Scientific Results in Press

High-Mass Proto-Stellar Candidates - I : The Sample and Initial Results

T.K.Sridharan(¹), H. Beuther(²), P. Schilke(²), K.M. Menten(²) and F. Wyrowski(³)
(¹)Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, MS 78, Cambridge, MA 02138, USA, (²)Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany, (³)Department of Astronomy, University of Maryland, College Park, USA

Abstract:
We describe a systematic program aimed at identifying and characterizing candidate high-mass proto-stellar objects (HMPOs). Our candidate sample consists of 69 objects selected by criteria based on those established by Ramesh & Sridharan (1997) using far-infrared, radio-continuum and molecular line data. Infrared-Astronomical-Satellite (IRAS) and Midcourse-Space-Experiment (MSX) data were used to study the larger scale environments of the candidate sources and to determine their total luminosities and dust temperatures.

To derive the physical and chemical properties of our target regions, we observed continuum and spectral line radiation at millimeter and radio wavelengths. We imaged the free-free and dust continuum emission at wavelengths of 3.6 cm and 1.2 mm, respectively, searched for H₂O and CH₃OH maser emission and observed the CO J=2-1 and several NH₃ lines toward all sources in our sample. Other molecular tracers were observed in a subsample.

While dust continuum emission was detected in all sources, most of them show only weak or no emission at 3.6 cm. Where detected, the cm emission is frequently found to be offset from the mm emission, indicating that the free-free and dust emissions arise from different subsources possibly belonging to the same (proto)cluster. A comparison of the luminosities derived from the cm emission with bolometric luminosities calculated from the IRAS far-infrared fluxes shows that the cm emission very likely traces the most massive source, whereas the whole cluster contributes to the far-infrared luminosity. Estimates of the accretion luminosity indicate that a significant fraction of the bolometric luminosity is still due to accretion processes. The earliest stages of HMPO evolution we seek to identify are represented by dust cores without radio emission.

Line wings due to outflow activity are nearly omnipresent in the CO observations, and the molecular line data indicate the presence of hot cores for several sources, where the abundances of various molecular species are elevated due to evaporation of icy grain mantles. Kinetic gas temperatures of 40 sources are derived from NH₃(1,1) and (2,2) data, and we compare the results with the dust temperatures obtained from the IRAS data.

Comparing the amount of dust, and hence the gas, associated with the HMPOs and with ultracompact H II regions (UCH IIs) we find that the two types of sources are clearly separated in mass-luminosity diagrams: for the same dust masses the UCH II regions have higher bolometric luminosities than HMPOs. We suggest that this is an evolutionary trend with the HMPOs being younger and reprocessing less (stellar) radiation in the IR than the more evolved UCH IIs regions.

These results indicate that a substantial fraction of our sample harbors HMPOs in a pre-UCH II region phase, the earliest known stage in the high-mass star formation process.

Published in ApJ 566, 931

High-Mass Proto-Stellar Candidates - II: Density structure from dust continuum and CS emission

H. Beuther(¹), P. Schilke(¹), K.M. Menten(¹), F. Motte(²), T.K.Sridharan(³) and F. Wyrowski(⁴)
(¹)Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121, Germany, (²)California Institute of Technology, MS 320-47, Pasadena, CA 91125, USA, (³)Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, MS 78, Cambridge, MA 02138, USA, (⁴)Department of Astronomy, University of Maryland, College Park, USA

Abstract:
We present a detailed 1.2 mm continuum and CS spectral line study of a large sample of 69 massive star forming regions in very early stages of evolution, most of them prior to building up an ultracompact H II region. The continuum data show a zoo of different morphologies and give detailed information on the spatial distributions, the masses, column densities and average densities of the whole sample.

Fitting the radial intensity profiles shows that three parameters are needed to describe the spatial distribution of the sources: constant emission from the center out to a few arcsec radius followed by a first power law intensity distribution which steepens further outside into a second power law distribution. The inner flat region is possibly caused by fragmentation of the large scale cores into smaller sub-sources, whereas the steeper outer power law distributions indicate finite sizes of the cores.

Separating the sources into sub-samples suggests that in the earliest stages prior to the onset of massive star formation the intensity radial distributions are rather flat resembling the structure of intensity peaks in more quiescent molecular clouds. Then in the subsequent collapse and accretion phase the intensity distributions become centrally peaked with steep power law indices. In this evolutionary stage the sources show also the broadest C³⁴S linewidth. During the following phase, when ultracompact H II regions evolve, the intensity power law radial distributions flatten out again. This is probably caused by the ignited massive stars in the center which disrupt the surrounding cores.

The mean inner power law intensity index m_i ( $I \propto r^{- m_i}$) is 1.2 corresponding to density indices p ( $n \propto r^{-p}$) of the density indices of 1.6. In total the density distribution of our massive star formations sites seem to be not too different from their low-mass counterparts, but we show that setting tight constrains on the density indices is very difficult and subject to many possible errors.

The local densities we derive from CS calculations are higher (up to one order of magnitude) than the mean densities we find via the mm-continuum. Such inhomogeneous density distribution reflects most likely the ubiquitous phenomenon of clumping and fragmentation in molecular clouds. Linewidth-mass relations show a departure from virial equilibrium in the stages of strongly collapsing cores.

Published in ApJ, 566, 945

Massive molecular outflows

H. Beuther(¹) - P. Schilke(¹) - T.K. Sridharan(³) - K.M. Menten(¹) - C.M. Walmsley(³) - F. Wyrowski(^1,4)
(¹)Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany, (²)Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, MS 78, Cambridge, MA 02138, USA, (³)Osservatorio Astrofisico di Arcetri, Largo E. Fermi, 50125 Firenze, Italy, (⁴)Department of Astronomy, University of Maryland, College Park, USA

Abstract:
With the aim of understanding the role of massive outflows in high-mass star formation, we mapped in the ¹²CO (J=2-1) transition 26 high-mass star-forming regions at very early stages of their evolution. At a spatial resolution of $11\hbox{$^{\prime\prime}$ }$ bipolar molecular outflows were found in 21 of them. The other five sources show confusing morphology but strong line wings. This high detection rate of bipolar structure proves that outflows common in low-mass sources are also ubiquitous phenomena in the formation process of massive stars. The flows are large, very massive and energetic, and the data indicate stronger collimation than previously thought. The dynamical timescales of the flows correspond well to the free-fall timescales of the associated cores. Comparing with correlations known for low-mass flows, we find continuity up to the high-mass regime suggesting similar flow-formation scenarios for all masses and luminosities. Accretion rate estimates in the $10^4 L_{\odot}$ range are around $10^{-4} M_{\odot} yr^{-1}$, higher than required for low-mass star formation, but consistent with high-mass star formation scenarios. Additionally, we find a tight correlation between the outflow mass and the core mass over many orders of magnitude. The strong correlation between those two quantities implies that the product of the accretion efficiency $f_{acc} = \dot{M}_{acc}/(M_{core}/t_{ff})$ and f_r (the ratio between jet mass loss rate and accretion rate), which equals the ratio between jet and core mass ( f_acc f_r = M_jet/M_core), is roughly constant for all core masses. This again indicates that the flow-formation processes are similar over a large range of masses. Additionally, we estimate median f_r and f_acc values of approximately 0.2 and 0.01, respectively, which is consistent with current jet-entrainment models. To summarize, the analysis of the bipolar outflow data strongly supports current theories which explain massive star formation by scaled up, but otherwise similar physical processes - mainly accretion - to their low-mass counterparts.

Published in A&A, 383, 892

Design and characterization of 225-370 GHz DSB and 250-360 GHz SSB full height waveguide SIS mixers

A.Navarrini(¹), B.Lazareff(¹), D.Billon-Pierron(¹) and I.Peron(¹)
(¹)Institut de Radio Astronomie Millimétrique, 300 rue de la Piscine, Domaine Universitaire de Grenoble, 38406 Saint-Martin d'Hères, France

Abstract:
We describe the design, construction, and characterization of two SIS mixers: a DSB mixer for the band 275-370 GHz, intended for band 7 of the ALMA frontend, and a SSB mixer, backshort tuned, intended for IRAM's Plateau de Bure interferometer, and covering the band 260-360 GHz. These two mixers share various common design features, such as a wideband single ended probe transition from full height waveguide to microstrip, and they use the same mixer chip. A significant challenge, especially for the SSB mixer, has been to achieve not only low noise, but also stable operation over the design band. The receiver noise for the DSB mixer is found to be below 50 K over 100 GHz of RF bandwidth, with a minimum as low as 27 K (uncorrected) at 336 GHz. The SSB receiver has a measured image rejection of order -14 dB over the design band, and its noise remains below 80 K (effectively a SSB receiver noise value).

Appeared in the Proceedings of the Thirteenth International Symposium on Space Terahertz Technology, March 26-28, 2002, Cambridge, MA, USA.

FE model based interpretation of telescope temperature variations

Michael Bremer(¹) and Juan Peñalver(²)
(¹)Institut de Radio Astronomie Millimétrique (IRAM Grenoble), 300 rue de la Piscine, F-38406 St Martin d'Hères, France, (²)Instituto de Radioastronomía Milimétrica (IRAM Granada), Avenida Divina Pastora 7, Núcleo Central E-18012 Granada, Spain

Abstract:
At the IRAM 30m telescope on Pico Veleta, Spain, an extended net of temperature sensors has been installed in 1996 and their data recorded since. A finite element (FE) model of the antenna has been used to analyse these measurements and to refine the sensor network. Details on the optimum choice of sensor locations will be presented, and how their readings are interpolated onto the model grid. From the model, structural deformations are obtained and converted into observable telescope parameters. These parameters, like focus, pointing and large-scale surface deformations, will be used to upgrade the real-time instrumental performance and to provide the astronomer with data for an eventual correction of observations.

Accepted for the Proceedings of the Workshop on Integrated Modeling of Telescopes, 5-7 February 2002, Lund, Sweden. Preprints: bremer@iram.fr

The Crab Nebula at 1.3 mm. Evidence for a new synchrotron component

Bandiera, R.(¹); Neri, R.(²); Cesaroni, R.(¹)
(¹)Osservatorio Astrofisico di Arcetri, Largo E.Fermi 5, 50125 Firenze, Italy, (²)IRAM, 300 rue de la Piscine, 38406 St-Martin-d'Hères, France

Abstract:
We present the results of 1.3 mm observations of the Crab Nebula, performed with the MPIfR bolometer arrays at the IRAM 30-m telescope. The maps obtained, of unprecedented quality at these wavelengths, allow a direct comparison with high-resolution radio maps. Although the spatial structure of the Crab Nebula does not change much from radio to millimetre wavelengths, we have detected significant spatial variations of the spectral index between 20 cm and 1.3 mm. The main effect is a spectral flattening in the inner region, which can be hardly explained just in terms of the evolution of a single population of synchrotron emitting electrons. We propose instead that this is the result of the emergence of a second synchrotron component, that we have tried to extract from the data. Shape and size of this component resemble those of the Crab Nebula in X-rays. However, while the more compact structure of the Crab Nebula in X-rays is commonly regarded as an effect of synchrotron downgrading, it cannot be explained why a similar structure is present also at millimetre wavelengths, where the electron lifetimes far exceed the nebular age. Our data, combined with published upper limits on spatial variations of the radio spectral index, also imply a low-energy cutoff for the distribution of electrons responsible for this additional synchrotron component. Although no model has been developed so far to explain the details of this component, one may verify that the total number of the electrons responsible for it is in agreement with what predicted by the classical pulsar-wind models, which otherwise are known to fail in accounting for the number of radio emitting electrons. This numerical coincidence can give indications about the origin of this component. We have also detected a spectral steepening at millimetre wavelengths in some elongated regions, whose positions match those of radio synchrotron filaments. The steepening is taken as the indication that magnetic fields in synchrotron filaments are stronger than the average nebular field.

Appeared in A&A 386, 1044

The structure and dynamics of the molecular envelope of M 2-56

Castro-Carrizo, A.(¹); Bujarrabal, V.(¹); Sànchez Contreras, C.(¹); Alcolea(¹), J.; Neri, R.(²)
(¹)Observatorio Astronomico Nacional (IGN), Apdo. 1143, 28800 Alcala de Henares, Spain, (²)IRAM, 300 rue de la Piscine, 38406, St-Martin-d'Hères, France

Abstract:
M 2-56 is a protoplanetary nebula (PPN) in which strong shocks are taking place, therefore, useful to study the post-AGB wind interaction. It is well known that molecular observations allow studying the mass distribution of PPNe, even in those regions that have been recently shocked. We present high-resolution maps of the emission of ¹²CO J=2-1 and J=1-0 in M 2-56. Such maps show a bipolar, molecular nebula that extends $\sim 28\hbox{$^{\prime\prime}$ }$ along the symmetry axis. The nebula is composed of two contiguous, incomplete shells located along the symmetry axis, which has an inclination of $\sim 17^{\circ}$ with respect to the plane of the sky. Those empty lobes intersect in the center of the nebula, where there is a small and dense ring perpendicular to the axis. This central ring expands radially at about 8 km/s and seems to be the remnant of the circumstellar envelope of the AGB star, that has not been accelerated by the interaction with the fast post-AGB jets. The radius of the central ring is of $\sim 4\,10^{16}$ cm, for a distance of 2.1 kpc (deduced from an analysis of the main properties of the object). At $\sim 4\,10^{17}$ cm from the nebular center, the tips of the lobes reach axial expansion velocities of $\sim 200$ km/s. We have developed a model for the spatio-kinematical distribution and the excitation conditions of the molecular gas in M 2-56. From the best fitting of the observations with the predictions of the model for both lines, we have estimated the physical conditions of the molecular nebula. It is found that the density varies from $5\,10^3$ to $0.6\,10^3$ cm^-3 from the nebular center to the lobe tips, and that the part of the lobes that has not been detected is probably composed of photodissociated gas, due to the effect of interstellar photons on low-density regions. The rotational temperature is estimated to be approximately constant, $\sim 13-16$ K. For the assumed geometry, a velocity field composed by a dominant radial component plus an axial contribution has been deduced. The emission of both lines is found to be optically thin, and therefore probes the whole molecular gas, which has a mass of $\sim 0.05$ M$_\odot$. The ``scalar'' momentum and the kinetic energy of the different regions of the molecular nebula have been calculated, finding that the high momentum won by the gas in the post-AGB phase cannot have been supplied by the radiation pressure mechanism. Although the central star of M 2-56 is not very hot yet ( $\sim 20 000$ K), this PPN has a large kinematical age, between 1000 yr and 1700 yr, in comparison with other PPNe that have hotter central stars. M 2-56 may not be a typical PPN, but an intermediate object between the known low-mass post-AGB nebulae and the standard PPNe.

Appeared in: A&A 386, 633