Shocked molecular hydrogen towards the Tornado nebula

1997

The physics of shocked outflows in molecular clouds is one of the fundamental astrophysical processes by which the cycle of s tar formation in our Galaxy is regulated. I outline the basis of our understanding of the star formation process and the violent o u t­ flow always associated with it, the physics of shocks in molecular gas, and the consequent excitation of molecular hydrogen (H2). It is demonstrated th a t molecular hydrogen is the best observational diagnostic of this hot, shocked molecular gas and an introduc­ tion is given to the observational techniques of near-infrared spectroscopy required in its measurement. I describe a detailed observational study of the physics of shocked H2 excitation and dynamics in the nearby massive star forming region of the Orion giant molecular cloud, the brightest source of its type, using the recently upgraded CGS4 near-IR spectrometer at UKIRT. We have demonstrated tha t integrated [Fell] 1.644/im line profiles in the Orion '‘bul­ let...

Arxiv preprint astro-ph/ …, 2000

We report the results of Hubble Space Telescope NICMOS and WFPC2 imaging of emission-line nebulae in the central galaxies of three clusters of galaxies purported to host massive cooling flows, Perseus (NGC1275), Abell 2597, and PKS0745-191. The spectral signature of vibrationally-excited molecular hydrogen has been seen in every galaxy searched thus far that is central to a cluster cooling flow with an optical emission line nebula. With the exquisite spatial resolution available to us with the Hubble Space Telescope, we have discovered that the vibrationally-excited molecular hydrogen gas extends several kpc from the centers of Abell 2597 and PKS0745-191, while the vibrationally-excited molecular hydrogen in NGC1275 appears to be mostly confined to its nucleus, with some extended emission < 1 kpc from the center. The molecular hydrogen in Abell 2597 and PKS0745-191 seems to be nearly co-spatial with the optical emission-line filaments in those systems. There may be a tiny jet visible in the 1.6 µm image of PKS0745-191. We also find significant dust absorption features in the 1.6 µm images of all three systems. The dust lanes are not strictly co-spatial with the emission-line filaments, but are aligned with and perhaps intermingled with them. The morphology of the emission-line systems suggests that the presence of vibrationally-excited -2molecular hydrogen is not purely an AGN-related property of cluster "coolingflow" nebulae, and that the optical and infrared emission-line gas, that is, the ionized and vibrationally-excited molecular gas have similar origins, if not also similar energy sources. The infrared molecular hydrogen lines are much too bright to be generated by gas simply cooling from a cooling flow; furthermore, the gas, because it is dusty, likely did not condense from the hot intracluster medium (ICM). We examine some candidates for heating the nebulae, including X-ray irradiation by the ICM, UV fluorescence by young stars, and shocks. UV heating by young stars provides the most satisfactory explanation for the H 2 emission in A2597; X-ray irradiation is energetically unlikely and strong shocks (v > ∼ 40 km s −1 ) are ruled out by the high H 2 /Hα ratios. If UV heating is the main energy input, a few billion solar masses of molecular gas is present in A2597 and PKS0745-191. UV irradiation models predict a significant amount of 1.0 − 2.0 micron emission line from higher excitation H 2 transitions and moderate far infrared luminosities (∼ 10 44 h −2 erg s −1 ) for A2597 and PKS0745-191. Even in the context of UV fluorescence models, the total amount of H 2 gas and star formation inferred from these observations is too small to account for the cooling flow rates and longevities inferred from X-ray observations. We note an interesting new constraint on cooling flow models: the radio sources do not provide a significant amount of shock heating, and therefore they cannot counterbalance the cooling of the X-ray gas in the cores of these clusters.

Astronomy and Astrophysics, 2002

We present high-resolution images obtained with the WFPC2, on board the HST, of the protoplanetary nebula (PPN) OH 231.8+4.2. Hα and NII line emission and scattered light in the continuum at 6750 and 7910Å were observed. We also discuss NIR NICMOS images from the HST archive. The images show with high accuracy the shape and excitation state of the shocks developed in the nebula. Our high-resolution images (and data from other works) allow a very detailed and quantitative description of the different nebular components and of the physical conditions in them. We interpret specific structures identified in our images using existing models of shock interaction. In the center of the nebula, there is a dense torus-or disk-like condensation continued by an hourglass-like structure, with relatively high densities (∼10 5-10 6 cm −3) and temperatures (∼30 K). Inside this torus we have identified the location of the central star, from SiO maser observations. Two shock regions are detected from the optical line emission images, respectively in the north and south lobes. In both regions, a forward and a backward shock are identified. The densities of this hot gas vary between 40 and 250 cm −3 , with the densest clumps being placed in the reverse shocks. The total mass of the shocked hot gas is ∼2×10 −3 M , both lobes showing similar masses in spite of their different extents. The relatively collimated jet that impinges on an originally slow shell, so producing the shocks, is identified from the scattered light images and in CO maps. This flow is significantly denser and cooler than the shocked Hα regions. Its density decreases with the distance to the star, with typical values ∼10 5-10 4 cm −3 , and its temperature ranges between about 25 and 8 K. We explain the high Hα emission of the backward shock assuming that it propagates in a diffuse gas component, entrained by the observed collimated flow and sharing its axial movement. The existence of shocks also in the collimated densest flow is suggested by the high abundance of some molecules like HCO + and its structure and kinematics in certain regions, but they are not seen in Hα emission, probably because of the absence of (well developed) hot components in this dense flow. We think that the exceptionally detailed and quantitative image derived for the wind interaction regions in OH 231.8+4.2 is a challenge to check and improve hydrodynamical models of wind interaction in PPNe.

The Astronomical Journal, 1998

Narrowband, (1È0) S(1) images of six luminous outÑow regions are presented and discussed. In Ðve H 2 of these regions, W75 N, S140 N, NGC 7538, AFGL 5180, and AFGL 490, shock features associated H 2 with molecular (CO) outÑows are observed. We have discovered faint, though extensive, bow shocks in the W75 N outÑow that indicate a total Ñow length of at least 3 pc. The Herbig-Haro knots that make up the HH 251È254 outÑow in S140 N are also observed in in addition, knots in the counterÑow H 2 ; are discovered. Copious emission is also observed throughout the NGC 7538 region ; Ðlamentary H 2 structures to the northeast of the central cluster (IRS 1) are probably photodissociation fronts, although a jetlike structure is observed associated with the IRS 9 CO outÑow. In AFGL 5180, a new, collimated jet is discovered to the east of the central cluster, and numerous knots and Ðlaments are observed H 2 around the cluster itself that could be associated with the known CO outÑow there. Last, line emis-H 2 sion is observed southwest of AFGL 490 ; in particular, a bright peak is found associated with a warm molecular clump in the CO outÑow.

1998

We used the University of New South Wales Infrared Fabry-Perot (UNSWIRF) to investigate the photodissociation region (PDR) associated with the`elephant trunk' features in the M16 H ii region (the Eagle nebula). Images were made in the H 2 1±0 S(1) and 2±1 S(1) lines at 2.122 and 2.248 mm, respectively, and in the H i Br g line at 2.166 mm. The trunk-like features have an average H 2 number density of ,10 4 cm À3 and are irradiated by a far-UV ®eld ,10 4´t he ambient interstellar value. The H 2 intensity pro®le across the trunks is consistent with a simple model in which cylindrical columns of gas are illuminated externally, primarily by a direct component (the stars of NGC 6611), with an additional contribution from an isotropic component (scattered light). We ®nd that most of the H 2 emission from the source is consistent with purely¯uorescent excitation, however a signi®cant fraction of the H 2 emission (,25 per cent) from the northernmost column shows evidence for`collisional¯uorescence', i.e. redistribution of H 2 level populations through collisions. This emission is con®ned to clumps up to ,0.01 pc in diameter, with densities >10 5 cm À3 , and perhaps > 10 6 cm À3 , ®lling at most a few per cent of the volume of the trunks. The line intensities and ratios are consistent with steady-state and not time-dependent PDR models.