Ultra Compact Research Articles

Design of Ultra Compact Polarization Splitter Based on the Complete Photonic Band Gap

A novel design of polarization splitter based on the complete photonic band gap has been proposed in this paper. The proposed Photonic Band Gap (PBG) polarization splitter is formed by two photonic crystal waveguides composed of dielectric rods in air in honeycomb structure for which complete photonic band gap is obtained using the plane wave expansion (PWE) method. The splitting properties (i.e. coupling length, extinction ratio and insertion loss) of PBG polarization splitter have numerically been investigated using the finite difference time domain (FDTD) method. It has been shown that polarization splitter of length as small as 32 μm can be designed at λ=1.55 μm. The proposed polarization splitter offers a large bandwidth of 120 nm.

Millimetre continuum observations of southern massive star formation regions -- I. SIMBA observations of cold cores

We report the results of a 1.2-mm continuum emission survey toward 131 star-forming complexes suspected of undergoing massive star formation. These regions have previously been identified as harbouring a methanol maser and/or a radio continuum source [ultracompact (UC) H II region], the presence of which is in most instances indicative of massive star formation. The 1.2-mm emission was mapped using the SIMBA instrument on the 15-m Swedish ESO Submillimetre Telescope (SEST). Emission is detected toward all of the methanol maser and UC H II regions targeted, as well as towards 20 others lying within the fields mapped, implying that these objects are associated with cold, deeply embedded objects. Interestingly, there are also 20 methanol maser sites and nine UC H II regions within the fields mapped which are devoid of millimetre continuum emission. In addition to the maser and UC H II regions detected, we have also identified 253 other sources within the SIMBA maps. All of these (253) are new sources, detected solely from their millimetre continuum emission. These ‘mm-only’ cores are devoid of the traditional indicators of massive star formation, (i.e. methanol/OH maser, UC H II regions or IRAS point sources). At least 45 per cent of these mm-only cores are also without mid-infrared Mid-course Space Experiment (MSX) emission. The ‘mm-only’ core may be an entirely new class of source that represents an earlier stage in the evolution of massive stars, prior to the onset of methanol maser emission. Or, they may harbour protoclusters which do not contain any high-mass stars (i.e. below the H II region limit). In total, 404 sources are detected, representing four classes of sources which are distinguished by the presence of the different combination of associated tracer/s. Their masses, estimated assuming a dust temperature of 20 K and adopting kinematic distances, range from 0.5 × 10 1 to 3.7 × 10 4 M� , with an average mass for the sample of 1.5 × 10 3 M� . The H2 number density (nH2 )o fthe source sample ranges from 1.4 × 10 3 to 1.9 × 10 6 cm −3 , with an average of 8.7 × 10 4 cm −3 . The average radius of the sample is 0.5 pc. The visual extinction ranges from 10 to 500 mag with an average of 80 mag, which implies a high degree of embedding. The surface density (� )v aries from 0.2 to 18.0 kg m −2 with an average of 2.8 kg m −2 . Analysis of the millimetre-only sources shows that they are less massive ( ¯

Relative Evolutionary Timescale of Hot Molecular Cores with Respect to Ultracompact HiiRegions

Using the Owens Valley and Nobeyama Radio Observatory interferometers, we carried out an unbiased search for hot molecular cores and ultracompact (UC) H II regions toward the high-mass star-forming region G19.61-0.23. In addition, we performed 1.2 mm imaging with SIMBA and retrieved 3.5 and 2 cm images from the VLA archive database. The newly obtained 3 mm image brings information on a cluster of high-mass (proto)stars located in the innermost and densest part of the parsec-scale clump detected in the 1.2 mm continuum. We identify a total of 10 high-mass young stellar objects: one hot core (HC) and nine UC H II regions, whose physical parameters are obtained from model fits to their continuum spectra. The ratio between the current and expected final radii of the UC H II regions ranges from 0.3 to 0.9, which leaves the possibility that all O-B stars formed simultaneously. Under the opposite assumption, namely, that star formation occurred randomly, we estimate that the HC lifetime is less than ~1/3 of that of UC H II regions on the basis of the source number ratio between them.

Objects earlier than precursors of UC HII regions: Inflow–signpost for a common way of star formation

Searching for objects in the earliest phases of star formation, e.g sources at the beginning of a gravitational collapse, are essential to our understanding of massive star formation. Today a number of precursors of ultra compact HII regions (PUCHs) have been found. Embedded in dense gas and dust, these PUCHs have a high bolometric luminosity but little or no 6 cm radio continuum emission (Molinari, et al. 2000; Beuther, et al. 2002). Evidence for Collapse was found in ultra compact (UC) HII regions and 12 water maser sources (Zhang, et al. 1998; Wu & Evans II 2003). This paper presents the identification of massive cores with no detectable infrared and radio sources. These kinds of cores usually have strong sub-mm emission. A special case is the SCUBA core JCMT 18354-0649S which has both infall and outflow motions as indicated by the profiles of high excitation molecular lines. This core is at a stage earlier than PUCHs. Blue profiles are also found in UC HII region, which indicates that material is still infalling in this phase. Our observations suggest that infall exists in different evolutionary stages for high mass star formation, similar to the low mass cases.

X-ray studies of massive star birth regions

I report X-ray features of young stage of intermediate and high mass stars. The giant molecular cloud Sagittarius B2 (Sgr B2) exhibits more than dozen X-ray sources, two are associated with the ultra compact (UC) HII complex, Sgr B2 Main. The sources show large absorption of $\gg$ 10 23 Hcm −2 , the largest among any known stellar X-ray sources. The Arches cluster exhibits 3 extremely bright (a few $\times10^{33}$ ergs s −1 ) X-ray sources associated with the infrared (IR) massive stars. The X-ray spectra have 1-3 keV temperature in thin thermal model. Elongated diffuse 6.4 keV line emission is found. The Monoceros R2 cloud exhibits half dozen X-ray sources associated with young high-mass IR stars. They show rapid time variability and a thin thermal spectrum of ${\sim }2$ keV temperature. Among 28 ASC A pointing on intermediate-mass pre-main-sequence stars, or Herbig Ae/Be stars (HAeBes), eleven are found to be plausible X-ray sources. The general X-ray properties of these HAeBes are; (1) the plasma temperature and time variability are higher than those of high mass main-sequence stars, and more similar to low mass stars; (2) the X-ray luminosities come to the upper end of low mass pre-main sequence stars, or in some cases exceed that; (3) the X-ray activity of HAeBe decreases at the age of about a few×10 6 years. Using above observational facts, I propose a unified picture of X-ray activity of young stars in the wide mass range.

Near and mid infrared observations of ultracompact HII regions

This review discusses near- and mid- infrared observations of Ultracompact (UC) HII regions. The importance of ISO mid-IR fine-structure nebular lines is emphasised, since only a small fraction of UCHII regions are observed directly in the near-IR. The reliability of contemporary atmospheric models for such indirect diagnostics is discussed, whilst a revised spectral type-temperature calibration is presented for Galactic O3 to B3 dwarfs. In particular, fine-structure line derived properties of G29.96–0.02 differ from the direct near-IR spectroscopic result and represents a serious discrepancy which needs to be addressed. Mid-IR MSX and Spitzer imaging permits the identification of those UCHII regions for which far-IR IRAS fluxes are reliable, relevant to the single versus cluster nature of individual sources. High spatial resolution imaging with ground-based 8m telescopes allows more direct tests, as recently applied to G70.29+1.60. Finally, recent Spitzer mid-IR observations of Giant HII regions are briefly discussed.

The Morphologies of Ultracompact H ii Regions in W49A and Sagittarius B2: The Prevalence of Shells and a Modified Classification Scheme

We have used Very Large Array (VLA) observations of the massive star-forming regions W49A and Sgr B2, obtained with resolutions from 20 to 004, to classify the morphologies of nearly 100 ultracompact (UC) H II regions. These high-resolution, multifrequency, multiconfiguration VLA observations motivate us to make several modifications of the existing morphological classification scheme for UC H II regions. In this work, we describe the modified morphology scheme and the criteria used in source classification. In particular, we drop the core-halo classification, add a bipolar classification, and change the shell classification to shell-like. We tally the percentage of each morphology found in the Sgr B2 and W49A regions and find broad agreement with the Galactic plane surveys in the distribution of morphologies for most types. However, we find that nearly a third of the sources in these regions are shell-like, which is a higher percentage by nearly a factor of 10 than found in the surveys of Galactic plane star-forming regions by Wood & Churchwell and Kurtz et al. This difference may be due to physical differences in the environments of these two extreme star-forming regions. Alternatively, differences in observational technique may be responsible.

High-mass star formation within the bright-rimmed cloud SFO 79

We report Radio Recombination Line (RRL) and continuum observations toward the IRAS point source 16362−4845, embedded within the Bright-Rimmed Cloud (BRC) SFO 79, a small molecular cloud lying at the edge of the HII region RCW 108. High resolution observations of the H92α hydrogen recombination line and of the continuum emission (3.6 and 6 cm) confirm the presence of a resolved Ultra Compact (UC) HII region embedded within the molecular cloud. The integrated radio fluxes suggest the source of the ionisation to be an O9 Zero Age Main Sequence (ZAMS) star. Millimetre observations of 12 CO, 13 CO and C 18 O( J = 1-0) molecular lines reveal the presence of a molecular condensation offset ∼30 �� to the north of the IRAS position on the boundary of the UC HII region. Analysis of 2MASS data has led to the identification of a small IR cluster of Young Stellar Objects (YSOs) that are positionally coincident with the UC HII region, lying to the south east of the peak of the radio emission. Moreover, the UC HII region appears to be extended in the direction of the IR cluster, which suggests that the radio emission and the IR cluster are in some way related to each other. MSX 8.3 µm and 21.3 µ mi mages have been used to trace the large scale structure of the BRC, revealing the presence of a Photo Dominated Region (PDR) and three embedded thermal sources within the molecular cloud. The PDR has a plane parallel morphology which correlates extremely well with the morphology of the ionised gas traced by the optical emission. The three thermal sources (labelled A, B, C) all lie at a similar projected distance from the interface between the HII region and the molecular gas of the cloud. Thermal sources A and C are positionally coincident with the IRAS point sources 16362-4845 and 16362-4841 respectively, both of which have IRAS colours consistent with the presence of UC HII regions. Given that UC HII regions are relatively short lived (∼10 5 yrs) it is reasonable to suggest that these two UC HII regions are of a similar age. The alignment of the three thermal sources along a line parallel to the bright rim suggests that they could have been triggered by the propagation of a plane parallel shock through the cloud.

Australia Telescope Compact Array Survey of Candidate Ultracompact and Buried HiiRegions in the Magellanic Clouds

We present a systematic survey for ultracompact (UC) H II regions in the Magellanic Clouds. Understanding the physics of massive star formation (MSF) is a critical astrophysical problem. The study of MSF began in our Galaxy with surveys of UC H II regions, but before now this has not been done for other galaxies. We selected candidates on the basis of their Infrared Astronomical Satellite (IRAS) colors and imaged them at 3 and 6 cm with the Australia Telescope Compact Array. Nearly all of the observed regions contain compact radio sources consistent with thermal emission. Many of the sources are related to optically visible H II regions, and often the radio emission traces the youngest and densest part of the H II region. The luminosity function and number distribution of Lyman continuum fluxes of the compact radio sources are consistent with standard stellar and cluster initial mass functions. This type of systematic assessment of IRAS diagnostics is important for interpreting Spitzer Space Telescope data, which will probe similar physical scales in nearby galaxies as IRAS did in the Magellanic Clouds.

Ultracompact Binaries as Bright X-Ray Sources in Elliptical Galaxies

Chandra observations of distant (D ~ 10 Mpc) elliptical galaxies have revealed large numbers of low-mass X-ray binaries (LMXBs) accreting at > 10-8 M☉ yr-1. The majority of these LMXBs reside in globular clusters (GCs), and it has been suggested that many of the field LMXBs also originated in GCs. We show here that ultracompact (UC) binaries with orbital periods of 8-10 minutes and He or C/O donors of 0.06-0.08 M☉ naturally provide the observed from gravitational radiation losses alone. Such systems are predicted to be formed in the dense GC environment, a hypothesis supported by the 11.4 minute binary 4U 1820-30, the brightest persistent LMXB in a Galactic GC. These binaries have short enough lifetimes ( 1038 ergs s-1) that we calculate their luminosity function under a steady state approximation. This yields a luminosity function slope in agreement with that observed for 6 × 1037 ergs s-1 < L < 5 × 1038 ergs s-1, encouraging us to use the observed numbers of LMXBs per GC mass to calculate the accumulated number of UC binaries. For a constant birthrate over 8 Gyr, the number of UC binaries that have evolved through this bright phase is ~4000 in a 107 M☉ GC, consistent with dynamical interaction calculations. Perhaps most importantly, if all UC binaries become millisecond radio pulsars, then the observed normalization agrees with the inferred number of millisecond radio pulsars in 47 Tuc and Galactic GCs in general.

Broad Radio Recombination Lines from Hypercompact HiiRegions

ABSTRACTThe H92 recombination line was observed toward six massive star formation regions (MSFRs), and theH76 line was observed toward one MSFR. All seven MSFRs were suspected of harboring hypercompact (HC)H ii regions. The goal was to detect broad-line sources and to investigate their properties. The sources wereselected according to their small sizes, high brightness temperatures, and rising continuum spectra (typicalspectral index þ 1, S / ) at centimeter wavelengths. Two of the HC Hii candidates, G25.5+0.2 and NGC7538 (IRS 1), were previously known to have extremely broad lines (line widths of 160 and 180 km s 1 ,respectively). Sixteen separate, compact, radio continuum components were detected, fourteen of which weredetected in either the H92 or H76 line. Eight sources have line widths (FWHMs) greater than 40 km s 1 ;typical ultracompact (UC) H ii region line widths are 25–30 km s 1 . These broad lines may be produced by acombination of thermal, turbulent, and electron impact broadening, and large-scale motions (rotation, expansion,jets, shocks, inflow, disk, etc.). On the basis of one line and a relatively low spatial resolution, we are unable todetermine the relative contributions from each mechanism. All the MSFRs in the current sample are composed oftwo or more continuum components. The large projected separations between the continuum components withina given MSFR indicate that they are unlikely to be gravitationally bound massive protostars. Possible origins ofthe observed intermediate-sloped power-law spectral energy distributions (SEDs) are discussed. It is suggestedthat hierarchal clumping in HC H ii regions may produce the observed power-law SEDs.Subject headings: H ii regions — radio lines: ISM — stars: formation1. INTRODUCTIONThe formation and earliest evolution of massive stars is oneof astrophysics’ least understood problems. The observationalrecord is far too incomplete to piece together a coherent theoryof massive star formation and subsequent evolution. Theconventional model assumes that ultracompact (UC) H iiregions represent the earliest manifestation of massive starsafter forming via rapid accretion of ambient gas onto a pro-tostellar hydrostatic core. Norberg & Maeder (2000) andBehrend & Maeder (2001) proposed a ‘‘growing accretionrate’’ scenario, in which the accretion rate increases as themass of the protostar increases. However, the hypothesis offormation via accretion for massive stars (M > 8 10 M )hasbeen questioned by Bonnell et al. (1998), who have proposedan alternative formation mechanism based on mergers oflower mass protostars in dense young clusters. The accretionhypothesis predicts infall of molecular gas to form an equa-torial, Keplerian, accretion disk accompanied by bipolar out-flows along the protostar’s spin axis. Massive bipolar outflowshave been observed toward numerous massive star formationregions (MSFRs; Shepherd & Churchwell 1996; Ridge 2000;Beuther et al. 2002, and references therein). However, thereexist only a few massive protostars with candidate accretiondisks (see Garay & Lizano 1999; Churchwell 2002; Shepherdet al. 2002, and references therein). The small number ofsources is not surprising, because the observations are difficultwith current telescopes, and the phase of rapid accretion islikelytobeshortlived(<10

Fornax compact object survey FCOS: On the nature of Ultra Compact Dwarf galaxies

The results of the Fornax Compact Object Survey (FCOS) are presented, complementing the first part of the FCOS described in Mieske et al. ([CITE]). The FCOS aims at investigating the nature of the Ultra Compact Dwarf galaxies (UCDs) recently discovered in the center of the Fornax cluster (Drinkwater et al. [CITE]). 280 unresolved objects with colours mag in the magnitude space covering UCDs and bright globular clusters ( mag) were observed spectroscopically. 54 new Fornax members were found, plus five of the seven already known UCDs. Their distribution in radial velocity, colour, magnitude and space was investigated. It is found that bright compact objects ( or mag), including the UCDs, have a higher mean velocity than faint compact objects ( mag) at 96% confidence. The mean velocity of the bright compact objects is consistent with that of the dwarf galaxy population in Fornax, but inconsistent with that of NGC 1399's globular cluster (GC) system at 93.5% confidence. The compact objects follow a colour magnitude relation with a slope very similar to that of normal dEs, but shifted about 0.2 mag redwards. The magnitude distribution of compact objects shows a fluent transition between UCDs and GCs. A slight overpopulation of objects for mag is detected with respect to the extrapolation of NGC 1399's GC luminosity function. The spatial distribution of bright compact objects is in comparison to the faint ones more extended at 88% confidence. All our findings are consistent with the threshing scenario (Bekki et al. [CITE]), suggesting that a substantial fraction of compact Fornax members brighter than mag could be created by threshing dE,Ns. Fainter than mag, the majority of the objects seem to be genuine GCs. Our results are also consistent with merged stellar super-clusters (Fellhauer & Kroupa [CITE]) as an alternative explanation for the bright compact objects.

Detection of Reactive Ions in the Ultracompact H ii Regions Monoceros R2 and G29.96-0.02

We report the first detection of the reactive ions CO+ and HOC+ toward ultracompact (UC) H II regions, particularly in Mon R2 and G29.96-0.02. We have observed two positions in Mon R2, namely, the peak of the UC H II region and the surrounding high-density molecular cloud. CO+ and HOC+ were detected at the UC H II region but not at the molecular cloud, as expected if both ions arise in the photodissociation region (PDR) surrounding the UC H II region. The measured column densities are on the order of 1011 cm-2 in both sources, which yields a strikingly low [HCO+]/[HOC+] abundance ratio of 460 in Mon R2. These values are similar to those found in some other well-known PDRs, like NGC 7023 and the Orion bar. We briefly discuss the chemical implications of these results.

Molecular Counterparts of Ultracompact HiiRegions with Extended Envelopes

We have carried out 13CO J = 1-0, CS, and C34S J = 2-1 and J = 3-2 line observations of molecular clouds associated with 16 ultracompact (UC) H II regions with extended envelopes. The molecular clouds are the ones that give birth to rich stellar clusters and/or very massive (O7-O4) stars. Our data show that the clouds are very clumpy and of irregular morphology. They usually have much larger masses, velocity dispersions, and fractions of dense gas than molecular clouds that form early B or late O stars. This is compatible with earlier findings that more massive stars form in more massive cores. The IR luminosity-to-mass ratio has a mean value of 9 L☉/M☉ and is little correlated with the cloud mass. Most molecular clouds have star formation efficiencies of 1%-2%. We find size-line width and size-density relations in the forms of Δv ∝ D0.4 and n(H2) ∝ D-1.2. 13CO cores are in general associated with compact H II regions regardless of the presence of UC H II regions therein. In contrast, CS cores are preferentially associated with compact H II regions that contain UC H II regions. As with the fact that the compact H II regions containing UC H II regions are more compact than those not associated with UC H II regions, these indicate that the former may be in an earlier evolutionary phase than the latter. The diffuse extended envelopes of H II regions often develop in the direction of decreasing molecular gas density. Based on detailed comparison of molecular line data with radio continuum and recombination line data, the extended ionized envelopes are likely the results of champagne flows in at least 10 sources in our sample. Together these results appear to support a published suggestion that the extended emission around UC H II regions can be naturally understood by combining the champagne flow model with the hierarchical structure of molecular clouds, taking into account various inclinations and low resolutions of our data. In addition, the blister model seems to be still applicable to most H II regions, even though massive stars usually form in the interiors rather than on the surfaces of molecular clouds. This is possible because massive star-forming clouds have hierarchical structure and irregular morphology.

Interferometric observations of FeO towards Sagittarius B2

We have used the Nobeyama Millimeter Array (NMA) to carry out aperture synthesis observations of the J = 5−4 ground state rotational transition of FeO molecule at 153.135273 GHz towards the galactic center H region Sagittarius B2 Main (Sgr B2 M). We confirm the detection of this line in absorption with the IRAM 30-m telescope by Walmsley et al. (2002). Due to the higher angular resolution (6. �� 5 × 3. �� 0) of our NMA observations, we were able to show that the absorption has a broader line width and a deeper apparent optical depth toward the central 9. �� 2 × 8. �� 0 area around the ultra-compact (UC) H regions. This suggests a higher column density of FeO towards the UCH regions associated with Sgr B2 M than along adjacent lines of sight sampled with the IRAM 30-m telescope. Our results will be a crucial step toward understanding not only the chemistry of iron-bearing species in the interstellar medium, but also the degree of depletion of heavy elements.

Resolution of Distance Ambiguities of Inner Galaxy Massive Star Formation Regions. I.

Fifty-four ultracompact (UC) H ii regions in the GLIMPSE survey region (jbj < 1 � and 30 � < l < 70 � ) were observed in H2CO and H110� using the 305 m Arecibo telescope. By analyzing H2CO absorption against the UC H ii region continuum emission, we resolve the distance ambiguity toward 44 sources. This determination is critical to measure global physical properties of UC H ii regions (e.g., luminosity, size, mass) and properties of the Galaxy (e.g., spiral structure, abundance gradients). We find that the distribution of UC H ii regions in this survey is consistent with a ‘‘ local spur,’’ the Perseus, Sagittarius, and Scutum arms as delineated by Taylor & Cordes. However, departures from model velocities produce distance uncertainties only slightly smaller than the proposed arm separations. Subject headings: Galaxy: disk — radio lines: general — stars: formation On-line material: machine-readable tables

A High Spectral Resolution VLBI Study of the 12 GHz Methanol Masers in W3(OH): Their Submilliarcsecond Structure and Clues on Saturation

This paper reports on the results of high velocity resolution (0.02 km s-1) VLBA observations of the 12 GHz methanol masers toward the ultracompact (UC) H II region W3(OH). About 50 maser features are detected, and their emission is resolved both in velocity and in space (with an angular resolution of ≈1 mas). The maser feature spectra are very well reproduced by single Gaussian profiles, and a narrow distribution for the FWHM line widths is found from 0.14 to 0.32 km s-1. The measured brightness temperatures distribute over the range 1010 to 2 × 1012 K. By performing elliptical Gaussian fits, the spot parameters (peak position, intensity, and FWHM sizes) in the high velocity resolution maps were derived. The highest brightness temperatures occur in correspondence with the smallest spots, with FWHM size ≤4 AU. For all the maser features, it is found that the channel peak positions vary smoothly with the velocity, delineating straight or arclike distributions projected onto the sky. In most cases, the channel (line-of-sight) velocities vary linearly with the peak positional shifts, with measured gradients in the range 0.05-0.65 km s-1 mas-1 (0.02-0.30 km s-1 AU-1). Most of the maser spot spectra have very small deviations from a Gaussian profile, ≤ 2 × 10-3, and a trend for decreasing deviations with increasing intensities is noted. In most cases, the kurtosis of the spectral profiles is positive, with the most accurate values concentrated in the range 0.1-0.5. Comparison of these results with recent calculations of the deviations from a Gaussian profile that occur across the line profile of a linear maser (as a result of the amplification process) suggests that the 12 GHz CH3OH masers are below the threshold, I0, at which line rebroadening occurs. However, the model predicts deviations from a Gaussian profile significantly smaller than the observed values, and this discrepancy needs to be investigated. In particular, the calculations of the deviations from a Gaussian profile should take into account the effects of a velocity gradient through the maser features (evidenced by our observations) that will influence the spectral profile. The maximum output given by the pump rate of saturated masers may account for the observational result that the highest intensities are found in correspondence with the smallest sky-projected spot areas.

Dwarf Galaxies in Clusters

Dwarf and Low Surface Brightness (LSB) galaxies are the most numerous type of galaxies in the local Universe. However ubiquitous they are, their properties remain widely unexplored. Dwarf galaxies range from Ultra Compact Objects (UCO), Dwarf Ellipticals (dE), Dwarf Spheroidals (dSph) to Dwarf Irregulars (dIrr) and are generally of low surface brightness. In this paper we focus on the relation between these objects and galaxy clusters. The discussion is centered on three main issues, the faint end of the cluster luminosity function, segregation of dwarfs in clusters and the dwarf galaxy – cluster dynamical state connection. A further analysis of the population and clustering properties of dwarf galaxies in clusters require better statistics. While concluding, we present, as well, future prospects on how to measure distances to dwarfs using the surface brightness fluctuation method so as to increase the number of confirmed dwarf member galaxies in more distant clusters.

Ultra-Compact HII Regions and Massive Star Formation

▪ Abstract This review discusses three main topics: evolution to the ultra-compact (UC) state; the properties of UC HII regions and their environments; and UC HII regions as probes of Galactic structure. The evolution to UC HII regions begins in giant molecular clouds that provide the natal material for prestellar cores that evolve into hot cores, the precursors of UC HII regions. The properties of each evolutionary phase are reviewed, with particular emphasis on those of hot cores. The observed properties of UC HII regions and their environments are summarized with emphasis on the physical processes that may produce the observed properties. The final section summarizes the use of UC HII regions as probes of Galactic structure: in particular, the Galactic population and distribution of newly formed massive stars, the location of spiral arms, and the average galactocentric temperature and abundance gradients.

The structure of molecular clumps around high-mass young stellar objects

We have used the IRAM 30-m and FCRAO 14-m telescopes to observe the molecular clumps associated with 12 ultracompact (UC) HII regions in the J=6-5, 8-7 and 13-12 rotational transitions of methyl-acetylene (CH3C2H). Under the assumption of LTE and optically thin emission, we have derived temperature estimates ranging from 30 to 56 K. We estimate that the clumps have diameters of 0.2-1.6 pc, H_2 densities of 10^5-10^6 {cm^{-3}}, and masses of 10^2-2 10^4 M_\odot. We compare these values with those obtained by other authors from different molecular tracers and find that the H_2 density and the temperature inside the clumps vary respectively like n_{H_2} ~ R^{-2.6} and T ~ R^{-0.5}, with R distance from the centre. We also find that the virial masses of the clumps are ~3 times less than those derived from the CH3C2H column densities: we show that a plausible explanation is that magnetic fields play an important role to stabilise the clumps, which are on the verge of gravitational collapse. Finally, we show that the CH3C2H line width increases for decreasing distance from the clump centre: this effect is consistent with infall in the inner regions of the clumps. We conclude that the clumps around UC HII regions are likely to be transient (~10^(5) yr) entities, remnants of isothermal spheres currently undergoing gravitational collapse: the high mass accretion rates (~10^{-2} M_\odot yr^{-1}) lead to massive star formation at the centre of such clumps.