV ˜ 12 mag seems to be required for the onset of starformation in the gas.Based on observations collected at the EuropeanSouthern Observatory, La Silla, Chile, program 71.C-0088.

Testing grain-surface chemistry in massive hot-core regions
Aims.We study the chemical origin of a set of complex organic moleculesthought to be produced by grain surface chemistry in high mass youngstellar objects (YSOs). Methods: A partial submillimeter line-surveywas performed toward 7 high-mass YSOs aimed at detecting H2CO, CH3OH,CH2CO, CH3CHO, C2H5OH, HCOOH, HNCO and NH2CHO. In addition, lines ofCH3CN, C2H5CN, CH3CCH, HCOOCH3, and CH3OCH3 were observed. Rotationtemperatures and beam-averaged column densities are determined. Tocorrect for beam dilution and determine abundances for hot gas, theradius and H2 column densities of gas at temperatures >100 K arecomputed using 850 μm dust continuum data and source luminosity. Results: Based on their rotation diagrams, molecules can be classifiedas either cold (<100 K) or hot (>100 K). This implies that complexorganics are present in at least two distinct regions. Furthermore, theabundances of the hot oxygen-bearing species are correlated, as arethose of HNCO and NH2CHO. This is suggestive of chemical relationshipswithin, but not between, those two groups of molecules. Conclusions:.The most likely explanation for the observed correlations of thevarious hot molecules is that they are "first generation" species thatoriginate from solid-state chemistry. This includes H2CO, CH3OH, C2H5OH,HCOOCH3, CH3OCH3, HNCO, NH2CHO, and possibly CH3CN, and C2H5CN. Thecorrelations between sources implies very similar conditions duringtheir formation or very similar doses of energetic processing. Coldspecies such as CH2CO, CH3CHO, and HCOOH, some of which are seen as icesalong the same lines of sight, are probably formed in the solid state aswell, but appear to be destroyed at higher temperatures. A low level ofnon-thermal desorption by cosmic rays can explain their low rotationtemperatures and relatively low abundances in the gas phase compared tothe solid state. The CH3CCH abundances can be fully explained by lowtemperature gas phase chemistry. No cold N-containing molecules arefound.Appendices are only available in electronic form at http://www.aanda.org

The Galactic distribution of magnetic fields in molecular clouds and HII regions
Aims.Magnetic fields exist on all scales in our Galaxy. There is acontroversy about whether the magnetic fields in molecular clouds arepreserved from the permeated magnetic fields in the interstellar medium(ISM) during cloud formation. We investigate this controversy usingavailable data in the light of the newly revealed magnetic fieldstructure of the Galactic disk obtained from pulsar rotation measures(RMs). Methods: We collected measurements of the magnetic fields inmolecular clouds, including Zeeman splitting data of OH masers in cloudsand OH or HI absorption or emission lines of clouds themselves. Results: The Zeeman data show structures in the sign distribution of theline-of-sight component of the magnetic field. Compared to thelarge-scale Galactic magnetic fields derived from pulsar RMs, the signdistribution of the Zeeman data shows similar large-scale fieldreversals. Previous such examinations were flawed in the over-simplifiedglobal model used for the large-scale magnetic fields in the Galacticdisk. Conclusions: .We conclude that the magnetic fields in the cloudsmay still "remember" the directions of magnetic fields in the GalacticISM to some extent, and could be used as complementary tracers of thelarge-scale magnetic structure. More Zeeman data of OH masers in widelydistributed clouds are required.Tables 1 and 2 are only available in electronic form athttp://www.aanda.org

Giant Molecular Association in Spiral Arms of M 31: I. Evidence for Dense Gas Formation via Spiral Shock Associated with Density Waves?
We present observations of 12CO (J = 1-0), 13CO (J= 1-0), and 12CO (J = 3-2) emissions toward a Giant MolecularAssociation (GMA) in the southern spiral arm of M 31 using the NRO 45 mand the ASTE 10 m telescopes. Observed regions are 3' × 4' (0.6kpc × 0.8 kpc) with an angular resolution of 16''-17'' for12CO (1-0) and 13CO (1-0), and 1.2' × 1.4'with 23'' for 12CO (3-2). The GMA has a size of a few 100 pcand a mass of 5.6×106 M_odot. The 12CO (1-0)to 13CO (1-0) integrated intensity ratio (R12/13)and the 12CO (3-2) to 12CO (1-0) ratio(R3-2/1-0), averaged over the entire region of the GMA, are∼ 10 and 0.3, respectively. These line ratios suggest gas densitiesof (3-6) × 102 cm-3 at a temperature of15-25 K, which are similar to, or slightly larger than, those of GMCs inthe Galactic disk. We found a radial gradient of R12/13within the GMA, ranging from 6 at the center to 14 at the edges. Thedistribution of R12/13 shows a smooth structure with anoverall density gradient. The GMA consists of two velocity components,blue (∼ -505 km s-1) and red (≥-490 kms-1). In both the 12CO (1-0) and 13CO(1-0) profiles, the blue component shows a strong peak intensity and anarrow velocity width, while the red is weaker and wider. TheR12/13 value of the red component is 5 and that of the blueis 16, indicating that the red component has a higher gas density.Moreover, we suggest that the red component is ``post-shock'' dense gasdecelerated by shock due to the density wave.

Chain-type structure in the H2O maser NGC 7538N
Not Available

Massive Quiescent Cores in Orion. II. Core Mass Function
We have surveyed submillimeter continuum emission from relativelyquiescent regions in the Orion molecular cloud to determine how the coremass function in a high-mass star-forming region compares to the stellarinitial mass function. Such studies are important for understanding theevolution of cores to stars, and for comparison to formation processesin high- and low-mass star-forming regions. We used the SHARC II cameraon the Caltech Submillimeter Observatory telescope to obtain 350 μmdata having angular resolution of about 9", which corresponds to 0.02 pcat the distance of Orion. Further data processing using a deconvolutionroutine enhances the resolution to about 3". Such high angularresolution allows a rare look into individually resolved densestructures in a massive star-forming region. Our analysis combining dustcontinuum and spectral line data defines a sample of 51 Orion molecularcores with masses ranging from 0.1 to 46 Msolar and a meanmass of 9.8 Msolar, which is 1 order of magnitude higher thanthe value found in typical low-mass star-forming regions, such asTaurus. The majority of these cores cannot be supported by thermalpressure or turbulence, and are probably supercritical. They are thuslikely precursors of protostars. The core mass function for the Orionquiescent cores can be fitted by a power law with an index equal to-0.85+/-0.21. This is significantly flatter than the Salpeter initialmass function and is also flatter than the core mass function found inlow and intermediate star-forming regions. When compared with othermassive star-forming regions such as NGC 7538, this slope is flatterthan the index derived for samples of cores with masses up to thousandsof Msolar. Closer inspection, however, indicates slopes inthose regions similar to our result if only cores in a similar massrange are considered. Based on the comparison between the mass functionof the Orion quiescent cores and those of cores in other regions, wefind that the core mass function is flatter in an environment affectedby ongoing high-mass star formation. Thus, it is likely thatenvironmental processes play a role in shaping the stellar IMF later inthe evolution of dense cores and the formation of stars in such regions.

First Detection of an H2CO 6 cm Maser Flare: A Burst in IRAS 18566+0408
We report the discovery of a short-duration (less than 3 months)outburst of the H2CO 6 cm maser in IRAS 18566+0408(G37.55+0.20). During the flare, the peak flux density of the maserincreased by a factor of 4; after less than a month, it decayed to thepreflare value. This is the first detection of a short, burstlikevariability of an H2CO 6 cm maser. The maser shows anasymmetric line profile that is consistent with the superposition of twoGaussian components. We did not detect a change in the velocity or theline width of the Gaussian components during the flare. If the twoGaussian components trace two separate maser regions, then very likelyan event outside the maser gas triggered simultaneous flares at twodifferent locations.

The Formaldehyde Masers in Sgr B2: Very Long Baseline Array and Very Large Array Observations
Observations of two of the formaldehyde (H2CO) masers (A andD) in Sgr B2 using the VLBA+Y27 (resolution ~0.01") and the VLA(resolution ~9") are presented. The VLBA observations show compactsources (<~10 mas, <~80 AU) with brightness temperatures>108 K. The maser sources are partially resolved in theVLBA observations. The flux densities in the VLBA observations are about1/2 those of the VLA, and the line widths are about 2/3 of the VLAvalues. The applicability of a core-halo model for the emissiondistribution is demonstrated. Comparison with earlier H2COabsorption observations and with ammonia (NH3) observationssuggests that H2CO masers form in shocked gas. Comparison ofthe integrated flux densities in current VLA observations with those inprevious observations indicates that (1) most of the masers have variedin the past 20 years, and (2) intensity variations are typically lessthan a factor of 2 compared to the 20 yr mean. No significant linear orcircular polarization is detected with either instrument.

Associations of H_2O and CH_3OH masers at milli-arcsec angular resolution in two high-mass YSOs
Context: While high-angular resolution observations have clarified thatwater masers originate from shocks associated with protostellar jets,different environments have been proposed in several sources to explainthe origin of CH3OH masers. Aims: We wish to investigate the nature ofthe CH3OH maser birthplace in SFRs and the association between the H2Oand CH3OH maser emission in the same YSO. Methods: We have conductedphase-reference VLBI observations of H2O and CH3OH masers toward twohigh-mass SFRs, Sh 2-255 IR and AFGL 5142. Results: In Sh 2-255 IRwater masers are aligned along a direction close to the orientation ofthe molecular outflow observed on angular scales of 1-10 arcsec, tracingpossibly the disk-wind emerging from the disk atmosphere. In AFGL 5142water masers trace expansion at the base of a protostellar jet, whilstmethanol masers are more probably tracing infalling than outflowing gas. Conclusions: .The results for AFGL 5142 suggest that water and methanolmasers trace different kinematic structures in the circumstellar gas.

Enhanced density and magnetic fields in interstellar OH masers
Aims.We have observed the 6030 and 6035 MHz transitions of OH inhigh-mass star-forming regions to obtain magnetic field estimates inboth maser emission and absorption. Methods: .Observations weretaken with the Effelsberg 100 m telescope. Results: .Ourobservations are consistent with previous results, although we do detecta new 6030 MHz maser feature near -70 km s-1 in the vicinityof W3(OH). In absorption we obtain a possible estimate of -1.1 ±0.3 mG for the average line-of-sight component of the magnetic field inthe absorbing OH gas in K3-50 and submilligauss upper limits for theline-of-sight field strength in DR 21 and W3. Conclusions: .Theseresults indicate that the magnetic field strength in the vicinity of OHmasers is higher than that of the surrounding, non-masing material,which in turn suggests that the density of masing OH regions is higherthan that of their surroundings.

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-LinkedInterferometer Network (MERLIN) are presented. The masers lie across thewestern face of the ultracompact HII region in extended filaments whichmay trace large-scale shocks. There is a complex interrelation betweenthe 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 thatstretches at least 3100 au across the face of the ultracompact HIIregion. The dominant 6.7-GHz methanol emission coincides with the radiocontinuum peak and is populated by masers with broad spectral lines. The6.7-GHz methanol emission is elongated at position angle 50° with astrong velocity gradient, and bears many similarities to the methanolmaser disc structure reported in NGC 7538. It is surrounded by arcs ofground state OH masers at 1.7 GHz and highly excited OH masers at 13.44GHz, some of which have the brightest methanol masers at their focus. Wesuggest that this region hosts the excitation centre for theultracompact HII region.

High-Mass Star Formation. III. The Functional Form of the Submillimeter Clump Mass Function
We investigate the mass function of cold, dusty clumps in 11 low- andhigh-mass star-forming regions. Using a homogeneous fitting technique,we analyze the shape of each region's clump mass function and examinethe commonalities among them. We find that the submillimeter continuumclump mass function in low-mass star-forming regions is typically bestfitted by a lognormal distribution, while that in high-mass star-formingregions is better fitted by a double power law. A single power-law clumpmass distribution is ruled out in all cases. Fitting all of the regionswith a double power law, we find that the mean power-law exponent at thehigh-mass end of each mass function isαhigh=-2.4+/-0.1, consistent with the Salpeter resultof α=-2.35. We find no region-to-region trend inαhigh with the mass scale of the clumps in a givenregion, as characterized by their median mass. Similarly, nonparametrictests show that the shape of the clump mass function does not changemuch from region to region, despite the obvious changes in the intrinsicmass scale. This result is consistent with the hypothesis that the clumpmass distribution is determined by a highly stochastic process, such asturbulent fragmentation. It may also suggest that the data reduction andanalysis techniques strongly affect the shape of the derived massfunction.

OH masers associated with bipolar outflow in ON1
Multi-Element Radio-Linked Interferometer Network polarizationmeasurements of the OH 18-cm masers in ON1 are presented. OH maseremission is detected in all four ground state lines. The bulk of themasers lie in an elongated distribution covering 2 arcsec at positionangle 130°, near the southern edge of the ultracompact HII region.The maser distribution is orthogonal to the bipolar outflow seen inHCO+ at position angle (PA) 44°. An isolated group of1720-MHz masers is also seen 6 arcsec to the east. The magnetic fielddeduced from Zeeman splitting of the OH maser lines shows a large-scaleorder, with field values ranging from -0.4 to -4.6 mG.

H2O maser flare in NGC 7538 S
We investigate the H2O maser flare that occurred in the source NGC 7538S during 1998 2005 in the radial-velocity range from -57 to -52 km s-1.We have found a large number of emission features, suggesting that themedium where the flare occurred is highly fragmented. We have identifiedfour spectral groups of emission features. All groups are most likelyassociated with the cluster of maser spots located in the center of anelongated structure and related to a massive rotating disk. The observedpattern of variations in the fluxes and radial velocities of thefeatures can be explained by the presence of inhomogeneities in themedium, which can form elongated structures like filaments or chains.The mean extent of this structure is estimated to be 6 8 AU. Two cyclesof maser activity have been observed, 1998 2002 and 2003 2005, which maybe determined by the cyclic activity of the central object, a massiveO-type protostar.

Outflows from the high-mass protostars NGC 7538 IRS1/2 observed with bispectrum speckle interferometry. Signatures of flow precession
Context.NGC 7538 IRS1 is a high-mass (30Mȯ) protostar with a CO outflow, an associatedultracompact H II region, and a linear methanol maser structure, whichmight trace a Keplerian-rotating circumstellar disk. The directions ofthe various associated axes are misaligned with each other. Aims.We investigate the near-infrared morphology of the source toclarify the relations among the various axes. Methods.K'-bandbispectrum speckle interferometry was performed at two 6-meter-classtelescopes - the BTA 6 m telescope and the 6.5 m MMT. Complementary IRACimages from the Spitzer Space Telescope Archive were used to relate thestructures detected with the outflow at larger scales. Results.High-dynamic range images show fan-shaped outflow structure inwhich we detect 18 stars and several blobs of diffuse emission. Weinterpret the misalignment of various outflow axes in the context of adisk precession model, including numerical hydrodynamic simulations ofthe molecular emission. The precession period is ~280 years and itshalf-opening angle is ~40°. A possible triggering mechanism isnon-coplanar tidal interaction of an (undiscovered) close companion withthe circumbinary protostellar disk. Our observations resolve the nearbymassive protostar NGC 7538 IRS2 as a close binarywith separation of 195 mas. We find indications for shock interactionbetween the outflow activities in IRS1 and IRS2. Finally, we findprominent sites of star formation at the interface between twobubble-like structures in NGC 7538, suggestive of a triggered starformation scenario. Conclusions.Indications of outflow precessionhave been discovered to date in a number of massive protostars, all withlarge precession angles (~20-45°). This might explain the differencebetween the outflow widths in low- and high-mass stars and add supportto a common collimation mechanism.

Astrophysics in 2005
We bring you, as usual, the Sun and Moon and stars, plus some galaxiesand a new section on astrobiology. Some highlights are short (the newlyidentified class of gamma-ray bursts, and the Deep Impact on Comet9P/Tempel 1), some long (the age of the universe, which will be found tohave the Earth at its center), and a few metonymic, for instance theterm ``down-sizing'' to describe the evolution of star formation rateswith redshift.

Radio Recombination Lines in Galactic H II Regions
We report radio recombination line (RRL) and continuum observations of asample of 106 Galactic H II regions made with the NRAO 140 Foot (43 m)radio telescope in Green Bank, West Virginia. We believe this to be themost sensitive RRL survey ever made for a sample this large. Most of oursource integration times range between 6 and 90 hr, yielding typical rmsnoise levels of ~1.0-3.5 mK. Our data result from two differentexperiments performed, calibrated, and analyzed in similar ways. A C IIsurvey was made at the 3.5 cm wavelength to obtain accurate measurementsof carbon radio recombination lines. When combined with atomic (C I) andmolecular (CO) data, these measurements will constrain the composition,structure, kinematics, and physical properties of the photodissociationregions that lie on the edges of H II regions. A second survey was madeat the 3.5 cm wavelength to determine the abundance of 3He inthe interstellar medium of the Milky Way. Together with measurements ofthe 3He+ hyperfine line, we get high-precision RRLparameters for H, 4He, and C. Here we discuss significantimprovements in these data with both longer integrations and newlyobserved sources.

High-Mass Star Formation. II. The Mass Function of Submillimeter Clumps in M17
We have mapped a ~5.5×5.5 pc portion of the M17 massivestar-forming region in both 850 and 450 μm dust continuum emissionusing the Submillimeter Common-User Bolometer Array (SCUBA) on the JamesClerk Maxwell Telescope (JCMT). The maps reveal more than 100 dustyclumps with deconvolved linear sizes of ~0.05-0.2 pc and masses of~0.8-120 Msolar, most of which are not associated with knownmid-infrared point sources. Fitting the clump mass function with adouble power law gives a mean power-law exponent ofαhigh=-2.4+/-0.3 for the high-mass power law,consistent with the exponent of the Salpeter stellar mass function. Weshow that a lognormal clump mass distribution with a peak at ~4Msolar produces as good a fit to the clump mass function asdoes a double power law. This 4 Msolar peak mass is wellabove the peak masses of both the stellar initial mass function and themass function of clumps in low-mass star-forming regions. Despite thedifference in intrinsic mass scale, the shape of the M17 clump massfunction appears to be consistent with the shape of the core massfunction in low-mass star-forming regions. Thus, we suggest that theclump mass function in high-mass star-forming regions may be a scaled upversion of that in low-mass regions, instead of its extension to highermasses.

A multiwavelength study of the massive star-forming region IRAS 06055+2039 (RAFGL 5179)
Aims.We present a multiwavelength study of the massive star-formingregion associated with IRAS 06055+2039. Methods: .Narrow-bandnear-infrared (NIR) observations were carried out with UKIRT-UFTI inmolecular hydrogen and Brγ lines to trace the shocked and ionizedgases, respectively. We have used 2MASS {J H K}s data tostudy the nature of the embedded cluster associated with IRAS06055+2039. The radio emission from the ionized gas was mapped at 610and 1280 MHz using the Giant Metrewave Radio Telescope (GMRT), India.Emission from warm dust and the unidentified infrared bands (UIBs) wasestimated using the mid-infrared (8-21 μm) data from the MSX survey.Submillimetre emission from the cold dust at 450 and 850 μm wasstudied using JCMT-SCUBA. Results: .For the infrared clusterassociated with IRAS 06055+2039, we obtain a power-law slope of0.43±0.09 for the K_s-band luminosity function (KLF), which is ingood agreement with other young embedded clusters. We estimate an age of2-3 Myr for this cluster. Apart from the diffuse emission, thehigh-resolution 1280 MHz map also shows the presence of several discretesources that possibly represent high-density clumps. The morphology ofshocked molecular hydrogen forms an arc towards the N-E of the centralIRAS point source and envelopes the radio emission. Submillimetreemission shows the presence of a dense cloud core that is probably at anearlier evolutionary stage compared to the ionized region with shockedmolecular gas lying between the two. The total mass of the cloud isestimated to be 7000-9000 Mȯ from the submillimetreemission at 450 and 850 μm. Conclusions: .The multiwavelengthstudy of this star-forming complex reveals an interesting scenario whereregions are at different stages in the evolution of star formation.

Full-Polarization Observations of OH Masers in Massive Star-forming Regions. II. Maser Properties and the Interpretation of Polarization
We analyze full-polarization VLBA data of ground-state OH masers in 18massive star-forming regions previously presented in a companion paper.We confirm results previously seen in the few individual sources studiedat milliarcsecond angular resolution. The OH masers often arise in theshocked neutral gas surrounding ultracompact H II regions. Magneticfields as deduced from OH maser Zeeman splitting are highly ordered onthe scale of a source and on the maser clustering scale of~1015 cm, which appears to be universal. OH masers aroundultracompact H II regions live ~104 yr before turning offabruptly, rather than weakening gradually. These masers have a widerange of polarization properties. At one extreme (e.g., W75 N),π-components are detected and the polarization position angles ofmaser spots show some organization. At the other extreme (e.g., W51e1/e2), almost no linear polarization is detected and partialdepolarization occurs. A typical source has properties intermediate tothese two extremes, with no clear pattern in the distribution ofpolarization position angles. This can be explained if Faraday rotationin a typical OH maser source is large on a maser amplification lengthbut small on a single (e-folding) gain length. Increasing or decreasingFaraday rotation by a factor of ~5 among different sources can explainthe observed variation in polarization properties. Pure π-components(in theory, 100% linearly polarized) are seldom seen. We suggest thatalmost all π-components acquire a significant amount of circularpolarization from low-gain stimulated emission of a σ-componentfrom velocity-coherent OH lying along the propagation path.

Detecting Interstellar Alkanes
Infrared spectra of long-chain n-alkane molecules, CH3(CH2)mCH3, with m>8, havebeen obtained in absorption and emission and are compared to spectra ofa variety of interstellar sources. Absorption spectra of these moleculesin the 3.4 μm region are remarkably similar, consisting of narrowfeatures at 3.384, 3.425, and 3.508 μm, independent of chain length.These features are not seen in absorption in interstellar clouds. Weestimate that the column density of CH2 groups in long-chainn-alkanes is typically N<(1.2-2.9)×1016cm-2 in VI Cyg 12. Given the narrow width of these features,this limit could be improved with higher resolution observations.Thermal spectra of these molecules show a number of characteristicfeatures near 6.8, 7.3, and 13.9 μm that should be useful indetermining whether large n-alkane molecules are present in interstellaremission sources.

Luminosity functions of YSO clusters in Sh-2 255, W3 main and NGC 7538 star forming regions
We have conducted deep near-infrared surveys of the Sh-2 255, W3 Mainand NGC 7538 massive star forming regions using simultaneous observations of the JHKs -band with the near-infrared camera SIRIUS onthe UH 88-inch telescope. The near-infrared surveys cover a total areaof ~72 square arcmin of three regions with 10-sigma limiting magnitudesof ~19.5, 18.4 and 17.3 in J, H and Ks-band, respectively.Based on the colour-colour and colour- magnitude diagrams and theirclustering properties, the candidate young stellar objects areidentified and their luminosity functions are constructed in Sh-2 255,W3 Main and NGC 7538. A large number of previously unreported redsources (H - K > 2) have also been detected around these regions. Weargue that these red stars are most probably pre-main sequence starswith intrinsic colour excesses. The detected young stellar objects showa clear clustering pattern in each region: the Class I-like sources aremostly clustered in molecular cloud region, while the Class II-likesources in or around more evolved optical H II regions. We find thatthe slopes of the Ks -band luminosity functions of Sh-2 255, W3 Main andNGC 7538 are lower than the typical values reported for the youngembedded clusters and their stellar populations are primarily composedof low mass pre-main sequence stars. From the slopes of the Ks -bandluminosity functions, we infer that Sh-2 255, W3 Main and NGC 7538 starforming regions are rather young (age 1 Myr).

Star formation associated with H II regions
Star formation associated with H II regions is briefly reviewed. Specialemphasis is laid on our series of observational studies on bright-rimmedclouds (BRCs), in which we found a phenomenon called "small-scalesequential star formation." In addition a new hypothesis is advocated onthe two modes of star formation associated with H II regions, i.e., thecluster and dispersed modes. The former gives birth to a rich clusterand in the associated H II region BRCs are formed only at a later stageof its evolution in the peripheries. In the latter mode no clusters oronly loose ones are formed, but BRCs can appear at earlier stages ininner part of the H II region. Presumably these modes depend on theinitial density distribution of the natal molecular cloud.

Discovery of two new methanol masers in NGC 7538. Locating of massive protostars
Context: .NGC 7538 is known to host a 6.7 and 12.2 GHz methanol masercospatial with a Ultra Compact (UC) H II region, IRS 1.Aims.We report onthe serendipitous discovery of two additional 6.7 GHz methanol masers inthe same region, not associated with IRS 1.Methods.Interferometry maserpositions are compared with recent single-dish and interferometrycontinuum observations.Results.The positions of the masers agree to highaccuracy with the 1.2 mm continuum peak emission in NGC 7538 IRS 9 andNGC 7538 S. This clear association is also confirmed by the positionalagreement of the masers with existing high resolution continuumobservations at cm and/or mm wavelengths.Conclusions.Making use of theestablished strong relation between methanol masers and high-mas starformation, we claim that we have accurately positioned the high-massprotostars within the regions where they are detected. The variety ofobjects hosting a 6.7 GHz methanol maser in NGC 7538 shows that thisemission probably traces different evolutionary stages within theprotostellar phase.

Abundance Gradients in the Galaxy
Six H II regions at galactocentric distances of R=10-15 kpc have beenobserved in the far-IR emission lines of [O III] (52 μm, 88 μm),[N III] (57 μm), and [S III] (19 μm) using the Kuiper AirborneObservatory. These observations have been combined with Very Large Arrayradio continuum observations of these sources to determine theabundances of O++, N++, and S++relative to hydrogen. In addition, eight of the most recent sets ofmeasurements of ionic line strengths in H II regions have beenreanalyzed in order to attempt to reconcile differences in opticalversus far-IR abundance determinations. We have in total 168 sets ofobservations of 117 H II regions in our analysis. The new analysisincluded updating the atomic constants (transition probabilities andcollision cross sections), recalculation of some of the physicalconditions in the H II regions (ne and Te), andthe use of new photoionization models to determine stellar effectivetemperatures of the exciting stars. We also use the most recent dataavailable for the distances for these objects, although for most westill rely on kinematic distance determinations. Our analysis findslittle indication of differences between optical and infraredobservations of the nitrogen abundances, but some differences are seenin the oxygen and sulfur abundances. A very significant offset continuesto be seen between optical and infrared measurements of the N/Oabundance ratio.

Submit a new article

Related links

  • - No Links Found -
Submit a new link

Member of following groups:

Observation and Astrometry data

Right ascension:23h13m45.70s
Apparent magnitude:99.9

Catalogs and designations:
Proper Names   (Edit)
NGC 2000.0NGC 7538

→ Request more catalogs and designations from VizieR