Dynamical Expansion of Ionization and Dissociation Front around a Massive Star. II. On the Generality of Triggered Star Formation

We analyze the dynamical expansion of the HII region, photodissociation region, and the swept-up shell, solving the UV- and FUV-radiative transfer, the thermal and chemical processes in the time-dependent hydrodynamics code. Following our previous paper, we investigate the time evolutions with various ambient number densities and central stars. Our calculations show that basic evolution is qualitatively similar among our models with different parameters. The molecular gas is finally accumulated in the shell, and the gravitational fragmentation of the shell is generally expected. The quantitative differences among models are well understood with analytic scaling relations. The detailed physical and chemical structure of the shell is mainly determined by the incident FUV flux and the column density of the shell, which also follow the scaling relations. The time of shell-fragmentation, and the mass of the gathered molecular gas are sensitive tothe ambient number density. In the case of the lower number density, the shell-fragmentation occurs over a longer timescale, and the accumulated molecular gas is more massive. The variations with different central stars are more moderate. The time of the shell-fragmentation differs by a factor of several with the various stars of M_* = 12-101 M_sun. According to our numerical results, we conclude that the expanding HII region should be an efficient trigger for star formation in molecular clouds if the mass of the ambient molecular material is large enough.Comment: 49 pages, including 17 figures ; Accepted for publication in Ap

Revealing the obscured supernova remnant Kes 32 with Chandra

I report here on the analysis and interpretation of a Chandra observation of the supernova remnant Kes 32. Kes 32 is rather weak in X-rays due to a large interstellar absorption, which is found to be ~4E22 cm^-2, larger than previously reported. Spectral analysis indicates that the ionization age of this object is very young, with n_e t ~ 4E9 cm^-3s, and a temperature of kT_e ~ 1 keV. The X-ray emission peaks at a smaller radius than in the radio. The low ionization age suggests that Kes 32 is a young remnant. However, a young age is in contradiction with the relatively large apparent size, which indicates an age of several thousand years, instead of a few hundred years. This problem is discussed in connection with Kes 32's unknown distance and its possible association with the Norma galactic arm.Comment: Accepted for publication in the Astrophysical Journal. 7 pages, 7 figure

Simple Models for Turbulent Self-Regulation in Galaxy Disks

We propose that turbulent heating, wave pressure and gas exchanges between different regions of disks play a dominant role in determining the preferred, quasi-equilibrium, self-similar states of gas disks on large-scales. We present simple families of analytic, thermohydrodynamic models for these global states, which include terms for turbulent pressure and Reynolds stresses. Star formation rates, phase balances, and hydrodynamic forces are all tightly coupled and balanced. The models have stratified radial flows, with the cold gas slowly flowing inward in the midplane of the disk, and with the warm/hot phases that surround the midplane flowing outward. The models suggest a number of results that are in accord with observation, as well as some novel predictions, including the following. 1) The large-scale gas density and thermal phase distributions in galaxy disks can be explained as the result of turbulent heating and spatial couplings. 2) The turbulent pressures and stresses that drive radial outflows in the warm gas also allow a reduced circular velocity there. This effect was observed by Swaters, Sancisi and van der Hulst in NGC 891, a particularly turbulent edge-on disk. The models predict that the effect should be universal in such disks. 3) They suggest that a star formation rate like the phenomenological Schmidt Law is the natural result of global thermohydrodynamical balance, and may not obtain in disks far from equilibrium. (Abridged)Comment: 37 pages, 1 gif figure, accepted for publication in the Astrophysical Journa

Superbubble evolution including the star-forming clouds: Is it possible to reconcile LMC observations with model predictions?

Here we present a possible solution to the apparent discrepancy between the observed properties of LMC bubbles and the standard, constant density bubble model. A two-dimensional model of a wind-driven bubble expanding from a flattened giant molecular cloud is examined. We conclude that the expansion velocities derived from spherically symmetric models are not always applicable to elongated young bubbles seen almost face-on due to the LMC orientation. In addition, an observational test to differentiate between spherical and elongated bubbles seen face-on is discussed.Comment: 25 pages, 7 figures, accepted to ApJ (September, 1999 issue

The LBT Panoramic View on the Recent Star-Formation Activity in IC2574

We present deep imaging of the star-forming dwarf galaxy IC2574 in the M81 group taken with the Large Binocular Telescope in order to study in detail the recent star-formation history of this galaxy and to constrain the stellar feedback on its HI gas. We identify the star-forming areas in the galaxy by removing a smooth disk component from the optical images. We construct pixel-by-pixel maps of stellar age and stellar mass surface density in these regions by comparing their observed colors with simple stellar populations synthesized with STARBURST99. We find that an older burst occurred about 100 Myr ago within the inner 4 kpc and that a younger burst happened in the last 10 Myr mostly at galactocentric radii between 4 and 8 kpc. We analyze the stellar populations residing in the known HI holes of IC2574. Our results indicate that, even at the remarkable photometric depth of the LBT data, there is no clear one-to-one association between the observed HI holes and the most recent bursts of star formation in IC2574. The stellar populations formed during the younger burst are usually located at the periphery of the HI holes and are seen to be younger than the holes dynamical age. The kinetic energy of the holes expansion is found to be on average 10% of the total stellar energy released by the stellar winds and supernova explosions of the young stellar populations within the holes. With the help of control apertures distributed across the galaxy we estimate that the kinetic energy stored in the HI gas in the form of its local velocity dispersion is about 35% of the total stellar energy.Comment: 16 pages, 14 figures, accepted for publication in Ap

A Test of the Standard Hypothesis for the Origin of the HI Holes in Holmberg II

The nearby irregular galaxy Holmberg II has been extensively mapped in HI using the Very Large Array (VLA), revealing intricate structure in its interstellar gas component (Puche et al. 1992). An analysis of these structures shows the neutral gas to contain a number of expanding HI holes. The formation of the HI holes has been attributed to multiple supernova events occurring within wind-blown shells around young, massive star clusters, with as many as 10-200 supernovae required to produce many of the holes. From the sizes and expansion velocities of the holes, Puche et al. assigned ages of ~10^7 to 10^8 years. If the supernova scenario for the formation of the HI holes is correct, it implies the existence of star clusters with a substantial population of late-B, A and F main sequence stars at the centers of the holes. Many of these clusters should be detectable in deep ground-based CCD images of the galaxy. In order to test the supernova hypothesis for the formation of the HI holes, we have obtained and analyzed deep broad-band BVR and narrow-band H-alpha images of Ho II. We compare the optical and HI data and search for evidence of the expected star clusters in and around the HI holes. We also use the HI data to constrain models of the expected remnant stellar population. We show that in several of the holes the observed upper limits for the remnant cluster brightness are strongly inconsistent with the SNe hypothesis described in Puche et al. Moreover, many of the HI holes are located in regions of very low optical surface brightness which show no indication of recent star formation. Here we present our findings and explore possible alternative explanations for the existence of the HI holes in Ho II, including the suggestion that some of the holes were produced by Gamma-ray burst events.Comment: 30 pages, including 6 tables and 3 images. To appear in Astron. Journal (June 1999

Identifying Gamma-Ray Burst Remnants in Nearby Galaxies

We study the spectral signatures arising from cooling and recombination of an interstellar medium whose equilibrium state has been altered over \sim 100 pc by the radiation of a Gamma-Ray Burst (GRB) and its afterglow. We identify signatures in the line diagnostics which are indicative of a photo-ionized GRB remnant which is \la 5 x 10^4 years old . We estimate that at least a few such remnants should be detectable in the Virgo cluster of galaxies. If the gamma-ray emission from GRBs is beamed to a fraction f_b of their sky, then the expected number of Virgo remnants is larger by a factor of f_b^{-1}. Virgo remnants can be resolved with arcsecond imaging, and are likely to be center-filled using narrow-band filters of high ionization lines (such as [O III] \lambda5007 or He II \lambda4686), and limb-brightened for low-ionization lines (such as [S II] \lambda6717). The non-relativistic blast wave might be visible separately, since it does not reach the outer edge of these young photo-ionized remnants. The remnants should show evidence for ionization cones if the prompt or afterglow UV emission from GRBs is beamed.Comment: 21 pages, 10 figures, submitted to Ap

The true nature of the alleged planetary nebula W16-185

We report the discovery of a small cluster of massive stars embedded in a NIR nebula in the direction of the IRAS15411-5352 point source, which is related to the alleged planetary nebula W16-185. The majority of the stars present large NIR excess characteristic of young stellar objects and have bright counterparts in the Spitzer IRAC images; the most luminous star (IRS1) is the NIR counterpart of the IRAS source. We found very strong unresolved Brgamma emission at the IRS1 position and more diluted and extended emission across the continuum nebula. From the sizes and electron volume densities we concluded that they represent ultra-compact and compact HII regions, respectively. Comparing the Brgamma emission with the 7 mm free-free emission, we estimated that the visual extinction ranges between 14 and 20 mag. We found that only one star (IRS1) can provide the number of UV photons necessary to ionize the nebula.Comment: 30 pages, 15 figures, 2 tables V3: minor grammatical changes. Figure 4 is available in pdf file. Accepted for publication in AJ, April / 200

Genic population structure and gene flow in the Northern Flicker (Colaptes Auratus) hybrid zone

The Yellow-shafted Flicker (Colaptes auratus auratus) and Red-shafted Flicker (C. a. cafer) form a stable, narrow hybrid zone on the western Great Plains of North America. Allozyme data were obtained from 31 structural gene loci for 33 samples representing 246 Northern Flickers from throughout the Great Plains. Flickers were approximately equivalent to other birds in terms of proportion of polymorphic loci (P = 0.207) and average heterozygosity (H = 0.056). There was no concordant variation between plumage characters and allelic frequencies. Gene-diversity analysis indicated that 92.5% of the genic variation occurred as within-deme heterozygosity (GD = 0.925), approximately 7% occurred among individual demes (GST= 0.07), and only 0.9% occurred among major river drainages (GST = 0.009). Even less diversity was found among parental and hybrid groups (GST = 0.002). There is substantial allozymic structuring of the Northern Flicker species population, but the structuring is not associated with the hybrid zone, and there is, at most, very weak structuring into riparian zones of habitat. The electrophoretic data support the inference that gene flow among Northern Flicker populations is high (Nm = 1.9-4.4/generation). If the high gene-flow estimates are correct, then geographical selection gradients would be the most likely mechanism maintaining the narrow hybrid zone of plumage and morphometric traits

Two Large HI Shells in the Outer Galaxy near l=279 degrees

As part of a survey of HI 21-cm emission in the Southern Milky Way, we have detected two large shells in the interstellar neutral hydrogen near l=279 deg. The center velocities are +36 and +59 km/s, which puts the shells at kinematic distances of 7 and 10 kpc. The larger shell is about 610 pc in diameter and very empty, with density contrast of at least 15 between the middle and the shell walls. It has expansion velocity of about 20 km/s and swept up mass of several million solar masses. The energy indicated by the expansion may be as high as 2.4 X 10^53 ergs. We estimate its age to be 15 to 20 million years. The smaller shell has diameter of about 400 pc, expansion velocity about 10 km/s and swept up mass of about 10^6 solar masses. Morphologically both regions appear to be shells, with high density regions mostly surrounding the voids, although the first appears to have channels of low density which connect with the halo above and below the HI layer. They lie on the edge of the Carina arm, which suggests that they may be expanding horizontally into the interarm region as well as vertically out of the disk. If this interpretation is correct, this is the first detection of an HI chimney which has blown out of both sides of the disk.Comment: 21 pages, 14 jpeg figures, accepted for publication in A