163 research outputs found
Spitzer-IRAC GLIMPSE of high mass protostellar objects. I Infrared point sources and nebulae
The GLIMPSE archive was used to obtain 3.6--8.0micron, point source
photometry and images for 381 massive protostellar candidates lying in the
Galactic mid-plane. The colours, magnitudes and spectral indicies of sources in
each of the 381 target fields were analysed and compared with the predictions
of 2D radiative transfer model simulations. Although no discernable embedded
clusters were found in any targets, multiple sources or associations of
redenned young stellar objects were found in many sources indicating
multiplicity at birth. The spectral index () of these point sources in
3.6--8.0mum bands display large values of =2--5. A color-magnitude
analog plot was used to identify 79 infrared counterparts to the HMPOs. Compact
nebulae are found in 75% of the detected sources with morphologies that can be
well described by core-halo, cometary, shell-like and bipolar geometries
similar to those observed in ultra-compact HII regions. The IRAC band SEDs of
the IR counterparts of HMPOs are best described to represent YSOs with a mass
range of 8--20\msun in their Class I stages when compared with 2D radiative
transfer models. They also suggest that the high values represent
reprocessed star/star+disk emission that is arising in the dense envelopes.
Thus we are witnessing the luminous envelopes around the protostars rather than
their photospheres or disks. We argue that the compact infrared nebulae likely
reflect the underlying physical structure of the dense cores and are found to
imitate the morphologies of known UCHII regions. Our results favour models of
continuuing accretion involving both molecular and ionised accretion components
to build the most massive stars rather than purely molecular rapid accretion
flows.Comment: 13 pages, 7 figures, accepted by A&
The structure of Onsala 1 star forming region
We present new high-sensitivity high-resolution mm-wave observations of the
Onsala 1 ultra-compact HII region that bring to light the internal structure of
this massive star forming cloud. The 1.2 mm continuum map obtained with the
IRAM 30-m radiotelescope (~11" resolution) shows a centrally peaked
condensation of 1' size (~0.5 pc at the assumed distance of 1.8 kpc) which has
been further investigated at higher resolution in the 3 mm continuum and in the
emission lines of H13CO+ J=1--0 and SiO J=2--1 with the IRAM Plateau de Bure
interferometer. The 3 mm data, with a resolution of ~5" X 4", displays a
unresolved continuum source at the peak of the extended 1.2 mm emission and
closely associated with the ultra-compact HII region. The H13CO+ map traces the
central condensation in good agreement with previous NH_3 maps of Zheng et al.
(1985). However, the velocity field of this central condensation, which was
previously thought to arise in a rapidly rotating structure, is better
explained in terms of the dense and compact component of a bipolar outflow.
This interpretation is confirmed by SiO and CO observations of the full region.
In fact, our new SiO data unveils the presence of multiple (at least 4)
outflows in the region. In particular, there is an important center of outflow
activity in the region at about 1' north of the UCHII region. Indeed the
different outflows are related to different members of the Onsala 1 cluster.
The data presented here support a scenario in which the phases of massive star
formation begin much later in the evolution of a cluster and/or UCHII region
last for much longer than 10^5 yrs.Comment: 6 pages and 3 figure
The N2D+/N2H+ ratio as an evolutionary tracer of Class 0 protostars
Deuterated ions are abundant in cold (T=10 K), dense (n=10^5 cm^-3) regions,
in which CO is frozen out onto dust grains. In such environments, the deuterium
fractionation of such ions can exceed the elemental abundance ratio of D/H by a
factor of 10^4. In this paper we use the deuterium fractionation to investigate
the evolutionary state of Class 0 protostars. In a sample of 20 protostellar
objects, we found a clear correlation between the N2D+/N2H+ ratio and
evolutionary tracers. As expected, the coolest, i.e. the youngest, objects show
the largest deuterium fractionation. Furthermore, we find that sources with a
high N2D+/N2H+ ratio show clear indication for infall.Comment: 19 pages, 12 figures, accepted by A&
SOLIS: XII. SVS13-A Class i chemical complexity as revealed by S-bearing species
Context. Recent results in astrochemistry have revealed that some molecules, such as interstellar complex organic species and deuterated species, can serve as valuable tools in the investigation of star-forming regions. Sulphuretted species can also be used to follow the chemical evolution of the early stages of a Sun-like star formation process. Aims. The goal is to obtain a census of S-bearing species using interferometric images towards SVS13-A, a Class I object associated with a hot corino that is rich in interstellar complex organic molecules. Methods. To this end, we used the NGC 1333 SVS13-A data at 3 mm and 1.4 mm obtained with the IRAM-NOEMA interferometer in the framework of the SOLIS (Seeds of Life in Space) Large Program. The line emission of S-bearing species was imaged and analyzed using local thermodynamic equilibrium (LTE) and large velocity gradient (LVG) approaches. Results. We imaged the spatial distribution on =300 au scale of the line emission of 32SO, 34SO, C32S, C34S, C33S, OCS, H2C32S, H2C34S, and NS. The low excitation (9 K) 32SO line traces: (i) the low-velocity SVS13-A outflow and (ii) the fast (up to 100 km s-1 away from the systemic velocity) collimated jet driven by the nearby SVS13-B Class 0 object. Conversely, the rest of the lines are confined in the inner SVS13-A region, where complex organics were previously imaged. More specifically, the non-LTE LVG analysis of SO, SO2, and H2CS indicates a hot corino origin (size in the 60 120 au range). Temperatures between 50 K and 300 K, as well as volume densities larger than 105 cm-3 have been derived. The abundances of the sulphuretted are in the following ranges: 0.3 6 ± 10-6 (CS), 7 ± 10-9 1 ± 10-7 (SO), 1 10 ± 10-7 (SO2), a few 10-10 (H2CS and OCS), and 10-10 10-9 (NS). The N(NS)/N(NS+) ratio is larger than 10, supporting the assessment that the NS+ ion is mainly formed in the extended envelope. Conclusions. The [H2CS]/[H2CO] ratio, once measured at high-spatial resolutions, increases with time (from Class 0 to Class II objects) by more than one order of magnitude (from =10-2 to a few 10-1). This suggests that [S]/[O] changes along the process of Sunlike star formation. Finally, the estimate of the [S]/[H] budget in SVS13-A is 2 17% of the Solar System value (1.8 ± 10-5), which is consistent with what was previously measured towards Class 0 objects (1 8%). This finding supports the notion that the enrichment of the sulphuretted species with respect to dark clouds remains constant from the Class 0 to the Class I stages of low-mass star formation. The present findings stress the importance of investigating the chemistry of star-forming regions using large observational surveys as well as sampling regions on the scale of the Solar System
Molecular outflows towards O-type young stellar objects
We have searched for massive molecular outflows in a sample of high-mass star
forming regions, and we have characterised both the outflow properties and
those of their associated molecular clumps. With a sample composed largely of
more luminous objects than previous ones, this work complements analogous
surveys performed by other authors by adding the missing highest luminosity
sources. The sample under study has been selected so as to favour the earliest
evolutionary phases of star formation, and is composed of very luminous objects
(L_bol > 2x10^4 L_sun and up to ~10^6 L_sun), possibly containing O-type stars.
Each source has been mapped in 13CO(2-1) and C18O(2-1) with the IRAM-30m
telescope on Pico Veleta (Spain). The whole sample shows high-velocity wings in
the 13CO(2-1) spectra, indicative of outflowing motions. In addition, we have
obtained outflow maps in 9 of our 11 sources, which display well-defined blue
and/or red lobes. For these sources, the outflow parameters have been derived
from the line wing 13CO(2-1) emission. An estimate of the clump masses from the
C18O(2-1) emission is also provided and found to be comparable to the virial
masses. From a comparison between our results and those found by other authors
at lower masses, it is clear that the outflow mechanical force increases with
the bolometric luminosity of the clump and with the ionising photon rate of the
associated HII regions, indicating that high-mass stars drive more powerful
outflows. A tight correlation between outflow mass and clump mass is also
found. Molecular outflows are found to be as common in massive star forming
regions as in low-mass star forming regions. This, added to the detection of a
few tentative large-scale rotating structures suggests that high-mass stars may
generally form via accretion, as low-mass stars.Comment: 16 pages, 10 figures, accepted by Astronomy and Astrophysic
Unveiling the Early-Stage Anatomy of a Protocluster Hub with ALMA
High-mass stars shape the interstellar medium in galaxies, and yet, largely because the initial conditions are poorly constrained, we do not know how they form. One possibility is that high-mass stars and star clusters form at the junction of filamentary networks, referred to as "hubs". In this letter we present the complex anatomy of a protocluster hub within an Infrared Dark Cloud (IRDC), G035.39-00.33, believed to be in an early phase of its evolution. We use high-angular resolution () and high-sensitivity ( mJy beam; M) 1.07 mm dust continuum observations from the Atacama Large Millimeter Array (ALMA) to identify a network of narrow, pc wide, filamentary structures. These are a factor of narrower than the proposed "quasi-universal" pc width of interstellar filaments. Additionally, 28 compact objects are reported, spanning a mass range . This indicates that at least some low-mass objects are forming coevally with more massive counterparts. Comparing to the popular "bead-on-a-string" analogy, the protocluster hub is poorly represented by a monolithic clump embedded within a single filament. Instead, it comprises multiple intra-hub filaments, each of which retains its integrity as an independent structure and possesses its own embedded core population
Omega-3 Fatty Acids Modify Human Cortical Visual Processing—A Double-Blind, Crossover Study
While cardiovascular and mood benefits of dietary omega-3 fatty acids such as docosahexaenoic acid (DHA) and eicosapentaenoic acid (EPA) are manifest, direct neurophysiological evidence of their effects on cortical activity is still limited. Hence we chose to examine the effects of two proprietary fish oil products with different EPA∶DHA ratios (EPA-rich, high EPA∶DHA; DHA-rich) on mental processing speed and visual evoked brain activity. We proposed that nonlinear multifocal visual evoked potentials (mfVEP) would be sensitive to any alteration of the neural function induced by omega-3 fatty acid supplementation, because the higher order kernel responses directly measure the degree of recovery of the neural system as a function of time following stimulation. Twenty-two healthy participants aged 18–34, with no known neurological or psychiatric disorder and not currently taking any nutritional supplementation, were recruited. A double-blind, crossover design was utilized, including a 30-day washout period, between two 30-day supplementation periods of the EPA-rich and DHA-rich diets (with order of diet randomized). Psychophysical choice reaction times and multi-focal nonlinear visual evoked potential (VEP) testing were performed at baseline (No Diet), and after each supplementation period. Following the EPA-rich supplementation, for stimulation at high luminance contrast, a significant reduction in the amplitude of the first slice of the second order VEP kernel response, previously related to activation in the magnocellular pathway, was observed. The correlations between the amplitude changes of short latency second and first order components were significantly different for the two supplementations. Significantly faster choice reaction times were observed psychophysically (compared with baseline performance) under the EPA-rich (but not DHA-rich) supplementation, while simple reaction times were not affected. The reduced nonlinearities observed under the EPA-rich diet suggest a mechanism involving more efficient neural recovery of magnocellular-like visual responses following cortical activation
Mother of Dragons: A Massive, quiescent core in the dragon cloud (IRDC G028.37+00.07)
Context: Core accretion models of massive star formation require the
existence of massive, starless cores within molecular clouds. Yet, only a small
number of candidates for such truly massive, monolithic cores are currently
known. Aims: Here, we analyse a massive core in the well-studied infrared dark
cloud (IRDC) called the "dragon cloud" (G028.37+00.07 or "Cloud C"). This core
(C2c1) sits at the end of a chain of a roughly equally spaced actively
star-forming cores near the center of the IRDC. Methods: We present new
high-angular resolution 1 mm ALMA dust continuum and molecular line
observations of the massive core. Results: The high-angular resolution
observations show that this region fragments into two cores C2c1a and C2c1b,
which retain significant background subtracted masses of 23 Msun and 2 Msun (31
Msun and 6 Msun without background subtraction), respectively. The cores do not
appear to fragment further on the scales of our highest angular resolution
images (0.200 arcsec, 0.005 pc ~ 1000 AU). We find that these cores are very
dense (nH2 > 10^6 cm-3) and have only trans-sonic non-thermal motions (Ms ~ 1).
Together the mass, density and internal motions imply a virial parameter of <
1, which suggests the cores are gravitationally unstable, unless supported by
strong magnetic fields with strengths of ~ 1 - 10 mG. From CO line
observations, we find that there is tentative evidence for a weak molecular
outflow towards the lower-mass core, yet the more massive core remains devoid
of any star formation indicators. Conclusions: We have presented evidence for
the existence of a massive, prestellar core, which has implications for
theories of massive star formation. This source warrants follow-up
higher-angular resolution observations to further assess its monolithic and
prestellar nature.Comment: 8+4 pages, 4+2 Figures, 2 Tables, accepted for publication in
Astronomy & Astrophysic
A <i>Herschel</i> and BIMA study of the sequential star formation near the W 48A H II region
We present the results of Herschel HOBYS (Herschel imaging survey of OB Young Stellar objects) photometric mapping combined with Berkeley Illinois Maryland Association (BIMA) observations and additional archival data, and perform an in-depth study of the evolutionary phases of the star-forming clumps in W 48A and their surroundings. Age estimates for the compact sources were derived from bolometric luminosities and envelope masses, which were obtained from the dust continuum emission, and agree within an order of magnitude with age estimates from molecular line and radio data. The clumps in W 48A are linearly aligned by age (east-old to west-young): we find a ultra-compact (UC) H II region, a young stellar object (YSO) with class II methanol maser emission, a YSO with a massive outflow and finally the NH2D prestellar cores from Pillai et al. This remarkable positioning reflects the (star) formation history of the region. We find that it is unlikely that the star formation in the W 48A molecular cloud was triggered by the UC H II region and discuss the Aquila supershell expansion as a major influence on the evolution of W 48A. We conclude that the combination of Herschel continuum data with interferometric molecular line and radio continuum data is important to derive trustworthy age estimates and interpret the origin of large-scale structures through kinematic information
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