## Monday 11/05/12

 Title: Tearing the Veil: interaction of the Orion Nebula with its neutral environment Authors: van der Werf, Paul P.; Goss, W. M.; O’Dell, C. R. Publication: eprint arXiv:1211.0470 Publication Date: 11/2012 Origin: ARXIV Keywords: Astrophysics – Galaxy Astrophysics Comment: accepted for publication in the Astrophysical Journal; version with full-resolution figures can be downloaded from http://www.strw.leidenuniv.nl/~pvdwerf/Orion/Orion.pdf Bibliographic Code: 2012arXiv1211.0470V

### Abstract

We present HI 21cm observations of the Orion Nebula, obtained with the Karl G. Jansky Very Large Array, at an angular resolution of 7.2"x5.7" and a velocity resolution of 0.77 km/s. Our data reveal HI absorption towards the radio continuum of the HII region, and HI emission arising from the Orion Bar photon-dominated region (PDR) and from the Orion-KL outflow. In the Orion Bar PDR, the HI signal peaks in the same layer as the H2 near-infrared vibrational line emission, in agreement with models of the photodissociation of H2. The gas temperature in this region is approximately 540K, and the HI abundance in the interclump gas in the PDR is 5-10% of the available hydrogen nuclei. Most of the gas in this region therefore remains molecular. Mechanical feedback on the Veil manifests itself through the interaction of ionized flow systems in the Orion Nebula, in particular the Herbig-Haro object HH202, with the Veil. These interactions give rise to prominent blueward velocity shifts of the gas in the Veil. The unambiguous evidence for interaction of this flow system with the Veil shows that the distance between the Veil and the Trapezium stars needs to be revised downwards to about 0.4pc. The depth of the ionized cavity is about 0.7pc, which is much smaller than the depth and the lateral extent of the Veil. Our results reaffirm the blister model for the M42 HII region, while also revealing its relation to the neutral environment on a larger scale.

 Title: A multi-wavelength view of the Galactic center dust ridge reveals little star formation Authors: Immer, K.; Menten, K. M.; Schuller, F.; Lis, D. C. Publication: eprint arXiv:1211.0455 Publication Date: 11/2012 Origin: ARXIV Keywords: Astrophysics – Galaxy Astrophysics Comment: 9 pages, 5 figures, accepted for publication in A&A Bibliographic Code: 2012arXiv1211.0455I

### Abstract

The Galactic center dust ridge consists of a narrow string of massive condensations identified in submillimeter dust continuum emission. To determine whether new high-mass stars are forming in this region, we performed new observations at 870 $\mu$m with the Atacama Pathfinder Experiment telescope and at 8.4 GHz with the Very Large Array. We complement our data with recent maser and mid-infrared results. The ridge’s clouds are dark at mid-infrared wavelengths, indicating the presence of cold, high column density material. In combination with existing temperature measurements in the dust ridge, we determined the masses of the largest clouds. The results show that the dust ridge contains a very massive reservoir of molecular material. We find five radio sources at 8.4 GHz in the general dust ridge vicinity but outside of the dust ridge clouds, which are probably all excited by massive young stars, whose properties we constrain. Our observations exclude the existence of zero age main sequence stars with spectral types earlier than B0.5 within the dust ridge clouds. The only indication of ongoing high-mass star formation inside the clouds are class II methanol masers that are found in two of the clouds. Except for a weak water maser, found in previous observations, no signs of star formation are detected in the massive cloud M0.25+0.012.