The southern dust pillars of the Carina Nebula

We present preliminary results from a detailed study towards four previously detected bright mid-infrared sources in the southern part of the Carina Nebula: G287.73--0.92, G287.84--0.82, G287.93--0.99 and G288.07--0.80. All of these sources are located at the heads of giant dust pillars that point towards the nearby massive star cluster, Trumpler 16. It is unclear if these pillars are the prime sites for a new generation of triggered star formation or if instead they are the only remaining parts of the nebula where ongoing star fromation can take place.Comment: 2 pages, to appear in the proceedings of "Hot Star Workshop III: The Earliest Phases of Massive Star Birth" (ed. P.A. Crowther

First-principles phase diagram calculations for the HfC–TiC, ZrC–TiC, and HfC–ZrC solid solutions

We report first-principles phase diagram calculations for the binary systems HfC–TiC, TiC–ZrC, and HfC–ZrC. Formation energies for superstructures of various bulk compositions were computed with a plane-wave pseudopotential method. They in turn were used as a basis for fitting cluster expansion Hamiltonians, both with and without approximations for excess vibrational free energies. Significant miscibility gaps are predicted for the systems TiC–ZrC and HfC–TiC, with consolute temperatures in excess of 2000 K. The HfC–ZrC system is predicted to be completely miscibile down to 185 K. Reductions in consolute temperature due to excess vibrational free energy are estimated to be ~7%, ~20%, and ~0%, for HfC–TiC, TiC–ZrC, and HfC–ZrC, respectively. Predicted miscibility gaps are symmetric for HfC–ZrC, almost symmetric for HfC–TiC and asymmetric for TiC–ZrC

Mapping warm molecular hydrogen with Spitzer's Infrared Array Camera (IRAC)

Photometric maps, obtained with Spitzer's Infrared Array Camera (IRAC), can provide a valuable probe of warm molecular hydrogen within the interstellar medium. IRAC maps of the supernova remnant IC443, extracted from the Spitzer archive, are strikingly similar to spectral line maps of the H2 pure rotational transitions that we obtained with the Infrared Spectrograph (IRS) instrument on Spitzer. IRS spectroscopy indicates that IRAC Bands 3 and 4 are indeed dominated by the H2 v=0-0 S(5) and S(7) transitions, respectively. Modeling of the H2 excitation suggests that Bands 1 and 2 are dominated by H2 v=1-0 O(5) and v=0-0 S(9). Large maps of the H2 emission in IC433, obtained with IRAC, show band ratios that are inconsistent with the presence of gas at a single temperature. The relative strengths of IRAC Bands 2, 3, and 4 are consistent with pure H2 emission from shocked material with a power-law distribution of gas temperatures. CO vibrational emissions do not contribute significantly to the observed Band 2 intensity. Assuming that the column density of H2 at temperatures T to T+dT is proportional to T raised to the power -b for temperatures up to 4000 K, we obtained a typical estimate of 4.5 for b. The power-law index, b, shows variations over the range 3 to 6 within the set of different sight-lines probed by the maps, with the majority of sight-lines showing b in the range 4 to 5. The observed power-law index is consistent with the predictions of simple models for paraboloidal bow shocks.Comment: 27 pages, including 11 figures. Accepted for publication in Ap

Rotational quenching rate coefficients for H_2 in collisions with H_2 from 2 to 10,000 K

Rate coefficients for rotational transitions in H_2 induced by H_2 impact are presented. Extensive quantum mechanical coupled-channel calculations based on a recently published (H_2)_2 potential energy surface were performed. The potential energy surface used here is presumed to be more reliable than surfaces used in previous work. Rotational transition cross sections with initial levels J <= 8 were computed for collision energies ranging between 0.0001 and 2.5 eV, and the corresponding rate coefficients were calculated for the temperature range 2 < T <10,000 K. In general, agreement with earlier calculations, which were limited to 100-6000 K, is good though discrepancies are found at the lowest and highest temperatures. Low-density-limit cooling functions due to para- and ortho-H_2 collisions are obtained from the collisional rate coefficients. Implications of the new results for non-thermal H_2 rotational distributions in molecular regions are also investigated

The molecular environment of massive star forming cores associated with Class II methanol maser emission

Methanol maser emission has proven to be an excellent signpost of regions undergoing massive star formation (MSF). To investigate their role as an evolutionary tracer, we have recently completed a large observing program with the ATCA to derive the dynamical and physical properties of molecular/ionised gas towards a sample of MSF regions traced by 6.7 GHz methanol maser emission. We find that the molecular gas in many of these regions breaks up into multiple sub-clumps which we separate into groups based on their association with/without methanol maser and cm continuum emission. The temperature and dynamic state of the molecular gas is markedly different between the groups. Based on these differences, we attempt to assess the evolutionary state of the cores in the groups and thus investigate the role of class II methanol masers as a tracer of MSF.Comment: 5 pages, 1 figure, IAU Symposium 242 Conference Proceeding

Infrared images of reflection nebulae and Orion's bar: Fluorescent molecular hydrogen and the 3.3 micron feature

Images were obtained of the (fluorescent) molecular hydrogen 1-0 S(1) line, and of the 3.3 micron emission feature, in Orion's Bar and three reflection nebulae. The emission from these species appears to come from the same spatial locations in all sources observed. This suggests that the 3.3 micron feature is excited by the same energetic UV-photons which cause the molecular hydrogen to fluoresce

Multiple Molecular H2 Outflows in AFGL 618

We report high spatial (0.5 arcsec) and high spectral (9 km/s) resolution spectro-imaging of the 2.12 micron H2 1-0 S(1) line in the proto-planetary nebula AFGL 618 using BEAR at the CFHT. The observations reveal the presence of multiple, high-velocity, molecular outflows that align with the remarkable optical jets seen in HST images. The structure and kinematics of the outflows show how jets interact with circumstellar gas and shape the environment in which planetary nebulae form.Comment: 14 pages, 5 figures. To appear in The Astrophysical Journal Letter