1,722 research outputs found

    Fragmentation, infall, and outflow around the showcase massive protostar NGC7538 IRS1 at 500 AU resolution

    Full text link
    Aims: Revealing the fragmentation, infall, and outflow processes in the immediate environment around massive young stellar objects is crucial for understanding the formation of the most massive stars. Methods: With this goal in mind we present the so far highest spatial-resolution thermal submm line and continuum observations toward the young high-mass protostar NGC7538 IRS1. Using the Plateau de Bure Interferometer in its most extended configuration at 843mum wavelength, we achieved a spatial resolution of 0.2"x0.17", corresponding to ~500AU at a distance of 2.7\,kpc. Results: For the first time, we have observed the fragmentation of the dense inner core of this region with at least three subsources within the inner 3000 AU. The outflow exhibits blue- and red-shifted emission on both sides of the central source indicating that the current orientation has to be close to the line-of-sight, which differs from other recent models. We observe rotational signatures in northeast-southwest direction; however, even on scales of 500 AU, we do not identify any Keplerian rotation signatures. This implies that during the early evolutionary stages any stable Keplerian inner disk has to be very small (<=500 AU). The high-energy line HCN(4-3)v2=1 (E_u/k=1050K) is detected over an extent of approximately 3000 AU. In addition to this, the detection of red-shifted absorption from this line toward the central dust continuum peak position allows us to estimate infall rates of ~1.8x10^(-3)Msun/yr on the smallest spatial scales. Although all that gas will not necessarily be accreted onto the central protostar, nevertheless, such inner core infall rates are among the best proxies of the actual accretion rates one can derive during the early embedded star formation phase. These data are consistent with collapse simulations and the observed high multiplicity of massive stars.Comment: Accepted for Astronomy & Astrophysics, 8 pages, also available at http://www.mpia.de/homes/beuther/papers.htm

    Photovoltaic effect in ferroelectric ceramics

    Get PDF
    The ceramic structure was simulated in a form that is more tractable to correlation between experiment and theory. Single crystals (of barium titanate) were fabricated in a simple corrugated structure in which the pedestals of the corrugation simulated the grain while the intervening cuts could be filled with materials simulating the grain boundaries. The observed photovoltages were extremely small (100 mv)

    Chemical evolution in the early phases of massive star formation II: Deuteration

    Full text link
    The chemical evolution in high-mass star-forming regions is still poorly constrained. Studying the evolution of deuterated molecules allows to differentiate between subsequent stages of high-mass star formation regions due to the strong temperature dependence of deuterium isotopic fractionation. We observed a sample of 59 sources including 19 infrared dark clouds, 20 high-mass protostellar objects, 11 hot molecular cores and 9 ultra-compact HII regions in the (3-2) transitions of the four deuterated molecules, DCN, DNC, DCO+ and N2D+ as well as their non-deuterated counterpart. The overall detection fraction of DCN, DNC and DCO+ is high and exceeds 50% for most of the stages. N2D+ was only detected in a few infrared dark clouds and high-mass protostellar objects. It can be related to problems in the bandpass at the frequency of the transition and to low abundances in the more evolved, warmer stages. We find median D/H ratios of ~0.02 for DCN, ~0.005 for DNC, ~0.0025 for DCO+ and ~0.02 for N2D+. While the D/H ratios of DNC, DCO+ and N2D+ decrease with time, DCN/HCN peaks at the hot molecular core stage. We only found weak correlations of the D/H ratios for N2D+ with the luminosity of the central source and the FWHM of the line, and no correlation with the H2 column density. In combination with a previously observed set of 14 other molecules (Paper I) we fitted the calculated column densities with an elaborate 1D physico-chemical model with time-dependent D-chemistry including ortho- and para-H2 states. Good overall fits to the observed data have been obtained the model. It is one of the first times that observations and modeling have been combined to derive chemically based best-fit models for the evolution of high-mass star formation including deuteration.Comment: 26 pages, 16 figures, accepted at A&

    The Herschel/PACS view of disks around low-mass stars in Chamaleon-I

    Full text link
    Circumstellar disks are expected to be the birthplaces of planets. The potential for forming one or more planets of various masses is essentially driven by the initial mass of the disks. We present and analyze Herschel/PACS observations of disk-bearing M-type stars that belong to the young ~2 Myr old Chamaleon-I star forming region. We used the radiative transfer code RADMC to successfully model the SED of 17 M-type stars detected at PACS wavelengths. We first discuss the relatively low detection rates of M5 and later spectral type stars with respect to the PACS sensitivity, and argue their disks masses, or flaring indices, are likely to be low. For M0 to M3 stars, we find a relatively broad range of disk masses, scale heights, and flaring indices. Via a parametrization of dust stratification, we can reproduce the peak fluxes of the 10 μ\mum emission feature observed with Spitzer/IRS, and find that disks around M-type stars may display signs of dust sedimentation. The Herschel/PACS observations of low-mass stars in Cha-I provide new constraints on their disk properties, overall suggesting that disk parameters for early M-type stars are comparable to those for more massive stars (e.g., comparable scale height and flaring angles). However, regions of the disks emitting at about 100 μ\mum may still be in the optically thick regime, preventing direct determination of disk masses. Thus the modeled disk masses should be considered as lower limits. Still, we are able to extend the wavelength coverage of SED models and start characterizing effects such as dust sedimentation, an effort leading the way towards ALMA observations of these low-mass stars

    Far-infrared photometric observations of the outer planets and satellites with Herschel-PACS

    Full text link
    We present all Herschel PACS photometer observations of Mars, Saturn, Uranus, Neptune, Callisto, Ganymede, and Titan. All measurements were carefully inspected for quality problems, were reduced in a (semi-)standard way, and were calibrated. The derived flux densities are tied to the standard PACS photometer response calibration, which is based on repeated measurements of five fiducial stars. The overall absolute flux uncertainty is dominated by the estimated 5% model uncertainty of the stellar models in the PACS wavelength range between 60 and 210 micron. A comparison with the corresponding planet and satellite models shows excellent agreement for Uranus, Neptune, and Titan, well within the specified 5%. Callisto is brighter than our model predictions by about 4-8%, Ganymede by about 14-21%. We discuss possible reasons for the model offsets. The measurements of these very bright point-like sources, together with observations of stars and asteroids, show the high reliability of the PACS photometer observations and the linear behavior of the PACS bolometer source fluxes over more than four orders of magnitude (from mJy levels up to more than 1000 Jy). Our results show the great potential of using the observed solar system targets for cross-calibration purposes with other ground-based, airborne, and space-based instruments and projects. At the same time, the PACS results will lead to improved model solutions for future calibration applications.Comment: 25 pages, 11 figures, 11 table

    Infrared variability, maser activity, and accretion of massive young stellar objects

    Get PDF
    Methanol and water masers indicate young stellar objects. They often exhibit flares, and a fraction shows periodic activity. Several mechanisms might explain this behavior but the lack of concurrent infrared (IR) data complicates to identify the cause. Recently, 6.7 GHz methanol maser flares were observed, triggered by accretion bursts of high-mass YSOs which confirmed the IR-pumping of these masers. This suggests that regular IR changes might lead to maser periodicity. Hence, we scrutinized space-based IR imaging of YSOs associated with periodic methanol masers. We succeeded to extract the IR light curve from NEOWISE data for the intermediate mass YSO G107.298+5.639. Thus, for the first time a relationship between the maser and IR variability could be established. While the IR light curve shows the same period of ~34.6 days as the masers, its shape is distinct from that of the maser flares. Possible reasons for the IR periodicity are discussed.Comment: 4 pages, 3 figures, to be published in: Proceedings IAU Symposium 336 "Astrophysical Masers: Unlocking the Mysteries of the Universe", Editors: A. Tarchi, M.J. Reid & P. Castangia, updated version with hyperlinks adde

    Characterization of Infrared Dark Clouds -- NH3_3 Observations of an Absorption-contrast Selected IRDC Sample

    Full text link
    Despite increasing research in massive star formation, little is known about its earliest stages. Infrared Dark Clouds (IRDCs) are cold, dense and massive enough to harbour the sites of future high-mass star formation. But up to now, mainly small samples have been observed and analysed. To understand the physical conditions during the early stages of high-mass star formation, it is necessary to learn more about the physical conditions and stability in relatively unevolved IRDCs. Thus, for characterising IRDCs studies of large samples are needed. We investigate a complete sample of 218 northern hemisphere high-contrast IRDCs using the ammonia (1,1)- and (2,2)-inversion transitions. We detected ammonia (1,1)-inversion transition lines in 109 of our IRDC candidates. Using the data we were able to study the physical conditions within the star-forming regions statistically. We compared them with the conditions in more evolved regions which have been observed in the same fashion as our sample sources. Our results show that IRDCs have, on average, rotation temperatures of 15 K, are turbulent (with line width FWHMs around 2 km s1^{-1}), have ammonia column densities on the order of 101410^{14} cm2^{-2} and molecular hydrogen column densities on the order of 102210^{22} cm2^{-2}. Their virial masses are between 100 and a few 1000 M_\odot. The comparison of bulk kinetic and potential energies indicate that the sources are close to virial equilibrium. IRDCs are on average cooler and less turbulent than a comparison sample of high-mass protostellar objects, and have lower ammonia column densities. Virial parameters indicate that the majority of IRDCs are currently stable, but are expected to collapse in the future.Comment: 21 pages, 11 figures, 7 tables. Paper accepted for publication in Astronomy & Astrophysic

    Chemical evolution in the early phases of massive star formation. I

    Full text link
    Understanding the chemical evolution of young (high-mass) star-forming regions is a central topic in star formation research. Chemistry is employed as a unique tool 1) to investigate the underlying physical processes and 2) to characterize the evolution of the chemical composition. We observed a sample of 59 high-mass star-forming regions at different evolutionary stages varying from the early starless phase of infrared dark clouds to high-mass protostellar objects to hot molecular cores and, finally, ultra-compact HII regions at 1mm and 3mm with the IRAM 30m telescope. We determined their large-scale chemical abundances and found that the chemical composition evolves along with the evolutionary stages. On average, the molecular abundances increase with time. We modeled the chemical evolution, using a 1D physical model where density and temperature vary from stage to stage coupled with an advanced gas-grain chemical model and derived the best-fit chi^2 values of all relevant parameters. A satisfying overall agreement between observed and modeled column densities for most of the molecules was obtained. With the best-fit model we also derived a chemical age for each stage, which gives the timescales for the transformation between two consecutive stages. The best-fit chemical ages are ~10,000 years for the IRDC stage, ~60,000 years for the HMPO stage, ~40,000 years for the HMC stage, and ~10,000 years for the UCHII stage. The total chemical timescale for the entire evolutionary sequence of the high-mass star formation process is on the order of 10^5 years, which is consistent with theoretical estimates. Furthermore, based on the approach of a multiple-line survey of unresolved data, we were able to constrain an intuitive and reasonable physical and chemical model. The results of this study can be used as chemical templates for the different evolutionary stages in high-mass star formation.Comment: 31 pages, 11 figures, 21 tables, accepted by A&A; typos adde
    corecore