Scientific Results in Press

Fragmentation in Massive Star Formation

Henrik Beuther(¹) and Peter Schilke(²)
(¹)Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA (²)Max-Planck-Institute für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany

Abstract:
Studies of evolved massive stars indicate that they form in a clustered mode. During the earliest evolutionary stages, these regions are embedded within their natal cores. Here, we show high-spatial-resolution interferometric dust continuum observations disentangling the cluster-like structure of a young massive star-forming region. The derived protocluster mass distribution is consistent with the stellar initial mass function. Thus, fragmentation of the initial massive cores may determine the initial mass function and the masses of the final stars. This implies that stars of all masses can form via accretion processes, and coalescence of intermediate-mass protostars appears not to be necessary.

Appeared in Science 303, 1167-1169 (February 2004)

Ethylene glycol in comet C/1995 O1 (Hale-Bopp)

J. Crovisier(¹), D. Bockelée-Morvan(¹), N. Biver(¹), P. Colom((¹), D. Despois(²) and D.C. Lis(³)
(¹)Observatoire de Paris, F-92195, Meudon, France, (²)Observatoire de Bordeaux, B.P. 89, F-33270, Floirac, France, (³)California Institute of Technology, MS 320-47, Pasadena, CA 91125, USA

Abstract:
We report the detection of ethylene glycol (HOCH₂CH₂OH) in comet C/1995 O1 (Hale-Bopp) from the analysis of archival radio spectra. About ten rotational lines of the molecule are observed in spectra obtained at the IRAM 30-m telescope and Plateau de Bure interferometer and at the Caltech Sumillimeter Observatory, in spring 1997. The identification was made just after the rotational lines of this molecule was included in the Cologne Database for Molecular Spectroscopy. The production rate of ethylene glycol is $\approx 0.25$% that of water, making it one of the most abundant organic molecules in cometary ices. This detection strengthens the similarity between interstellar and cometary material. It outlines the possible role of cometary impacts in the origin of life by seeding the early Earth with prebiotic molecules.

Accepted for publication in A&A Letters

Dense gas in nearby galaxies
XVI. The nuclear starburst environment in NGC 4945

M. Wang(^1,2), C. Henkel(¹), Y.-N. Chin(³), J. B. Whiteoak(⁴), M. Hunt Cunningham(⁵), R. Mauersberger(⁶), and D. Muders(¹)
(¹)Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, D-53121 Bonn, Germany, (²)Purple Mountain Observatory, Chinese Academy of Sciences, 210008 Nanjing, China, (³)Department of Physics, Tamkang University, 251-37 Tamsui, Taipei County, Taiwan, (⁴)Australia Telescope National Facility, CSIRO Radiophysics Labs., P.O. Box 76, Epping, NSW 2121, Australia, (⁵)School of Physics, UNSW, 2052 Sydney, Australia, (⁶)IRAM, Avenida Divina Pastora 7, Local 20, E-18012 Granada, Spain

Abstract:
A multi-line millimeter-wave study of the nearby starburst galaxy NGC 4945 has been carried out using the Swedish-ESO Submillimeter Telescope (SEST). The study covers the frequency range from 82 GHz to 354 GHz and includes 80 transitions of 19 molecules. 1.3 mm continuum data of the nuclear source are also presented. An analysis of CO and 1.3 mm continuum fluxes indicates that the conversion factor between H₂ column density and CO J=1-0integrated intensity is smaller than in the galactic disk by factors of 5-10. A large number of molecular species indicate the presence of a prominent high density interstellar gas component characterized by $n_{H_2}\approx 10^5$cm^-3. Some spectra show Gaussian profiles. Others exhibit two main velocity components, one at $\approx 450$ km s^-1, the other at $\approx 710$ km s^-1. While the gas in the former component has a higher linewidth, the latter component arises from gas that is more highly excited as is indicated by HCN, HCO+ and CN spectra. Abundances of molecular species are calculated and compared with abundances observed toward the starburst galaxies NGC 253 and M 82 and galactic sources. Apparent is an 'overabundance' of HNC in the nuclear environment of NGC 4945. While the HNC/HCN J=1-0 line intensity ratio is $\approx 0.5$, the HNC/HCN abundance ratio is $\approx 1$. From a comparison of K_a=0 and 1 HNCO line intensities, an upper limit to the background radiation of 30 K is derived. While HCN is subthermally excited (T $_{ex}\approx 8$ K), CN is even less excited (T $_{ex}\approx 3-4$ K), indicating that it arises from a less dense gas component and that its N=2-1 line can be optically thin even though its N=1-0 emission is moderately optically thick. Overall, fractional abundances of NGC 4945 suggest that the starburst has reached a stage of evolution that is intermediate between those observed in NGC 253 and M 82. Carbon, nitrogen, oxygen and sulfur isotope ratios are also determined. Within the limits of uncertainty, carbon and oxygen isotope ratios appear to be the same in the nuclear regions of NGC 4945 and NGC 253. High ¹⁸O/¹⁷O, low ¹⁶O/¹⁸O and ¹⁴N/¹⁵N and perhaps also low ³²S/³⁴S ratios ( $6.4\pm 0.3$, $195\pm 45$, $105\pm 25$ and $13.5\pm 2.5$ in NGC 4945, respectively) appear to be characteristic properties of a starburst environment in which massive stars have had sufficient time to affect the isotopic composition of the surrounding interstellar medium.

Accepted for publication in A&A

Rotating Disks in High-Mass Young Stellar Objects

Beltrán M. T.(¹), Cesaroni R.(¹), Neri R.(²), Codella C.(³), Furuya R. S.(^1,4), Testi L.(¹), Olmi, L.(⁵)
(¹)Osservatorio Astrofisico di Arcetri, Istituto Nazionale di Astrofisica, Largo Enrico Fermi 5, I-50125 Florence, Italy, (²)Institut de Radioastronomie Millimétrique, 300 Rue de la Piscine, F-38406 Saint Martin d'Hères, France, (³)Istituto di Radioastronomia, CNR, Sezione di Firenze, Largo Enrico Fermi 5, I-50125 Florence, Italy, (⁴)Division of Physics, Mathematics, and Astronomy, California Institute of Technology, MS 105-24, Pasadena, CA 91125, (⁵)Istituto di Radioastronomia, CNR, Sezione di Firenze, Largo Enrico Fermi 5, I-50125 Florence, Italy

Abstract:
We report on the detection of four rotating massive disks in two regions of high-mass star formation. The disks are perpendicular to known bipolar outflows and turn out to be unstable but long-lived. We infer that accretion onto the embedded (proto)stars must proceed through the disks with rates of $\approx 10^{-2} \mbox{M$_\odot$ }yr^{-1}$.

Appeared in: ApJ 601, L187-L190

A search for evolved dust in Herbig Ae stars

Natta A.(¹), Testi L.(¹), Neri R.(²), Shepherd D. S.(³), Wilner D. J.(⁴)
(¹)Osservatorio Astrofisico di Arcetri, INAF, Largo E.Fermi 5, 50125 Firenze, Italy, (²)IRAM, 300 rue de la Piscine, 38406 St Martin d'Heres, France, (³)National Radio Astronomy Observatory, PO Box O, Socorro, NM 87801, USA, (⁴)Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA

Abstract:
We present observations of six isolated, pre-main-sequence, intermediate mass stars selected for shallow spectra at submillimeter wavelengths at 1.3, 2.6, 7.0, and 36 millimeters from the IRAM PdBI and the VLA. We analyze the new observations of these stars (HD 34282, HD 35187, HD 142666, HD 143006, HD 150193, HD 163296) together with similar observations of three additional stars from the literature (CQ Tau, UX Ori, TW Hya), in the context of self-consistent irradiated disk models. Our aim is to constrain the wavelength dependence of the dust opacity and the total dust mass in the disks. The shallow wavelength dependence of the opacity is confirmed and for a few stars extended to significantly longer wavelengths. For any plausible dust properties, this requires grain growth from interstellar sizes to maximum sizes of at least a few millimeters, and very likely to several centimeters or more. For four of the stars (HD 34282, HD 163296, CQ Tau, TW Hya), the millimeter emission has been spatially resolved, and the large disk radii (>100 AU) rule out that high optical depths play a role. The mass of dust that has been processed into large grains is substantial, and in some cases implies a disk mass comparable to the mass of the central star.

Appeared in: A&A 416, 179-186

L1157: Interaction of the molecular outflow with the Class 0 environment

Beltrán M.T.(^1,2), Gueth F.(³), Guilloteau S.(³), Dutrey A.(⁴)
(¹)Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA, (²)Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy, (³)Institut de Radio Astronomie Millimétrique, 300 rue de la Piscine, 38406 Saint Martin d'Hères, France, (⁴)Laboratoire d'Astrophysique de l'Observatoire de Grenoble, BP 53, 38041 Grenoble, France

Abstract:
We present high angular resolution interferometric observations of the dust continuum at 2.7 and 1.3 mm, and of the HC₃N (J=12-11) and C¹⁸O (J=2-1) emission around L1157-mm, a Class 0 object that drives a spectacular molecular outflow. The millimeter dust emission is clearly resolved into two components, a flattened compact source of $\approx 450\times 250$ AU at 1.3 mm, and mass $\approx 0.1$ M$_\odot$