A New Model for the Spiral Structure of the Galaxy. Superposition of 2+4-armed patterns

We investigate the possibility of describing the spiral pattern of the Milky Way in terms of a model of superposition 2- and 4-armed wave harmonics (the simplest description, besides pure modes). Two complementary methods are used: a study of stellar kinematics, and direct tracing of positions of spiral arms. In the first method, the parameters of the galactic rotation curve and the free parameters of the spiral density waves were obtained from Cepheid kinematics, under different assumptions. To turn visible the structure corresponding to these models, we computed the evolution of an ensemble of N-particles, simulating the ISM clouds, in the perturbed galactic gravitational field. In the second method, we present a new analysis of the longitude-velocity (l-v) diagram of the sample of galactic HII regions, converting positions of spiral arms in the galactic plane into locii of these arms in the l-v diagram. Both methods indicate that the ``self-sustained'' model, in which the 2-armed and 4-armed mode have different pitch angles (6 arcdeg and 12 arcdeg, respectively) is a good description of the disk structure. An important conclusion is that the Sun happens to be practically at the corotation circle. As an additional result of our study, we propose an independent test for localization of the corotation circle in a spiral galaxy: a gap in the radial distribution of interstellar gas has to be observed in the corotation region.Comment: 17 pages, 9 figures, Latex, uses aas2pp4.st

New Herbig-Haro Objects and Giant Outflows in Orion

We present the results of a photographic and CCD imaging survey for Herbig-Haro (HH) objects in the L1630 and L1641 giant molecular clouds in Orion. The new HH flows were initially identified from a deep H-alpha film from the recently commissioned AAO/UKST H-alpha Survey of the southern sky. Our scanned H-alpha and broad band R images highlight both the improved resolution of the H-alpha survey and the excellent contrast of the H-alpha flux with respect to the broad band R. Comparative IVN survey images allow us to distinguish between emission and reflection nebulosity. Our CCD H-alpha, [SII], continuum and I band images confirm the presence of a parsec-scale HH flow associated with the Ori I-2 cometary globule and several parsec-scale strings of HH emission centred on the L1641-N infrared cluster. Several smaller outflows display one-sided jets. Our results indicate that for declinations south of -6 degrees in L1641, parsec-scale flows appear to be the major force in the large-scale movement of optical dust and molecular gas.Comment: 14 pages, Latex using MN style, 21 figures in JPEG format. Higher resolution figures available from S.L. Mader. Accepted by MNRAS. Email contact for higher resolution images: [email protected]