Outflows in Regions of Star Formation

The high spatial and spectral resolution offered by the new generation of infrared spectrometers at ESO is optimally suited for the observational study of outflows from young stellar objects. Models of interstellar shock waves would benefit from observations of spectrally resolved line profiles. This applies also to attempts of measuring the rotation rates of jets very close to their driving source, which in general suffer considerable extinction. Observations of forbidden lines of ionised iron, [Fe II], could be used to accomplish this. The possibility of using rotational lines of molecular hydrogen, H2, to study the temporal evolution of outflow and disk gas is discussed. Similarly, high resolution IR observations of fluorescent water lines, H2O, open up the possibility to access outflow and disk water.Comment: 12 pages, 3 Postscript figures, uses cropmark.sty, physprbb.sty, sprmindx.sty, subeqnar.sty, svmult.cls to appear in: High Resolution Infrared Spectroscopy in Astronomy, H.U. Kaufl, R. Siebebnorgen & A. Moorwood (eds.), Garching, Germany, 18-21 November 2003, ESO Astrophysics Symposia, Springe

The ISO-LWS map of the Serpens cloud core. II. The line spectra

We present spectrophotometric ISO imaging with the LWS and the CAM-CVF of the Serpens molecular cloud core. The LWS map is centred on the far infrared and submillimetre source SMM1 and its size is 8' x 8'. The fine structure line emission in [OI] and [CII] is extended and can be successfully modelled to originate in a PDR with G_0 = 15 and n(H2) about 10^4 - 10^5 cm^-3. Extended emission is also observed in the rotational line emission of H2O and high-J CO. However, lack of sufficient angular resolution prevents us from excluding the possibility that the emssion regions of these lines are point like, which could be linked to the embedded objects SMM9 and SMM4. Toward the Class0 source SMM1, the LWS observations reveal, in addition to fine structure line emission, a rich spectrum of molecular lines. The sub-thermally excited and optically thick CO, H2O and OH lines are tracing an about 10^3 AU source with temperatures higher than 300 K and densities above 10^6 cm^-3. We show that geometry is of concern for the correct interpretation of the data and based on 2D-radiative transfer modelling of the disk/torus around SMM1, which successfully reproduces the entire observed SED and the observed line profiles of CO isotopomers, we can exclude the disk to be the source of the LWS-molecular line emission. The CAM-CVF permits us to see a region of rotational H2 emission. This H2 gas has a temperature of 10^3 K, which suggests that the heating of the gas is achieved through relatively slow shocks. Although we are not able to establish any firm conclusion regarding the detailed nature of the shock waves, our observations of the molecular line emission from SMM1 can be explainable in terms of an admixture of J-shocks and of C-shocks.Comment: 20 pages, 20 figures, accepted for publication in A&

Are Giant Planets Forming Around HR 4796A?

We have obtained FUSE and HST STIS spectra of HR 4796A, a nearby 8 Myr old main sequence star that possesses a dusty circumstellar disk whose inclination has been constrained from high resolution near-infrared observations to be ~17 deg from edge-on. We searched for circumstellar absorption in the ground states of C II at 1036.3 A, O I at 1039.2 A, Zn II at 2026.1 A, Lyman series H2, and CO (A-X) and failed to detect any of these species. We place upper limits on the column densities and infer upper limits on the gas masses assuming that the gas is in hydrostatic equilibrium, is well-mixed, and has a temperature, Tgas ~ 65 K. Our measurements suggest that this system possesses very little molecular gas. Therefore, we infer an upper limit for the gas:dust ratio (<4.0) assuming that the gas is atomic. We measure less gas in this system than is required to form the envelope of Jupiter.Comment: 10 pages, 3 figures (including 1 color figure), accepted for publication in Ap

Oxygen in dense interstellar gas - the oxygen abundance of the star forming core rho Oph A

Oxygen is the third most abundant element in the universe, but its chemistry in the interstellar medium is still not well understood. In order to critically examine the entire oxygen budget, we attempt here initially to estimate the abundance of atomic oxygen, O, in the only one region, where molecular oxygen, O2, has been detected to date. We analyse ISOCAM-CVF spectral image data toward rho Oph A to derive the temperatures and column densities of H2 at the locations of ISO-LWS observations of two [OI] 3P_J lines. The intensity ratios of the (J=1-2) 63um to (J=0-1) 145um lines largely exceed ten, attesting to the fact that these lines are optically thin. This is confirmed by radiative transfer calculations, making these lines suitable for abundance determinations. For that purpose, we calculate line strengths and compare them to the LWS observations. Excess [OI] emission is observed to be associated with the molecular outflow from VLA 1623. For this region, we determine the physical parameters, T and N(H2), from the CAM observations and the gas density, n(H2), is determined from the flux ratio of the [O I]63um and [O I]145um lines. For the oxygen abundance, our analysis leads to essentially three possibilities: (1) Extended low density gas with standard ISM O-abundance, (2) Compact high density gas with standard ISM O-abundance and (3) Extended high density gas with reduced oxygen abundance, [O/H] ~ 2E-5. As option (1) disregards valid [O I] 145um data, we do not find it very compelling; we favour option (3), as lower abundances are expected as a result of chemical cloud evolution, but we are not able to dismiss option (2) entirely. Observations at higher angular resolution than offered by the LWS are required to decide between these possibilities.Comment: Accepted for publication in A&

Multi-line detection of O_2 toward ρ Ophiuchi A

Context. Models of pure gas-phase chemistry in well-shielded regions of molecular clouds predict relatively high levels of molecular oxygen, O_2, and water, H_(2)O. These high abundances imply high cooling rates, leading to relatively short timescales for the evolution of gravitationally unstable dense cores, forming stars and planets. Contrary to expectations, the dedicated space missions SWAS and Odin typically found only very small amounts of water vapour and essentially no O_2 in the dense star-forming interstellar medium. Aims. Only toward ρOph   A did Odin detect a very weak line of O_2 at 119 GHz in a beam of size 10 arcmin. The line emission of related molecules changes on angular scales of the order of some tens of arcseconds, requiring a larger telescope aperture such as that of the Herschel Space Observatory to resolve the O2 emission and pinpoint its origin. Methods. We use the Heterodyne Instrument for the Far Infrared (HIFI) aboard Herschel to obtain high resolution O_2 spectra toward selected positions in the ρOph A   core. These data are analysed using standard techniques for O_2 excitation and compared to recent PDR-like chemical cloud models. Results. The N_J = 3_(3) − 1_(2) line at 487.2 GHz is clearly detected toward all three observed positions in the ρOph A  core. In addition, an oversampled map of the 5_(4)−3_(4) transition at 773.8 GHz reveals the detection of the line in only half of the observed area. On the basis of their ratios, the temperature of the O_2 emitting gas appears to vary quite substantially, with warm gas (≳ 50K) being adjacent to a much colder region, of temperatures lower than 30 K. Conclusions. The exploited models predict that the O_2 column densities are sensitive to the prevailing dust temperatures, but rather insensitive to the temperatures of the gas. In agreement with these models, the observationally determined O_2 column densities do not seem to depend strongly on the derived gas temperatures, but fall into the range N(O_2) = 3 to ≳ 6 × 10^(15) cm^(-2). Beam-averaged O2 abundances are about 5 × 10^(-8) relative to H_2. Combining the HIFI data with earlier Odin observations yields a source size at 119 GHz in the range of 4 to 5 arcmin, encompassing the entire ρOph A core. We speculate that one of the reasons for the generally very low detection rate of O2 is the short period of time during which O_2 molecules are reasonably abundant in molecular clouds

Enhanced OH in C-type shock waves in molecular clouds

Cosmic-ray and X-ray ionisations in molecular gas produce a weak far-ultraviolet flux through the radiative decay of H2 molecules that have been excited by collisions with energetic electrons (the Prasad-Tarafdar mechanism). I consider the effect of this dissociating flux on the oxygen chemistry in C-type shocks. Typically a few percent of the water molecules produced within the shock front are dissociated before the gas has cooled to 50K. The resulting column density of warm OH rises from 10^15 to 10^16 cm^-2 as the ionisation rate is increased from 10^-17 (typical of dark clouds) to 10^-15 s^-1 (adjacent to supernova remnants). These column densities produce substantial emission in the far-infrared rotational transitions of OH, and are consistent with the OH/H2O ratios inferred from ISO observations of emission from molecular shocks. For high ionisation rates the column of warm OH is sufficient to explain the OH(1720 MHz) masers that occur where molecular clouds are being shocked by supernova remnants. The predicted abundance of OH throughout the shock front will enable C-type shocks to be examined with high spectral resolution through radio observations of the four hyperfine ground state transitions of OH at 18cm and heterodyne measurements of emission in the FIR (e.g. from SOFIA)Comment: 5 pp incl 3 figs, LaTeX, uses emulateapj.sty; ApJ Letters in press. Revised 2nd paragraph of discussio

Detection of interstellar hydrogen peroxide

The molecular species hydrogen peroxide, HOOH, is likely to be a key ingredient in the oxygen and water chemistry in the interstellar medium. Our aim with this investigation is to determine how abundant HOOH is in the cloud core {\rho} Oph A. By observing several transitions of HOOH in the (sub)millimeter regime we seek to identify the molecule and also to determine the excitation conditions through a multilevel excitation analysis. We have detected three spectral lines toward the SM1 position of {\rho} Oph A at velocity-corrected frequencies that coincide very closely with those measured from laboratory spectroscopy of HOOH. A fourth line was detected at the 4{\sigma} level. We also found through mapping observations that the HOOH emission extends (about 0.05 pc) over the densest part of the {\rho} Oph A cloud core. We derive an abundance of HOOH relative to that of H_2 in the SM1 core of about 1\times10^(-10). To our knowledge, this is the first reported detection of HOOH in the interstellar medium.Comment: 5 pages, 4 figures, accepted for publication in Astronomy & Astrophysics, new version corrects a typo in Table 1 (and consequently in Fig 4

The 1.2 Millimeter Image of the beta Pictoris Disk

We present millimeter imaging observations in the 1200 micron continuum of the disk around beta Pictoris. With the 25 arcsec beam, the beta Pic disk is unresolved perpendicularly to the disk plane (< 10 arcsec), but slightly resolved in the northeast-southwest direction (26 arcsec). Peak emission is observed at the stellar position. A secondary maximum is found 1000 AU along the disk plane in the southwest, which does not positionally coincide with a similar feature reported earlier at 850 micron. Arguments are presented which could be seen in support of the reality of these features. The observed submm/mm emission is consistent with thermal emission from dust grains, which are significantly larger than those generally found in the interstellar medium, including mm-size particles, and thus more reminiscent of the dust observed in protostellar disks. Modelling the observed scattered light in the visible and the emission in the submm/mm provides evidence for the particles dominating the scattering in the visible/NIR and those primarily responsible for the thermal emission at longer wavelengths belonging to different populations.Comment: 6 pages, 3 postscript figures, accepted for publication in Astronomy and Astrophysic

Odin detection of O2

We present the detection of molecular oxygen with Odin toward the dense molecular core rho Oph A, which is part of a region of active star formation. The observed spectral line is the (N,J = 1,1-1,0) ground state transition of molecular oxygen at 119 GHz (2.5 mm wavelength). The center of the line is at the LSR velocity of a number of optically thin lines from other species in the region and the O2 line also has a very similar, narrow, line width. Within the 10 arcmin beam, the integrated line intensity is 28 mK km/s, which corresponds to 5 sigma of the rms noise. A standard LTE analysis results in an O2 abundance of 5E(-8), with an uncertainty of at least a factor of two. We show that standard methods, however, do not apply in this case, as the coupling of the Odin beam to the source structure needs to be accounted for. Preliminary model results indicate O2 abundances to be higher by one order of magnitude than suggested by the standard case. This model predicts the 487 GHz line of O2 to be easily detectable by the future Herschel-HIFI facility, but to be out of reach for observations on a shorter time scale with the Odin space observatory.Comment: 8 pages, 4 Postscript figures (3 in colour), uses iaus.cl

O18O and C18O observations of rho Oph A

Observations of the (N_J=1_1-1_0) ground state transition of O_2 with the Odin satellite resulted in a about 5 sigma detection toward the dense core rho Oph A. At the frequency of the line, 119 GHz, the Odin telescope has a beam width of 10', larger than the size of the dense core, so that the precise nature of the emitting source and its exact location and extent are unknown. The current investigation is intended to remedy this. Telluric absorption makes ground based O_2 observations essentially impossible and observations had to be done from space. mm-wave telescopes on space platforms were necessarily small, which resulted in large, several arcminutes wide, beam patterns. Although the Earth's atmosphere is entirely opaque to low-lying O_2 transitions, it allows ground based observations of the much rarer O18O in favourable conditions and at much higher angular resolution with larger telescopes. In addition, rho Oph A exhibits both multiple radial velocity systems and considerable velocity gradients. Extensive mapping of the region in the proxy C18O (J=3-2) line can be expected to help identify the O_2 source on the basis of its line shape and Doppler velocity. Line opacities were determined from observations of optically thin 13C18O (J=3-2) at selected positions. During several observing periods, two C18O intensity maxima in rho Oph A were searched for in the 16O18O (2_1-0_1) line at 234 GHz with the 12m APEX telescope. Our observations resulted in an upper limit on the integrated O18O intensity of < 0.01 K km/s (3 sigma) into the 26.5" beam. We conclude that the source of observed O_2 emission is most likely confined to the central regions of the rho Oph A cloud. In this limited area, implied O_2 abundances could thus be higher than previously reported, by up to two orders of magnitude.Comment: 7 pages, 6 figures (5 colour), Astronomy & Astrophysic