124 research outputs found
Multiwavelength observations of massive star-forming regions selected in the far-infrared
The lack of observations of the earliest stages in high-mass star formation motivated the selection of massive star-forming regions using the 170 µm ISOPHOT Serendipity Survey. The evaluation of comprehensive follow-up observations, covering near-infrared to (sub-)millimetre wavelengths, identifies massive clumps and characterises the star-forming content in detail. The clumps comprise a large fraction of cold dusty material at temperatures between 12 and 22 K, and several have masses of 100 solar masses or more. Star formation has initiated in every clump, as they harbour embedded sources detected in the mid-infrared that represent low- to intermediate-mass young stellar objects. One case study uses millimetre interferometry and discovers two compact cores of about 15 solar masses embedded in a massive clump. They are driving energetic outflows and may accrete at high rates, and thus represent examples for the first stages of forming intermediate- to high-mass stars. The importance of high spatial resolution in the infrared for the study of high-mass star formation drives dedicated observing programmes with the Herschel Space Observatory. Furthermore, an active part in the preparation of the JWST MIRI instrument was taken because in particular its unprecedented imaging capabilities will allow to constrain crucial properties of the investigated sources
Infrared spectroscopy of intermediate mass young stellar objects
In this paper we present Spitzer Infrared Spectrograph spectroscopy for 14
intermediate-mass young stellar objects. We use Spitzer spectroscopy to
investigate the physical properties of these sources and their environments.
Our sample can be divided into two types of objects: young isolated, embedded
objects with spectra that are dominated by ice and silicate absorption bands,
and more evolved objects that are dominated by extended emission from
polycyclic aromatic hydrocarbons (PAHs) and pure H2 rotational lines. We are
able to constrain the illuminating FUV fields by classifying the PAH bands
below 9micron. For most of the sources we are able to detect several atomic
fine structure lines. In particular, the [NeII] line appearing in two regions
could originate from unresolved photodissociation regions (PDRs) or J-shocks.
We relate the identified spectral features to observations obtained from NIR
through submillimeter imaging. The spatial extent of several H2 and PAH bands
is matched with morphologies identified in previous Spitzer/IRAC observations.
This also allows us to distinguish between the different H2 excitation
mechanisms. In addition, we calculate the optical extinction from the silicate
bands and use this to constrain the spectral energy distribution fit, allowing
us to estimate the masses of these YSOs.Comment: 21 pages, 26 figures, accepted to Ap
Kinematic structure of massive star-forming regions. I. accretion along filaments
Context. The mid- and far-infrared view on high-mass star formation, in particular with the results from the Herschel space observatory, has shed light on many aspects of massive star formation. However, these continuum studies lack kinematic information. Aims: We study the kinematics of the molecular gas in high-mass star-forming regions. Methods: We complemented the PACS and SPIRE far-infrared data of 16 high-mass star-forming regions from the Herschel key project EPoS with N2H+ molecular line data from the MOPRA and Nobeyama 45 m telescope. Using the full N2H+ hyperfine structure, we produced column density, velocity, and linewidth maps. These were correlated with PACS 70 μm images and PACS point sources. In addition, we searched for velocity gradients. Results: For several regions, the data suggest that the linewidth on the scale of clumps is dominated by outflows or unresolved velocity gradients. IRDC 18454 and G11.11 show two velocity components along several lines of sight. We find that all regions with a diameter larger than 1 pc show either velocity gradients or fragment into independent structures with distinct velocities. The velocity profiles of three regions with a smooth gradient are consistent with gas flows along the filament, suggesting accretion flows onto the densest regions. Conclusions: We show that the kinematics of several regions have a significant and complex velocity structure. For three filaments, we suggest that gas flows toward the more massive clumps are present
Cluster-formation in the Rosette molecular cloud at the junctions of filaments
For many years feedback processes generated by OB-stars in molecular clouds,
including expanding ionization fronts, stellar winds, or UV-radiation, have
been proposed to trigger subsequent star formation. However, hydrodynamic
models including radiation and gravity show that UV-illumination has little or
no impact on the global dynamical evolution of the cloud. The Rosette molecular
cloud, irradiated by the NGC2244 cluster, is a template region for triggered
star-formation, and we investigated its spatial and density structure by
applying a curvelet analysis, a filament-tracing algorithm (DisPerSE), and
probability density functions (PDFs) on Herschel column density maps, obtained
within the HOBYS key program. The analysis reveals not only the filamentary
structure of the cloud but also that all known infrared clusters except one lie
at junctions of filaments, as predicted by turbulence simulations. The PDFs of
sub-regions in the cloud show systematic differences. The two UV-exposed
regions have a double-peaked PDF we interprete as caused by shock compression.
The deviations of the PDF from the log-normal shape typically associated with
low- and high-mass star-forming regions at Av~3-4m and 8-10m, respectively, are
found here within the very same cloud. This shows that there is no fundamental
difference in the density structure of low- and high-mass star-forming regions.
We conclude that star-formation in Rosette - and probably in high-mass
star-forming clouds in general - is not globally triggered by the impact of
UV-radiation. Moreover, star formation takes place in filaments that arose from
the primordial turbulent structure built up during the formation of the cloud.
Clusters form at filament mergers, but star formation can be locally induced in
the direct interaction zone between an expanding HII--region and the molecular
cloud.Comment: A&A Letter, in pres
Kinematic and thermal structure at the onset of high-mass star formation
Context. Even though high-mass stars are crucial for understanding a diversity of processes within our galaxy and beyond, their formation and initial conditions are still poorly constrained.
Aims: We want to understand the kinematic and thermal properties of young massive gas clumps prior to and at the earliest evolutionary stages of high-mass star formation. Do we find signatures of gravitational collapse? Do we find temperature gradients in the vicinity or absence of infrared emission sources? Do we find coherent velocity structures toward the center of the dense and cold gas clumps?
Methods: To determine kinematics and gas temperatures, we used ammonia, because it is known to be a good tracer and thermometer of dense gas. We observed the NH3 (1, 1) and (2, 2) lines within six very young high-mass star-forming regions comprised of infrared dark clouds (IRDCs), along with ISO-selected far-infrared emission sources (ISOSS) with the Karl G. Jansky Very Large Array (VLA) and the Effelsberg 100 m Telescope.
Results: The molecular line data allows us to study velocity structures, linewidths, and gas temperatures at high spatial resolution of 3-5'', corresponding to ~0.05 pc at a typical source distance of 2.5 kpc. We find on average cold gas clumps with temperatures in the range between 10 K and 30 K. The observations do not reveal a clear correlation between infrared emission peaks and ammonia temperature peaks. Several infrared emission sources show ammonia temperature peaks up to 30 K, whereas other infrared emission sources show no enhanced kinetic gas temperature in their surrounding. We report an upper limit for the linewidth of ~1.3 km s-1, at the spectral resolution limit of our VLA observation. This indicates a relatively low level of turbulence on the scale of the observations. Velocity gradients are present in almost all regions with typical velocity differences of 1 to 2 km s-1 and gradients of 5 to 10 km s-1 pc-1. These velocity gradients are smooth in most cases, but there is one exceptional source (ISOSS23053), for which we find several velocity components with a steep velocity gradient toward the clump centers that is larger than 30 km s-1 pc-1. This steep velocity gradient is consistent with recent models of cloud collapse. Furthermore, we report a spatial correlation of ammonia and cold dust, but we also find decreasing ammonia emission close to infrared emission sources
The Earliest Phases of Star Formation (EPoS): A Herschel Key Program - The precursors to high-mass stars and clusters
(Abridged) We present an overview of the sample of high-mass star and cluster
forming regions observed as part of the Earliest Phases of Star Formation
(EPoS) Herschel Guaranteed Time Key Program. A sample of 45 infrared-dark
clouds (IRDCs) were mapped at PACS 70, 100, and 160 micron and SPIRE 250, 350,
and 500 micron. In this paper, we characterize a population of cores which
appear in the PACS bands and place them into context with their host cloud and
investigate their evolutionary stage. We construct spectral energy
distributions (SEDs) of 496 cores which appear in all PACS bands, 34% of which
lack counterparts at 24 micron. From single-temperature modified blackbody fits
of the SEDs, we derive the temperature, luminosity, and mass of each core.
These properties predominantly reflect the conditions in the cold, outer
regions. Taking into account optical depth effects and performing simple
radiative transfer models, we explore the origin of emission at PACS
wavelengths. The core population has a median temperature of 20K and has masses
and luminosities that span four to five orders of magnitude. Cores with a
counterpart at 24 micron are warmer and bluer on average than cores without a
24 micron counterpart. We conclude that cores bright at 24 micron are on
average more advanced in their evolution, where a central protostar(s) have
heated the outer bulk of the core, than 24 micron-dark cores. The 24 micron
emission itself can arise in instances where our line of sight aligns with an
exposed part of the warm inner core. About 10% of the total cloud mass is found
in a given cloud's core population. We uncover over 300 further candidate cores
which are dark until 100 micron. These are candidate starless objects, and
further observations will help us determine the nature of these very cold
cores.Comment: Accepted for publication in A&A, 81 pages, 68 figures. For full
resolution image gallery (Appendix B), see
http://www.mpia.de/~ragan/epos.htm
Globules and pillars seen in the [CII] 158 micron line with SOFIA
Molecular globules and pillars are spectacular features, found only in the
interface region between a molecular cloud and an HII-region. Impacting
Far-ultraviolet (FUV) radiation creates photon dominated regions (PDRs) on
their surfaces that can be traced by typical cooling lines. With the GREAT
receiver onboard SOFIA we mapped and spectrally resolved the [CII] 158 micron
atomic fine-structure line and the highly excited 12CO J=11-10 molecular line
from three objects in Cygnus X (a pillar, a globule, and a strong IRAS source).
We focus here on the globule and compare our data with existing Spitzer data
and recent Herschel Open-Time PACS data. Extended [CII] emission and more
compact CO-emission was found in the globule. We ascribe this emission mainly
to an internal PDR, created by a possibly embedded star-cluster with at least
one early B-star. However, external PDR emission caused by the excitation by
the Cyg OB2 association cannot be fully excluded. The velocity-resolved [CII]
emission traces the emission of PDR surfaces, possible rotation of the globule,
and high-velocity outflowing gas. The globule shows a velocity shift of ~2 km/s
with respect to the expanding HII-region, which can be understood as the
residual turbulence of the molecular cloud from which the globule arose. This
scenario is compatible with recent numerical simulations that emphazise the
effect of turbulence. It is remarkable that an isolated globule shows these
strong dynamical features traced by the [CII]-line, but it demands more
observational studies to verify if there is indeed an embedded cluster of
B-stars.Comment: Letter accepted by A&A (SOFIA special issue
The <i>Herschel</i> view of the massive star-forming region NGC 6334
Aims: Fundamental to any theory of high-mass star formation are gravity and turbulence. Their relative importance, which probably changes during cloud evolution, is not known. By investigating the spatial and density structure of the high-mass star-forming complex NGC 6334 we aim to disentangle the contributions of turbulence and gravity.
Methods: We used Herschel PACS and SPIRE imaging observations from the HOBYS key programme at wavelengths of 160, 250, 350, and 500 μm to construct dust temperature and column density maps. Using probability distribution functions (PDFs) of the column density determined for the whole complex and for four distinct sub-regions (distinguished on the basis of differences in the column density, temperature, and radiation field), we characterize the density structure of the complex. We investigate the spatial structure using the Δ-variance, which probes the relative amount of structure on different size scales and traces possible energy injection mechanisms into the molecular cloud.
Results: The Δ-variance analysis suggests that the significant scales of a few parsec that were found are caused by energy injection due to expanding HII regions, which are numerous, and by the lengths of filaments seen everywhere in the complex. The column density PDFs have a lognormal shape at low densities and a clearly defined power law at high densities for all sub-regions whose slope is linked to the exponent α of an equivalent spherical density distribution. In particular with α = 2.37, the central sub-region is largly dominated by gravity, caused by individual collapsing dense cores and global collapse of a larger region. The collapse is faster than free-fall (which would lead only to α = 2) and thus requires a more dynamic scenario (external compression, flows). The column density PDFs suggest that the different sub-regions are at different evolutionary stages, especially the central sub-region, which seems to be in a more evolved stage
The spine of the swan: A Herschel study of the DR21 ridge and filaments in Cygnus X
In order to characterise the cloud structures responsible for the formation
of high-mass stars, we present Herschel observations of the DR21 environment.
Maps of the column density and dust temperature unveil the structure of the
DR21 ridge and several connected filaments. The ridge has column densities
larger than 1e23/cm^2 over a region of 2.3 pc^2. It shows substructured column
density profiles and branching into two major filaments in the north. The
masses in the studied filaments range between 130 and 1400 Msun whereas the
mass in the ridge is 15000 Msun. The accretion of these filaments onto the DR21
ridge, suggested by a previous molecular line study, could provide a continuous
mass inflow to the ridge. In contrast to the striations seen in e.g., the
Taurus region, these filaments are gravitationally unstable and form cores and
protostars. These cores formed in the filaments potentially fall into the
ridge. Both inflow and collisions of cores could be important to drive the
observed high-mass star formation. The evolutionary gradient of star formation
running from DR21 in the south to the northern branching is traced by
decreasing dust temperature. This evolution and the ridge structure can be
explained by two main filamentary components of the ridge that merged first in
the south.Comment: 8 pages, 5 figures, accepted for publication as a Letter in Astronomy
and Astrophysic
Herschel observations of embedded protostellar clusters in the Rosette Molecular Cloud
The Herschel OB young stellar objects survey (HOBYS) has observed the Rosette
molecular cloud, providing an unprecedented view of its star formation
activity. These new far-infrared data reveal a population of compact young
stellar objects whose physical properties we aim to characterise. We compiled a
sample of protostars and their spectral energy distributions that covers the
near-infrared to submillimetre wavelength range. These were used to constrain
key properties in the protostellar evolution, bolometric luminosity, and
envelope mass and to build an evolutionary diagram. Several clusters are
distinguished including the cloud centre, the embedded clusters in the vicinity
of luminous infrared sources, and the interaction region. The analysed
protostellar population in Rosette ranges from 0.1 to about 15 Msun with
luminosities between 1 and 150 Lsun, which extends the evolutionary diagram
from low-mass protostars into the high-mass regime. Some sources lack
counterparts at near- to mid-infrared wavelengths, indicating extreme youth.
The central cluster and the Phelps & Lada 7 cluster appear less evolved than
the remainder of the analysed protostellar population. For the central cluster,
we find indications that about 25% of the protostars classified as Class I from
near- to mid-infrared data are actually candidate Class 0 objects. As a
showcase for protostellar evolution, we analysed four protostars of low- to
intermediate-mass in a single dense core, and they represent different
evolutionary stages from Class 0 to Class I. Their mid- to far-infrared
spectral slopes flatten towards the Class I stage, and the 160 to 70um flux
ratio is greatest for the presumed Class 0 source. This shows that the Herschel
observations characterise the earliest stages of protostellar evolution in
detail.Comment: Astronomy & Astrophysics letter, 6 pages, 4 figures, accepted for
publication in the Special Issue for Herschel first result
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