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1.
We have detected the rotational lines of HCOOCH3 toward a Class 0 low-mass protostar, NGC1333 IRAS4B, which is reported to be extremely young according to the dynamical age of the molecular outflow (a few 100 yr). This suggests that the complex organic molecules appear from the very early stage of protostellar evolution. On the other hand, the complex organic molecules are not detected in a more evolved protostar, L1527. We have also found a similar trend in a massive star forming region, NGC2264. The HCOOCH3 emission is almost absent toward IRS1, whereas it is concentrated near MMS3, which is younger than IRS1. In addition, the HCOOCH3 intensity peak is slightly shifted from the dust emission peak, as is seen in the Orion KL Compact Ridge, giving an important clue to solve its origin.  相似文献   

2.
A review is presented of the earliest stages of protostellar evolution. Observations of prestellar cores, which are believed to represent the initial conditions for protostellar collapse, depart significantly from the scale-free density distribution which is usually taken as the starting point for the formation of a low-mass protostar. Pre-stellar cores are observed to have radial density profiles which have flat inner regions, steepening towards their edges. This is seen to qualitatively match the predictions of the Bonnor-Ebert stability criterion for pressure-bounded self-gravitating gas clouds. From these initial conditions, theoretical modelling of cores threaded by magnetic fields predicts that quasi-static evolution by the process of ambipolar diffusion will lead to a significantly different starting point for collapse than the static singular isothermal sphere.This departure from a scale-free density distribution for the initial conditions has recently been shown to produce an ensuing protostellar collapse which has a non-constant accretion rate. Recently published observations of low-mass protostars in the Ophiuchi cluster are demonstrated to be consistent with such a non-constant protostellar mass accretion rate, contrary to the standard protostellar collapse model. Instead, the data appear consistent with an initially high accretion rate, which subsequently decays. The initial phase of high accretion rate is labelled the main accretion phase, during which 50 per cent of the circumstellar envelope mass is accreted in 10 per cent of the total accretion time, and which is equated observationally with Class 0 objects. The subsequent phase with roughly an order of magnitude lower accretion rate is labelled the late accretion phase, during which the remainder of the envelope mass is accreted in the remaining 90 per cent of the total accretion time, at an order of magnitude lower accretion rate, and which is equated observationally with Class I objects. The growth of circumstellar discs begins in the Class 0 stage, and proceeds through the Class I and II stages. Published data of the Taurus star-forming region currently available appear also to be consistent with this scenario.  相似文献   

3.
We present our most recent results from an ongoing study of the Class 0 source Barnard 1c in Perseus. This source is of particular interest because it exhibits evidence of strong alignment of grains all the way to the core’s centre, which is contrary to all other low-mass protostellar cores observed to date. Our goal is to clarify the source of poor alignment in other sources by identifying the source of strong alignment in B1c. A central cavity has been identified in N2H+ emission; its anticorrelation with C18O emission suggests that heating in the centre has released CO from grain mantles, in turn destroying N2H+. We present sensitivity-limited, high spatial resolution polarimetry data from the SubMillimeter Array and discuss the potential implications of these data.  相似文献   

4.
We investigate the formation by accretion of massive primordial protostars in the range 10 to 300 M ⊙. The high accretion rate used in the models (M ⊙ = 4.4 x 10-3 M⊙ yr-1) causes the structure and evolution to differ significantly from those of both present-day protostars and primordial zero-age main sequence stars. The stellar surface is not visible throughout most of the main accretion phase, since a photosphere is formed in the in falling envelope. Significant nuclear burning does not take place until a protostellar mass of about 80 M ⊙. As the interior luminosity approaches the Eddington luminosity, the protostellar radius rapidly expands owing to the radiation pressure. Eventually, a final swelling occurs when the stellar mass reaches about 300 M ⊙. This expansion is likely to signal the end of the main accretion phase, thus setting an upper limit to the protostellar mass formed in these conditions. This revised version was published online in September 2006 with corrections to the Cover Date.  相似文献   

5.
We have performed millimeter- and submilli- meter-wave survey observations using the Nobeyama millimeter array (NMA) and the Atacama Submillimeter Telescope Experiment (ASTE) in one of the nearest intermediate-mass (IM) star-forming regions: Orion Molecular Cloud-2/3 (OMC-2/3). Using the high-resolution capabilities offered by the NMA (∼several arcsec), we observed dust continuum and H13CO+(1–0) emission in 12 pre- and proto-stellar candidates identified previously in single-dish millimeter observations. We unveiled the evolutionary changes with variations of the morphology and velocity structure of the dense envelopes traced by the H13CO+(1–0) emission. Furthermore, using the high-sensitivity capabilities offered by the ASTE, we searched for large-scale molecular outflows associated with these pre- and proto-stellar candidates observed with the NMA. As a result of the CO(3–2) observations, we detected six molecular outflows associated with the dense gas envelopes traced by H13CO+(1–0) and 3.3 mm continuum emission. The estimated CO outflow momentum increases with the evolutionary sequence from early to late type of the protostellar cores. We also found that the 24 μm flux increases as the dense gas evolutionary sequence. We propose that the enhancement of the 24 μm flux is caused by the growth of the cavity (i.e. the CO outflow destroys the envelope) as the evolutionary sequence. Our results show that the dissipation of the dense gas envelope plays an essential role in the evolution of the IM protostars. The extremely high-sensitivity and high-angular resolution offered by ALMA will reveal unprecedented details of the inner ∼50 AU of these protostars, which will provide us a break through in the classic scenario of IM star/disk formation.  相似文献   

6.
Close links between jet evolution and protostellar evolution are beginning to be understood. Firstly, stellar jets are reviewed here, establishing the accretion-outflow connection. Then, outflows from young stars are reviewed, suggesting a synchronised development in the star and outflow. This yields a unification scheme in which rising molecular jets dominate the early protostellar epoch, followed by a jet-driven outflow stage and, finally, a bow-driven ballistic stage. This scheme is quantified, yielding the systematic changes in the bolometric, mechanical and shock luminosities and the cross-over phase from dense molecular jets to light atomic jets. This revised version was published online in July 2006 with corrections to the Cover Date.  相似文献   

7.
Magnetically mediated disk outflows are a leading paradigm to explain winds and jets in a variety of astrophysical sources, but where do the fields come from? Since accretion of mean magnetic flux may be disfavored in a thin turbulent disk, and only fields generated with sufficiently large scale can escape before being shredded by turbulence, in situ field production is desirable. Nonlinear helical inverse dynamo theory can provide the desired fields for coronae and outflows. We discuss the implications for contemporary protostellar disks, where the (magneto-rotational instability (MRI)) can drive turbulence in the inner regions, and primordial protostellar disks, where gravitational instability drives the turbulence. We emphasize that helical dynamos are compatible with the magneto-rotational instability, and clarify the relationship between the two.  相似文献   

8.
We present a submillimetre continuum survey for accretion discs around seven embedded protostars in the Perseus and Serpens molecular clouds. Observations were made at frequencies between 339 and 357 GHz using the James Clerk Maxwell Telescope–Caltech Submillimeter Observatory single-baseline interferometer on Mauna Kea, Hawaii. All the objects in our survey show compact dust emission on scales ≲1 arcsec, assumed to arise in a circumstellar accretion disc. We compare the properties of this compact component with evolutionary indicators, such as the ratio of compact to extended emission, and bolometric temperature. We find that discs of mass ∼0.01 M⊙ have formed by the Class 0 stage, and that similar mass discs are observed in Class I and Class II sources. A trend is observed whereby the ratio of compact to extended emission in our sources increases from Class 0 to Class II sources. For three of the objects in the survey, NGC 1333 IRAS2:CR1 and SVS13 in Perseus, and FIRS1 in Serpens, the signal-to-noise ratio is sufficient to allow us to model the brightness distributions with elliptical Gaussian and power-law disc models. The Gaussian fits give semimajor half-power radii of approximately 90 to 140 au, at the assumed distance of 350 pc to the Perseus and Serpens clouds.  相似文献   

9.
Infrared dark clouds (IRDCs) are cold, dense molecular clouds identified as extinction features against the bright mid-infrared Galactic background. Our recent 1.2 mm continuum emission survey of IRDCs reveals many compact (<0.5 pc) and massive (10–2100 M⊙) cores within them. These prestellar cores hold the key to understanding IRDCs and their role in star formation. Here, we present high angular resolution spectral-line and mm/sub-mm continuum images obtained with the IRAM Plateau de Bure Interferometer and the Sub-Millimeter Array towards three high-mass IRDC cores. The high angular resolution images reveal that two of the cores are resolved into multiple, compact protostellar condensations, while the remaining core contains a single, compact protostellar condensation with a very rich molecular spectrum, indicating that it is a hot molecular core. The derived gas masses for these condensations suggest that each core is forming at least one high-mass protostar, while two of the cores are also forming lower-mass protostars. The close proximity of multiple protostars of disparate mass indicates that these IRDCs are in the earliest evolutionary states in the formation of stellar clusters.  相似文献   

10.
Most stars produce spectacular jets during their formation. There are thousands of young stars within 500 pc of the Sun and many power jets. Thus protostellar jets may be the most common type of collimated astrophysical outflow. Shocks powered by outflows excite many emission lines, exhibit a rich variety of structure, and motions with velocities ranging from 50 to over 500 km s−1. Due to their relative proximity, proper motions and structural changes can be observed in less than a year. I review the general properties of protostellar jets, summarize some results from recent narrow-band imaging surveys of entire clouds, discuss irradiated jets, and end with some comments concerning outflows from high-mass young stellar objects. Protostellar outflows are ideal laboratories for the exploration of the jet physics.  相似文献   

11.
We have obtained infrared colors and limiting magnitudes from 1.25–4.8µm for a sample of 26 of the cm continuum radio sources located in the core of the Oph molecular cloud. Their colors demonstrate that the majority of the sources appear to be heavily reddened objects surrounded by circumstellar accretion disks. In these cases the radio emission most likely diagnoses accretion driven energetic outflow phenomena: either ionized winds or possibly synchrotron emission from shocked gas associated with stellar jets.  相似文献   

12.
The scale invariance model (Heinz, S. and Sunyaev, R.A.: 2003, MNRAS 343, L59) can be used to derive robust scaling relations between the radio luminosity from accreting black holes and the black hole mass and accretion rate. These relations agree well with the recently found “fundamental plane” of black hole activity (Merloni, A., Heinz, S. and Di Matteo, T.: 2003, MNRAS 345, 1057). This relation provides a new, powerful tool for the comparison of jets from black holes of different masses and accretion rates. The regression coefficients of this relation contain information about the nature of the X-ray emission mechanism driving the correlation. We argue that X-ray synchrotron emission from the base of the jets is unlikely to be the dominant contribution to the X-ray spectrum in most of the sources.  相似文献   

13.
The “strange star - NDAF” model (NDAF: Neutrino Dominated Accretion Flow) is proposed as an alternative central engine of gamma-ray bursts for unifying the interpretation of the prompt emission and postburst activities of gamma-ray bursts. The structure of NDAF around a strange star is calculated. Different from other central compact objects, the strange star will feed back the phase transition energy of strangization on the accretion flow, with neutrinos as energy carriers. The friction between NDAF and strange star is ignored in this paper. The results indicate: firstly, the structure of NDAF around a strange star is sensitive to accretion rate; secondly, if accretion rate is larger than 0.18 M? s-1, the “strange star - NDAF” model can unify the explanation on the prompt emission and postburst activities of gamma-ray bursts, and the range of allowable accretion rates is wider than that in frictionless “neutron star - NDAF” models; thirdly, the range of annihilation energy of “strange star - NDAF” model is very wide, when the accretion rate is higher than 0.3 M? s-1, the annihilation energy is greater than 1051 erg; finally, if the accretion rate is greater than 0.3 M? s-1, the annihilation energy of “strange star - NDAF” model is larger than what of “black hole - NDAF” model at the same accretion rate by more than one order of magnitude, it is favorable to explaining some extremely energetic gamma-ray bursts.  相似文献   

14.
High velocity jets from young stars interact with the surrounding molecular environment and molecular outflows quite possibly are the result. This interaction can take place through the formation of a turbulent mixing layer. Models have been constructed (following Cant/'o and Raga) of a plane mixing layer in the boundary between a high velocity, atomic wind (i.e., the stellar jet) and a stationary, molecular environment, computed considering a detailed chemical network.The chemical composition of the mixing layer initially corresponds to the direct mixture of the (atomic) jet and (molecular) environmental material. However, we find that the mixing layer is hot (with temperatures exceeding 104 K), and the surprising only partial dissociation of H2 means that a number of molecules are either created or survive in the high velocity gas. This contrasts with the slower, cooler flows that have tended to be termed a molecular outflow.The emission from such atomic jet/molecular environment mixing layers is dominated by emission in the rotational and vibrational lines of H2. As a result of the high temperatures and velocities (ranging from zero to the jet velocity) of these mixing layers, the predicted H2 emission line spectrum has interesting characteristics.  相似文献   

15.
The extended X-ray emission observed at arcsec scales along the propagation trajectory of the precessing relativistic jets of the Galactic microquasar SS 433 features a broad emission line, with the position of the centroid being significantly different for the approaching and receding jets (≈7.3 and ≈6.4 keV, respectively). These observed line positions are at odds with the predictions of the kinematic model for any of the plausible bright spectral lines in this band, raising the question of their identification. Here we address this issue by taking into account time delays of the emission coming from the receding regions of the jets relative to that from the approaching ones, which cause a substantial phase shift and distortion of the predicted line positions for the extended (~1017 cm) emission compared to the X-ray and optical lines observed from the central source (emitted at distances ~1011 and ~1015 cm, respectively). We demonstrate that the observed line positions are fully consistent with the Fe XXVI Lyα (E 0 = 6.96 keV) line emerging from a region of size ~6 × 1016 cm along the jet. This supports the idea that intensive reheating of the jets up to temperatures >10 keV takes place at these distances, probably as a result of partial deceleration of the jets due to interaction with the surrounding medium, which might cause collisions between discrete dense blobs inside the jets.  相似文献   

16.
I examine the question of purely accreting protostars, and set limits to the breakout time of a protostellar wind within the accretion flow forming the new star. Hypothesizing a wind launched from the protostellar surface, three temporal phases are derived: a crushed wind, a trapped wind, and an escaping wind. In the current model, evolution from one phase to the next is a consequence of the growing anisotropy of the infalling flow, a natural outcome of the collapse of a rotating cloud core. During the crushed wind phase, infall overcomes the wind at all solid angles, and the accretion directly strikes the protostellar surface. The trapped phase consists of a wind sufficiently strong to push material back from the stellar surface, but too weak to carry the heavy, shocked and swept-up infall out of the star's gravitational potential. Unless the wind turns on impulsively, a significant fraction of the pre-breakout life of the protostar may be spent in this trapped wind phase in which gas is launched from the protostar but is pulled back, crashing onto the protostellar and disk surfaces. It may be that some `starless cores' contain as-yet undetected, very young accreting protostars, and that episodic luminosity fluctuations associated with this trapped wind could be observed.  相似文献   

17.
Whether jets from newly forming stars rotate is a fundamental question in star formation research. Theoretical models propose jet rotation as a means of removing angular momentum from the young star and disk system, thus allowing accretion. While widely accepted, this idea has not yet been tested observationally due to the high resolution requirement of examining jets close to their launching point. Previous findings from the Hubble Space Telescope Imaging Spectrograph (HST/STIS) and Owens Valley Radio Observatory (OVRO) give indications of same rotation of the jet and disk respectively, of T Tauri star DG Tau. We report preliminary findings from STIS data for 3 of 8 sources in a current survey to establish conclusively whether protostellar jets rotate. The results were positive, yielding evidence of radial velocity differences about the axis at the base of all three jets of 10–25 km s?1.  相似文献   

18.
The ultraviolet spectra of the star RU Lup obtained with the Hubble Space Telescope are analyzed. Emission lines are identified. The presence of absorption components with a nearly zero residual intensity in the Mg II resonance doublet lines is indicative of mass outflow with a velocity V ∞?300 km s?1. These lines also exhibit a broad (?1400 km s?1 at the base) component originating in the star itself. The profiles of the (optically thin) Si II] and Si III]1892 Å lines for the first time unequivocally prove that these lines originate in an accretion shock wave rather than in the chromosphere, with the gas infall velocity being V 0?400 km s?1. The intensity ratio of the C IV 1550 Å and Si IV 1400 Å resonance doublet components was found to be close to unity, suggesting a high accreted-gas density, logN 0>12.5. Molecular H2 Lyman lines formed in the stellar wind were detected. The H I Lα luminosity of RU Lup was found from their intensities to exceed 10% of L bol. Radiation pressure in the Lα line on atomic hydrogen may play a significant role in the initial acceleration of stellar-wind matter, but the effect of Lα emission on the dynamics of molecular gas is negligible.  相似文献   

19.
《New Astronomy Reviews》2000,44(4-6):235-240
The analysis of the long-slit spectral observations of 40 Wolf-Rayet (WR) galaxies with heavy element mass fraction ranging over two orders of magnitudes from Z⊙/50 to 2Z⊙ are presented. We derive the number of O stars from the luminosity of the Hβ emission line, the number of early carbon Wolf-Rayet stars (WCE) from the luminosity of the red bump (broad CIV λ5808 emission) and the number of late nitrogen Wolf-Rayet stars (WNL) from the luminosity of the blue bump (broad emission near λ4650). We identified some of weak WR emission lines, most often the N III λ4512 and Si III λ4565 lines, which have very rarely or never been seen and discussed before in WR galaxies. A new technique for deriving the number of WNL stars (WN7–WN8) from the N III λ4512 and the number of WN9–WN11 from Si III λ4565 emission lines has been proposed. This technique is potentially more precise than the blue bump method because it does not suffer from contamination of WCE and early WN (WNE) stars and nebular gaseous emission. We find that the fraction of WR stars relative to all massive stars increases with increasing metallicity, in agreement with predictions of evolutionary synthesis models. The relative number ratios N(WC)/N(WN) and the equivalent widths of the blue and red bumps derived from observations are also in satisfactory agreement with theoretical predictions, except for the most metal-deficient WR galaxies. A possible source of disagreement is too low a line emission luminosity adopted for a single WCE star in low-metallicity models.  相似文献   

20.
We demonstrate that even in the absence of flares there are very often volumes of hot plasma in the corona above active regions with temperatures in excess of 10 million degrees. Characteristics of this hot plasma and its time variations seem to be different in active regions of different phase of development. These hot plasma regions are sources of very weak, but clearly recognizable, X-ray emission above 3.5 keV. Long-lived X-ray brightenings, 104 times weaker than a flare, but lasting up to 10 hr occur predominantly along the H ∥ = 0 line, apparently low in the corona. After major flares, long-lived X-ray emission is also radiated from tops of arches extending high into the corona. Some other long-lived sources, far from the H∥ = 0 line, may be associated with newly emerging flux. Short-lived X-ray sources, with fluxes ranging from subflare levels to 10?3 times the flare flux, last for 2 to more than 30 min and are probably microflares. They seem to be most frequent in growing young active regions and appear often in areas with newly emerging flux.  相似文献   

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