Small HII Region Research Articles

Outflow-confined H ii Regions. II. The Early Break-out Phase

Abstract In this series of papers, we model the formation and evolution of the photoionized region and its observational signatures during massive star formation. Here, we focus on the early breakout of the photoionized region into the outflow cavity. Using results of 3D magnetohydrodynamic-outflow simulations and protostellar evolution calculations, we perform a post-processing radiative transfer. The photoionized region first appears at a protostellar mass of in our fiducial model and is confined to within 10–100 au by the dense inner outflow, which is similar to some of the observed very small hypercompact H ii regions. Since the ionizing luminosity of the massive protostar increases dramatically as the Kelvin–Helmholtz (KH) contraction proceeds, the photoionized region breaks out to the entire outflow region in ≲10,000 year. Accordingly, the radio free–free emission brightens significantly in this stage. In our fiducial model, the radio luminosity at 10 GHz changes from at to at , while the infrared luminosity increases by less than a factor of two. The radio spectral index also changes in the break-out phase from the optically thick value of ∼2 to the partially optically thin value of ∼0.6. Additionally, we demonstrate that short-timescale variation in the free–free flux would be induced by an accretion burst. The outflow density is enhanced in the accretion burst phase, which leads to a smaller ionized region and weaker free–free emission. The radio luminosity may decrease by one order of magnitude during such bursts, while the infrared luminosity is much less affected because internal protostellar luminosity dominates over accretion luminosity after the KH contraction starts. Such a variability may be observable on timescales as short 10–100 year if accretion bursts are driven by disk instabilities.

Distinguishing between HII regions and planetary nebulae with Hi-GAL, WISE, MIPSGAL, and GLIMPSE

HII regions and planetary nebulae (PNe) both emit at radio and infrared (IR) wavelengths, and angularly small HII regions can be mistaken for PNe. This problem of classification is most severe for HII regions in an early evolutionary stage, those that are extremely distant, or those that are both young and distant. Using data from the Herschel Hi-GAL survey, as well as WISE and the Spitzer MIPSGAL and GLIMPSE surveys, we wish to establish characteristic IR colors that can be used to distinguish between HII regions and PNe. We perform aperture photometry measurements for a sample of 126 HII regions and 43 PNe at wavelengths from 8.0 micron to 500 micron. We find that HII regions and PNe have distinct IR colors. The most robust discriminating color criteria are [F12/F8] < 0.3, [F160/F12] > 1.3, and [F160/F24] > 0.8 (or alternately [F160/F22] > 0.8), where the brackets indicate the log of the flux ratio. All three of these criteria are individually satisfied by over 98% of our sample of HII regions and by about 10% of our sample of PNe. Combinations of these colors are more robust in separating the two populations; for example all HII regions and no PNe satisfy [F12/F8] < 0.4 and [F160/F22] > 0.8. When applied to objects of unknown classification, these criteria prove useful in separating the two populations. The dispersion in color is relatively small for HII regions; this suggests that any evolution in these colors with time for HII regions must be relatively modest. The spectral energy distributions (SEDs) of HII regions can be separated into "warm" and "cold" components. The "cold" component is well-fit by a grey-body of temperature 25K. The SEDs of nearly two-thirds of our sample of HII regions peak at 160 micron and one third peak at 70 micron. For PNe, 67% of the SEDs peak at 70 micron, 23% peak at either 22 micron or 24 micron, and 9% (two sources) peak at 160 micron.

MOLECULAR CLOUDS IN SUPERSHELLS: A CASE STUDY OF THREE OBJECTS IN THE WALLS OF GSH 287+04-17 AND GSH 277+00+36

We present an in-depth case study of three molecular clouds associated with the walls of the Galactic supershells GSH 287+04-17 and GSH 277+00+36. These clouds have been identified in previous work as examples in which molecular gas is either being formed or destroyed due to the influence of the shells. 12CO(J=1-0), 13CO(J=1-0) and C18O(J=1-0) mapping observations with the Mopra telescope provide detailed information on the distribution and properties of the molecular gas, enabling an improved discussion of its relationship to the wider environment in which it resides. We find that massive star formation is occurring in molecular gas likely formed in-situ in the shell wall, at a Galactic altitude of ~200 pc. This second-generation star formation activity is dominating its local environment; driving the expansion of a small HII region which is blistering out of the atomic shell wall. We also find new morphological evidence of disruption in two smaller entrained molecular clouds thought to pre-date the shells. We suggest that at the present post-interaction epoch, the lifetime of this surviving molecular material is no longer strongly determined by the shells themselves.

THE SECOND SURVEY OF THE MOLECULAR CLOUDS IN THE LARGE MAGELLANIC CLOUD BY NANTEN. II. STAR FORMATION

We studied star formation activities in the molecular clouds in the Large Magellanic Cloud. We have utilized the second catalog of 272 molecular clouds obtained by NANTEN to compare the cloud distribution with signatures of massive star formation including stellar clusters, and optical and radio HII regions. We find that the molecular clouds are classified into three types according to the activities of massive star formation; Type I shows no signature of massive star formation, Type II is associated with relatively small HII region(s) and Type III with both HII region(s) and young stellar cluster(s). The radio continuum sources were used to confirm that Type I GMCs do not host optically hidden HII regions. These signatures of massive star formation show a good spatial correlation with the molecular clouds in a sense they are located within ~100 pc of the molecular clouds. Among possible ideas to explain the GMC Types, we favor that the Types indicate an evolutionary sequence; i.e., the youngest phase is Type I, followed by Type II and the last phase is Type III, where the most active star formation takes place leading to cloud dispersal. The number of the three types of GMCs should be proportional to the time scale of each evolutionary stage if a steady state of massive star and cluster formation is a good approximation. By adopting the time scale of the youngest stellar clusters, 10 Myrs, we roughly estimate the timescales of Types I, II and III to be 6 Myrs, 13 Myrs and 7 Myrs, respectively, corresponding to a lifetime of 20-30 Myrs for the GMCs with a mass above the completeness limit, 5 x 10^4 Msun.

ON THE ORIGIN OF THE HII REGIONS ASSOCIATED WITH MASSIVE AND COMPACT SUPERSTAR CLUSTERS

This contribution to the IV Korea-Mexico meeting deals with the hydrodynamics of the matter reinserted within super star clusters (SSCs) by both stellar winds and supernova explosions, results recently printed in The Astrophysical Journal (Silich et al. 2007). The motivation of such a project arose from the persistent presence of the small mass and compact HII regions that sit right on top of many massive and compact SSCs, from which one expects a large mechanical energy power. The data used for our calculations appear only recently (see Smith et al. 2006) for the massive and compact SSC M82-A1. We presented in our paper the calculated flow, derived through analytical and semi-analytical methods, which led to almost identical results. We have found out that the only way of accommodating a compact HII region (4.5 pc in radius, in the case of M82-A1) on top of a 6.3 Myr old and massive (> <TEX>$10^6M_{\bigodot}$</TEX>) SSC with a half light radius of 3 pc, requires of two assumptions: a very low heating efficiency (< 10%) within the cluster, what leads to a bimodal solution (see Tenorio-Tagle et al. 2007) and a high pressure in the surrounding medium.

The Formation of Massive Stars: Accretion, Disks, and the Development of Hypercompact HiiRegions

The hypothesis that massive stars form by accretion can be investigated by simple analytical calculations that describe the effect that the formation of a massive star has on its own accretion flow. Within a simple accretion model that includes angular momentum, that of gas flow on ballistic trajectories around a star, the increasing ionization of a massive star growing by accretion produces a three-stage evolutionary sequence. The ionization first forms a small quasi-spherical HII region gravitationally trapped within the accretion flow. At this stage the flow of ionized gas is entirely inward. As the ionization increases, the HII region transitions to a bipolar morphology in which the inflow is replaced by outflow within a narrow range of angle with about the bipolar axis. At higher rates of ionization, the opening angle of the outflow region progressively increases. Eventually, in the third stage, the accretion is confined to a thin region about an equatorial disk. Throughout this early evolution, the HII region is of hypercompact to ultracompact size depending on the mass of the enclosed star or stars. These small HII regions whose dynamics are dominated by stellar gravitation and accretion are different than compact and larger HII regions whose dynamics are dominated by the thermal pressure of the ionized gas.

Properties of the close binary and circumbinary torus of the Red Rectangle

New diffraction-limited speckle images of the Red Rectangle in the wavelength range 2.1-3.3 µm with angular resolutions of 44-68 mas (Tuthill et al. 2002) and previous speckle images at 0.7-2.2 µm (Osterbart et al. 1997; Men'shchikov et al. 1998) revealed well-resolved bright bipolar outflow lobes and long X-shaped spikes originating deep inside the outflow cavities. This set of high-resolution images stimulated us to reanalyze all infrared observations of the Red Rectangle using our two-dimensional radiative transfer code. The high-resolution images imply a geometrically and optically thick torus-like density distribution with bipolar conical cavities and are inconsistent with the flat disk geometry frequently used to visualize bipolar nebulae. The new detailed modeling, together with estimates of the interstellar extinction in the direction of the Red Rectangle enabled us to more accurately determine one of the key parameters, the distance D ≈ 710pc with model uncertainties of 70pc, which is twice as far as the commonly used estimate of 330pc. The central binary is surrounded by a compact, massive (M ≈ 1.2M⊙), very dense dusty torus with hydrogen densities reaching nH ≈ 2.5 ×10 12 cm −3 (dust-to-gas mass ratiod/� ≈ 0.01). The model implies that most of the dust mass in the dense torus is in very large particles and, on scales of more than an arcsecond, the polar outflow regions are denser than the surrounding medium. The bright component of the spectroscopic binary HD44179 is a post-AGB star with mass M⋆ ≈ 0.57M⊙, luminosity L⋆ ≈ 6000L⊙, and effective temperature T⋆ ≈ 7750K. Based on the orbital elements of the binary, we identify its invisible component with a helium white dwarf with MWD ≈ 0.35M⊙, LWD ∼ 100L⊙, and TWD ∼ 6 ×10 4 K. The hot white dwarf ionizes the low-density bipolar outflow cavities inside the dense torus, producing a small HII region observed at radio wavelengths. We propose an evolutionary scenario for the formation of the Red Rectangle nebula, in which the binary initially had 2.3 and 1.9M⊙ components at a separation of ∼ 130R⊙. The nebula was formed in the ejection of a common envelope after Roche lobe overflow by the present post-AGB star.

An Atlas of H(alpha) Emitting Regions in M33: A Systematic Search for SS433 Star Candidates

We report finding charts and accurate positions for 432 compact H-alpha emitting regions in the Local Group galaxy M 33 (NGC 598), in an effort to isolate candidates for an SS433-like stellar system. The objects were extracted from narrow band images, centered in the rest-frame H-alpha (lambda 6563 A) and in the red continuum at 6100 A. The atlas is complete down to V approximately equal to 20 and includes 279 compact HII regions and 153 line emitting point-like sources. The point-like sources undoubtedly include a variety of objects: very small HII regions, early type stars with intense stellar winds, and Wolf-Rayet stars, but should also contain objects with the characteristics of SS433. This extensive survey of compact H-alpha regions in M 33 is a first step towards the identification of peculiar stellar systems like SS433 in external galaxies.

High resolution fabry-perot and multi-pupil spectral observations of the genuine blue compact dwarf galaxy IZW18

We present the observations of the BCDG IZW18 performed with a Fabry-Perot interferometer at the CFH 3.6 m telescope and with a multi-pupil spectrograph at the SAO (Russia) 6 m telescope. The morphological structure of the galaxy in emission lines and in continuum, the velocity field of the ionized gas, and [OIII]/H β ratio distribution along the NW component have been investigated. Besides the NW and SE HII components, we find a population of small HII regions. Continuum maps show that the peaks of the stellar light distribution are displaced with respect to the emission lines maxima. the velocity field shows peculiar motions superposed on an approximately regular background implying solid body rotation. Emission line profiles exhibit an asymmetric structure, except for the NW compact component. The [OIII]/H β ratio decreases from the center of the NW component to its edge with the gradient of 1.86 kpc−1.

Stromgren Sphere in the Expanding Universe and a Possible Seed of Structure Formation

12 for typical parameters of the universe model. The electron temperature Te in the HII region is kept equal to the cosmic background radiation temperature Tr by the Compton process. After this epoch, the ionisation front expands continuously and the quasi-stationary Stromgren sphere is not formed. Te can increase above T, in this later stage. Transition feature is investigated in an analytic manner. In the case of the quasi-stationary Stromgren sphere, the HII region may generate a dense shell by the expansion due to pressure unbalance and a fragmentation of this shell could trigger further structure formation. This process occurs only for the small HII region.

The dust distribution in some smallHII regions

Based on the calibrated maps in the Hα andV bands, simple shell models for the distributions of ionized gas and dust are calculated for the smallHii regions S237 and S254–S257. In deriving the dust distribution from theV-band maps, it is assumed that scattering particles are made of dirty ice. The results of calculation show that a dust depletion zone should be placed in the central region of each of the observedHii regions. The formation of this dust depletion zone and the evolutionary state of theseHii regions are briefly discussed.

Radio observations of small Hii regions with identified exciting stars

The radio maps of ten small Hii regions at λλ=2.8, 6 and 11 cm, obtained with the 100-m MPIR radiotelescope, are given. A comparison of the radio fluxes obtained with parameters of exciting stars is carried out. For four compact Hii regions — namely, S146, S152, S237 and S297 — we present the results of observations with the radiotelescope of the U.S.S.R. Academy of Sciences, RATAN-600. The paper also presents estimates of the physical conditions in nebulae, and conclusions are drawn about the possible stage of their evolution.

Interacting Galaxies: The Kinematics of NGC 4038/39 and the HI Bridge Between M81 and NGC 3077

Computer simulations of galaxy encounters (Toomre 1974 and references therein) have strongly supported the idea that the luminous bridges and tails in many multiple galaxies are a result of gravitational interaction. The need for an observational test of the kinematics of these models has prompted observations of NGC 4038/39 (the “antennae”) in the 21 cm line of neutral hydrogen (HI) with the Westerbork Synthesis Radio Telescope (WSRT) at an angular resolution of 100″×150″ (α×δ). A detailed description of the observations and data reduction is given elsewhere (van der Hulst, 1977). Neutral hydrogen is definitely associated with the tails as was already indicated by the filled aperture observations of Huchtmeier and Bohnenstengel (1975) at the lower resolution of 8′.7. The distribution of HI column density is shown in Figure 1 superposed on a print of a plate obtained by F. Schweizer with the 4 m telescope at Cerro Tololo. Owing to the low declination of the object (−19°) the WSRT map is affected by several sidelobes; the main effect is an attenuation of extended east-west structures, for which reason the HI along the southern tail in Fig. 1 shows a gap. Computer experiments and a comparison with the filled aperture observations show that the WSRT data are entirely consistent with an HI distribution which is uniform along this tail and increases with about a factor 2 at the extreme end, where the tail is very blue and small HII regions are present (Schweizer, elsewhere in this volume). The tails contain 2.9 × 109M⊙of HI (assuming a distance of 20 Mpc), 85% of which is in the southern and more prominent tail. About 1.1 × 109M⊙of HI is associated with the galaxies which are not spatially resolved at the present angular resolution.