In recent years, a plethora of observations with high spectral resolution of sub-millimetre and far-infrared transitions of methylidene (CH), conducted with Herschel and SOFIA, have demonstrated this ...radical to be a valuable proxy for molecular hydrogen that can be used for characterising molecular gas within the interstellar medium on a Galactic scale, including the CO-dark component. We report the discovery of the 13CH isotopologue in the interstellar medium using the upGREAT receiver on board SOFIA. We have detected the three hyperfine structure components of the ≈2 THz frequency transition from its X2Π1∕2 ground-state towards the high-mass star-forming regions Sgr B2(M), G34.26+0.15, W49(N), and W51E and determined 13CH column densities. The ubiquity of molecules containing carbon in the interstellar medium has turned the determination of the ratio between the abundances of the two stable isotopes of carbon, 12C/13C, into a cornerstone for Galactic chemical evolution studies. Whilst displaying a rising gradient with galactocentric distance, this ratio, when measured using observations of different molecules (CO, H2CO, and others), shows systematic variations depending on the tracer used. These observed inconsistencies may arise from optical depth effects, chemical fractionation, or isotope-selective photo-dissociation. Formed from C+ either through UV-driven or turbulence-driven chemistry, CH reflects the fractionation of C+, and does not show any significant fractionation effects, unlike other molecules that were previously used to determine the 12C/13C isotopic ratio. This makes it an ideal tracer for the 12C/13C ratio throughout the Galaxy. By comparing the derived column densities of 13CH with previously obtained SOFIA data of the corresponding transitions of the main isotopologue 12CH, we therefore derive 12C/13C isotopic ratios toward Sgr B2(M), G34.26+0.15, W49(N) and W51E. Adding our values derived from 12∕13CH to previous calculations of the Galactic isotopic gradient, we derive a revised value of 12C/13C = 5.87(0.45)RGC + 13.25(2.94).
Abstract
Hub–filament systems (HFSs) being the potential sites of formation of star clusters and high-mass stars, provide a testbed for the current theories that attempt to explain star formation ...globally. It is thus important to study a large number of HFSs using both intensity and velocity information to constrain these objects better observationally. Here, we present a study of the HFS associated with G6.55-0.1 using newly obtained observations of the radio continuum and the
J
= 2–1 transition of CO,
13
CO, and C
18
O. The radio continuum maps show multiple peaks that coincide with far-infrared dust continuum peaks, indicating the presence of more than one young massive star in the hub of the HFS. We used the velocity information from the C
18
O(2–1) map to (a) show that the source G6.55-0.1 is not physically associated with the supernova remnant W28 and (b) disentangle and identify the velocity components genuinely associated with G6.55-0.1. Among the velocity-coherent structures identified in the region, we conclude that only the two filaments at 13.8 and 17.3 km s
−1
contribute a total mass accretion rate of 3000
M
⊙
Myr
−1
to the hub. Both the filaments also show a V-shaped structure, characteristic of gravitational collapse, in their velocity profile at the location of the hub. The estimated mass per unit length of the segments of the filaments is smaller than the critical line masses derived from virial equilibrium considerations. This suggests that the filaments are not gravitationally collapsing as a whole, although their inner parts clearly show evidence of collapse in the form of young star-forming cores. We further conclude that the observed velocity gradients are consistent with the gravitational collapse of the main source in the region as estimated from its mass and size.
Abstract
The mass assembly in star-forming regions arises from the hierarchical structure in molecular clouds in tandem with fragmentation at different scales. In this paper, we present a study of ...the fragmentation of massive clumps covering a range of evolutionary states, selected from the ATLASGAL survey, using the compact configuration of the Submillimeter Array. The observations reveal a wide diversity in the fragmentation properties with about 60% of the sources showing limited to no fragmentation at the 2″ scale, or a physical scale of 0.015–0.09 pc. We also find several examples where the cores detected with the Submillimeter Array are significantly offset from the clump potential, suggesting that initial fragmentation does not result in the formation of a large number of Jeans mass fragments. The fraction of the clump mass that is in compact structures is seen to increase with source evolution. We also see a significant correlation between the maximum mass of a fragment and the bolometric luminosity of the parent clump. These suggest that massive star formation proceeds through clump fed core accretion, with the initial fragmentation being dependent on the density structure of the clumps and/or magnetic fields.
Abstract
In the last years there has been a substantial increase in the number of the reported massive and luminous star-forming regions with related explosive outflows thanks to the superb ...sensitivity and angular resolution provided by the new radio, infrared, and optical facilities. Here, we report one more explosive outflow related with the massive and bright star-forming region IRAS 12326−6245 using Band 6 sensitive and high-angular-resolution (∼0.″2) Atacama Large Millimeter/Submillimeter Array observations. We find over 10 molecular and collimated well-defined streamers, with Hubble–Lemaitre–like expansion motions, and pointing right to the center of a dusty and molecular shell (reported for the first time here) localized in the northern part of the UC H
ii
region known as G301.1A. The estimated kinematic age and energy for the explosion are ∼700 yr and 10
48
erg, respectively. Taking into account the recently reported explosive outflows together with IRAS 12326−6245, we estimate an event rate of once every 90 yr in our Galaxy, similar to the formation rate of massive stars.
The formation mechanism of brown dwarfs (BDs) is one of the long-standing problems in star formation because the typical Jeans mass in molecular clouds is too large to form these substellar objects. ...To answer this question, it is crucial to study a BD in the embedded phase. IRAS 16253-2429 is classified as a very low-luminosity object (VeLLO) with an internal luminosity of <0.1 L sub(middot in circle). VeLLOs are believed to be very low-mass protostars or even proto-BDs. We observed the jet/outflow driven by IRAS 16253-2429 in CO (2-1), (6-5), and (7-6) using the IRAM 30 m and Atacama Pathfinder Experiment telescopes and the Submillimeter Array (SMA) in order to study its dynamical features and physical properties. Our SMA map reveals two protostellar jets, indicating the existence of a proto-binary system as implied by the precessing jet detected in H sub(2) emission. We detect a wiggling pattern in the position-velocity diagrams along the jet axes, which is likely due to the binary orbital motion. Based on this information, we derive the current mass of the binary as ~0.032 M sub(middot in circle). Given the low envelope mass, IRAS 16253-2429 will form a binary that probably consist of one or two BDs. Furthermore, we found that the outflow force as well as the mass accretion rate are very low based on the multi-transition CO observations, which suggests that the final masses of the binary components are at the stellar/substellar boundary. Since IRAS 16253 is located in an isolated environment, we suggest that BDs can form through fragmentation and collapse, similar to low-mass stars.
We present Submillimeter Array (SMA) observations in the CO J = 3-2, SiO J = 5-4 and 8-7, and SO 98-87 lines, as well as Atacama Pathfinder EXperiment observations in the CO J = 6-5 line, of an ...extremely high-velocity and jet-like outflow in high-mass star-forming region HH 80-81. The outflow is known to contain two prominent molecular bullets, namely B1 and B2, discovered from our previous SMA CO J = 2-1 observations. While B1 is detected in all the CO, SiO, and SO lines, B2 is only detected in CO lines. The CO 3-2/2-1 line ratio in B1 is clearly greater than that in B2. We perform a large velocity gradient analysis of the CO lines and derive a temperature of 70-210 K for B1 and 20-50 K for B2. Taking into account the differences in the velocity, distance from the central source, excitation conditions, and chemistry between the two bullets, we suggest that the bullets are better explained by direct ejections from the innermost vicinity of the central high-mass protostar, and that we are more likely observing the molecular component of a primary wind rather than entrained or swept-up material from the ambient cloud. These findings further support our previous suggestions that the molecular bullets indicate an episodic, disk-mediated accretion in the high-mass star formation process.
Circumstellar disks are an essential ingredient of the formation of low-mass stars. It is unclear, however, whether the accretion-disk paradigm can also account for the formation of stars more ...massive than about 10 solar masses, in which strong radiation pressure might halt mass infall. Massive stars may form by stellar merging, although more recent theoretical investigations suggest that the radiative-pressure limit may be overcome by considering more complex, non-spherical infall geometries. Clear observational evidence, such as the detection of compact dusty disks around massive young stellar objects, is needed to identify unambiguously the formation mode of the most massive stars. Here we report near-infrared interferometric observations that spatially resolve the astronomical-unit-scale distribution of hot material around a high-mass (∼20 solar masses) young stellar object. The image shows an elongated structure with a size of ∼13 × 19 astronomical units, consistent with a disk seen at an inclination angle of ∼45°. Using geometric and detailed physical models, we found a radial temperature gradient in the disk, with a dust-free region less than 9.5 astronomical units from the star, qualitatively and quantitatively similar to the disks observed in low-mass star formation. Perpendicular to the disk plane we observed a molecular outflow and two bow shocks, indicating that a bipolar outflow emanates from the inner regions of the system.
Celotno besedilo
Dostopno za:
DOBA, IJS, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
ABSTRACT
We present Combined Array for Research in Millimeter-wave Astronomy (CARMA) CO (
) observations and
Herschel
PACS spectroscopy, characterizing the outflow properties toward extremely young ...and deeply embedded protostars in the Orion molecular clouds. The sample comprises a subset of the Orion protostars known as the PACS Bright Red Sources (PBRS; Stutz et al.). We observed 14 PBRS with CARMA and 8 of these 14 with
Herschel
, acquiring full spectral scans from 55 to 200
μ
m. Outflows are detected in CO (
) from 8 of 14 PBRS, with two additional tentative detections; outflows are also detected from the outbursting protostar HOPS 223 (V2775 Ori) and the Class I protostar HOPS 68. The outflows have a range of morphologies; some are spatially compact, <10,000 au in extent, while others extend beyond the primary beam. The outflow velocities and morphologies are consistent with being dominated by intermediate inclination angles (80° ≥
i
≥ 20°). This confirms the interpretation of the very red 24–70
μ
m colors of the PBRS as a signpost of high envelope densities, with only one (possibly two) cases of the red colors resulting from edge-on inclinations. We detect high-
J
(
J
up
> 13) CO lines and/or H
2
O lines from 5 of 8 PBRS and only for those with detected CO outflows. The far-infrared CO rotation temperatures of the detected PBRS are marginally colder (∼230 K) than those observed for most protostars (∼300 K), and only one of these five PBRS has detected O
i
63
μ
m emission. The high envelope densities could be obscuring some O
i
emission and cause a ∼20 K reduction to the CO rotation temperatures.
Abstract We investigate the physical structure and conditions of photodissociation regions (PDRs) and molecular gas within the Pillars of Creation in the Eagle Nebula using SOFIA FEEDBACK ...observations of the C ii 158 μ m line. These observations are velocity resolved to 0.5 km s −1 and are analyzed alongside a collection of complimentary data with similar spatial and spectral resolution: the O i 63 μ m line, also observed with SOFIA, and rotational lines of CO, HCN, HCO + , CS, and N 2 H + . Using the superb spectral resolution of SOFIA, APEX, CARMA, and BIMA, we reveal the relationships between the warm PDR and cool molecular gas layers in context of the Pillars’ kinematic structure. We assemble a geometric picture of the Pillars and their surroundings informed by illumination patterns and kinematic relationships and derive physical conditions in the PDRs associated with the Pillars. We estimate an average molecular gas density n H 2 ∼ 1.3 × 10 5 cm −3 and an average atomic gas density n H ∼ 1.8 × 10 4 cm −3 and infer that the ionized, atomic, and molecular phases are in pressure equilibrium if the atomic gas is magnetically supported. We find pillar masses of 103, 78, 103, and 18 M ⊙ for P1a, P1b, P2, and P3, respectively, and evaporation times of ∼1–2 Myr. The dense clumps at the tops of the pillars are currently supported by the magnetic field. Our analysis suggests that ambipolar diffusion is rapid and these clumps are likely to collapse within their photoevaporation timescales.
We mapped the kinetic temperature structure of Orion KL in a ∼20″ (∼8000 au) sized region with para-H2CS 707 − 606, 726 − 625, and 725 − 624 making use of Atacama Large Millimeter/submillimeter Array ...Band 6 Science Verification data. The kinetic temperatures obtained with a resolution of 1 65 × 1 14 (∼550 au) are deduced by modeling the measured averaged velocity-integrated intensity ratios of para-H2CS 726 − 625/707 − 606 and 725 − 624/707 − 606 with a RADEX non-LTE model. The kinetic temperatures of the dense gas, derived from the para-H2CS line ratios at a spatial density of 107 cm−3, are high, ranging from 43 to >500 K with an unweighted average of ∼170 K. There is no evidence for internal sources playing an important role in the heating of the various structures identified in previous work, namely the elongated ridge, the northwestern clump, and the eastern region of the compact ridge, while the high temperatures in the western region of the compact ridge may be dominated by internal massive star formation. Significant gradients of kinetic temperature along molecular filaments traced by H2CS indicate that the dense gas is heated by the shocks induced by the enigmatic explosive event which occurred several hundred years ago and greatly affected the energetics of the Orion KL region. Thus, with the notable exception of the western region of the compact ridge, the high temperatures of the dense gas in Orion KL are probably caused by shocks from the explosive event, leading to a dominant component of externally heated dense gas.
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