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1.
In this study we investigated the effects of external trigger on the characteristics of young stellar objects (YSOs) associated with cometary globules (CGs). We made optical spectroscopy of stars associated with star-forming CGs. We find that the masses of the most massive stars associated with CGs are correlated with the masses of the parent cloud but they are systematically larger than expected for clouds of similar mass from the relation M max-star=0.33M cl 0.43 given by Larson (Mon. Not. R. Astron. Soc. 200:159, 1982). We have also estimated the luminosities of the IRAS sources found associated with CGs as a function of cloud mass and then compared them with those of the IRAS sources found associated with isolated opacity class 6 clouds (isolated and relatively away from large star forming regions). We find that the luminosities of IRAS sources associated with CGs are larger than those of the opacity class 6 clouds. These findings support results from recent simulations in which it was shown that the Radiation Driven Implosion (RDI) process, believed to be responsible for the cometary morphology and star formation, can increase the luminosity 1–2 orders of magnitudes higher than those of protostars formed without external triggering due to an increase in accretion rates. Thus implying that the massive stars can have profound influence on the star formation in clouds located in their vicinity.  相似文献   

2.
We review some of the recent water maser proper motion measurements in star-forming regions performed through VLBI multi-epoch observations. These observations are starting to reveal exciting perspectives, providing the full kinematics of the gas within the outflows/circumstellar disks around YSOs at scales of AUs, discovering new phenomena (e.g., isotropic mass ejections, water maser micro-structures exhibiting remarkable coherent and well ordered spatio-kinematical behavior at AU scale), and opening new, puzzling questions related to the early stellar evolution.  相似文献   

3.
We present results from a survey of the Rosette Molecular Cloud (RMC) using both the Infrared Array Camera (IRAC) and Multiband Imaging Photometer for Spitzer (MIPS) onboard the Spitzer Space Telescope . We have mapped a region of active star formation covering an area approximately 1° by 1.5° including several previously known clusters. Spectral energy distributions (SEDs) fitted to our data combined with that from Two Micron All Sky Survey (2MASS) are used to identify young stellar objects (YSOs) with infrared (IR) excesses. We find that roughly 50 per cent of the sources are forming in clustered environments and identify seven clusters of IR excess sources including four that were previously unknown. We investigate evidence for triggering of star formation due to the ionization front, identified in Brackett-α emission, associated with the young open cluster NGC 2244. Although the position of several of the clusters of IR excess sources are coincident with the ionization front, the bulk of the youngest YSOs are located far from the ionization front, in clusters located along the mid-plane of the cloud. We conclude that although triggering from the H  ii nebula is a possible origin for some of the recent star formation, the majority of the active star formation is occurring in already dense regions of the cloud not compressed by the expansion of the H  ii region.  相似文献   

4.
We report the first infrared proper motion measurements of the HerbigHaro objects in OMC-1 using a 4-yr time baseline. The [Fe  ii ]-emitting bullets are moving of the order of 0.08 arcsec per year, or at about 170 km s1. The direction of motion is similar to that inferred from their morphology. The proper motions of H2-emitting wakes behind the [Fe  ii ] bullets, and of newly found H2 bullets, are also measured. H2 bullets have smaller proper motion than [Fe  ii ] bullets, while H2 wakes with leading [Fe  ii ] bullets appear to move at similar speeds to their associated bullets. A few instances of variability in the emission can be attributed to dense, stationary clumps in the ambient cloud being overrun, setting up a reverse-oriented bullet. Differential motion between [Fe  ii ] bullets and their trailing H2 wakes is not observed, suggesting that these are not separating, and also that they have reached a steady-state configuration over at least 100 yr. The most distant bullets have, on average, larger proper motions, but are not consistent with free expansion. Nevertheless, an impulsive, or short-lived (<<1000 yr), duration for their origin seems likely.  相似文献   

5.
As part of a new southern sky survey for faint high proper motion stars based on Automatic Plate Measuring (APM) measurements of UK Schmidt Telescope plates, we have found a large number of previously unknown brighter objects. Spectroscopic follow-up observations with the European Southern Observatory New Technology Telescope of 15 of these new, relatively bright     high proper motion stars     show one-third of them to be nearby     . Among the nearby stars is an M6 dwarf with strong emission lines at a spectroscopic distance of about 11 pc and an M4 dwarf at about 13 pc. Coupled with earlier South African Astronomical Observatory spectroscopic observations of three similar bright high proper motion stars, the success rate of finding nearby stars     is about 45 per cent. All newly discovered nearby stars have disc kinematics confirmed by radial velocity measurements from our spectra. In addition there are several high-velocity stars with halo kinematics in the sample, mainly subdwarfs, at about 60 to 110 pc distance. These high-velocity stars are interesting targets for further study of the Galactic escape velocity. One of the detected nearby high proper motion stars was formerly thought to be an M giant in the Small Magellanic Cloud. The spectrum of one M3 star shows a strong blue continuum, which is likely to signify the presence of a hot companion. Spectroscopic follow-up observations of high proper motion stars are shown to be an effective tool in the search for the missing stars in the Solar neighbourhood. Candidates for more extensive trigonometric parallax determination can be selected on the basis of the spectroscopic distance estimates.  相似文献   

6.
The UK Infrared Telescope Infrared Deep Sky Survey (UKIDSS) is the first of a new generation of infrared surveys. Here, we combine the data from two UKIDSS components, the Large Area Survey (LAS) and the Galactic Cluster Survey (GCS), with Two-Micron All-Sky Survey (2MASS) data to produce an infrared proper motion survey for low-mass stars and brown dwarfs. In total, we detect 267 low-mass stars and brown dwarfs with significant proper motions. We recover all 10 known single L dwarfs and the one known T dwarf above the 2MASS detection limit in our LAS survey area and identify eight additional new candidate L dwarfs. We also find one new candidate L dwarf in our GCS sample. Our sample also contains objects from 11 potential common proper motion binaries. Finally, we test our proper motions and find that while the LAS objects have proper motions consistent with absolute proper motions, the GCS stars may have proper motions which are significantly underestimated. This is possibly due to the bulk motion of some of the local astrometric reference stars used in the proper motion determination.  相似文献   

7.
质量是恒星最基本、最重要的物理量之一,它在恒星形成和演化过程中起着至关重要的作用.年轻星天体的形成机制以及早期演化过程是彻底解决恒星形成问题的关键所在,而年轻星天体形成过程中的物质下落、质量吸积以及外流等物理过程都与年轻星天体的质量密切相关.该文介绍了确定年轻星天体质量的一些常用方法,包括利用开普勒定律的动力学方法、赫罗图方法、年轻星天体脉动方法和磁层吸积方法等.并对这些方法的适用范围及测量精度进行了评述,还综述了各种方法的研究进展及未来可能的发展.  相似文献   

8.
We present preliminary results of our X‐shooter survey in star forming regions. In this contribution we focus on subsamples of young stellar and sub‐stellar objects (YSOs) in the Lupus star forming region and in the TW Hya association. We show that the X‐shooter spectra are suitable for conducting several parallel studies such as YSO + disk fundamental parameters, accretion and outflow activity in the very low‐mass (VLM) and sub‐stellar regimes, as well as magnetic activity in young VLM YSOs, and Li abundance determinations. The capabilities of X‐shooter in terms of wide spectral coverage, resolution and limiting magnitudes, allow us to assess simultaneously the accretion/outflow, magnetic activity, and disk diagnostics, from the UV and optical to the near‐IR, avoiding ambiguities due to possible YSO variability (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)  相似文献   

9.
A deep Hα image of interlocking filamentary arcs of nebulosity has been obtained with a wide-field (≈30° diameter) narrow-band filter camera combined with a charge-coupled device as a detector. The resultant mosaic of images, extending to a galactic latitude of −65°, has been corrected for field distortions and had galactic coordinates superimposed on it to permit accurate correlations with the most recent H  i (21 cm), X-ray (0.75 keV) and FIR ( IRAS 100 μm) maps.
Furthermore, an upper limit of 0.13 arcsec yr−1 to the expansion proper motion of the primary 25° long nebulous arc has been obtained by comparing a recent Hα image obtained with the San Pedro Martir telescope of its filamentary edge with that on a Palomar Observatory Sky Survey E plate obtained in 1951.
It is concluded that these filamentary arcs are the superimposed images of separate shells (driven by supernova explosions and/or stellar winds) rather than the edges of a single 'superbubble' stretching from Barnard's Arc (and the Orion Nebula) to these high galactic latitudes. The proper motion measurement argues against the primary Hα-emitting arc being associated with the giant radio loop (Loop 2) except in extraordinary circumstances.  相似文献   

10.
There is currently a clear discrepancy in the proper motions measured on different angular scales in the approaching radio jets of the black hole X-ray binary GRS 1915+105. Lower velocities were measured with the Very Large Array (VLA) prior to 1996 than were subsequently found from higher resolution observations made with the Very Long Baseline Array and the Multi-Element Radio Linked Interferometer Network. We initiated an observing campaign to use all three arrays to attempt to track the motion of the jet knots from the 2006 February outburst of the source, giving us unprecedented simultaneous coverage of all angular scales, from milliarcsecond scales out to arcsecond scales. The derived proper motion, which was dominated by the VLA measurements, was found to be 17.0 mas d−1, demonstrating that there has been no significant permanent change in the properties of the jets since 1994. We find no conclusive evidence for deceleration of the jet knots, unless this occurs within 70 mas of the core. We discuss possible causes for the varying proper motions recorded in the literature.  相似文献   

11.
The globule CB 34, which harbours a cluster of class 0 young stellar object (YSO) protostars, has been investigated through a multiline SO and SiO survey at millimetre wavelengths. The SO data reveal that the globule consists of three quiescent high-density (∼105 cm−3) clumps, labelled A, B and C, with sizes of ∼  0.2–0.3 pc  . The SiO data provide evidence for high-velocity gas across the globule. Most likely, the high-velocity gas is distributed in three distinct high-velocity outflows associated with the YSOs in each of the three clumps. High-velocity SO features have been detected only towards the two brightest SiO outflows. These broad SO components exhibit spatial and spectral distributions which are consistent with those of the SiO emission, so they can also be used as tracers of the outflows.
The comparison between the spatial and spectral properties of the SO and SiO emissions in the three clumps suggests different evolutionary stages for the embedded YSOs. In particular, the YSO associated with clump C exhibits some peculiarities, namely smaller SiO linewidths, lower SiO column densities, a lack of extended SiO structure and of SO wings, and the presence of a SO spatial distribution which is displaced with respect to the location of the YSO. This behaviour is well explained if the SiO and SO molecules which were produced at high velocities in the shocked region have been destroyed or slowed down because of the interaction with the ambient medium, and the chemistry is dominated again by low-temperature reactions. Thus our observations strongly suggest that the YSO in clump C is in a more evolved phase than the other members of the cluster.  相似文献   

12.
江治波  杨戟 《天文学进展》2000,18(4):320-335
分子氢的红外振动发射线是显现年轻星质量外流的重要谱线之一。自Gautier等人1976年在猎户座发现年轻星质量外流的分子氢发射开始,人们在银河系内几乎所有的恒星形成区都发现了这种线发射。研究表明,分子氢发射与年轻星周围的其它活动现象（如分子外流和光学喷流）之间有着非常密切的联系。红外和光学喷流代表了年轻星剧烈活动的两个侧面,是喷流与周围介质相互作用强弱不同的表现,这种作用还拖带周围介质,产生分子外流,光学、红外喷流和分子外流组成了恒星形成区壮观的景象,它们是恒星形成活动的重要标志。随着红外探测技术的飞速发展,对年轻星外流活动现象的观测越来越丰富的详细,使人们对这种现象的本质越来越了解。在20世纪90年代NICMOS等大阵列红外探测器投入使用后,红外成像观测有了长足的进步。目前已在70个左右的区域里发现了H2发射,这一数字还在迅速增加,今后的研究主要可能向两个方向发展。其一是高分辨观测,进一步了解H2发射的结构以及与光学喷流和分子外流之间的关系;其二是天观测,了解银河系内的恒星形成H2区发射的大尺度结构和恒星形成的统计分布规律。  相似文献   

13.
We present the largest publicly available catalogue of compact groups (CGs) of galaxies identified using the original selection criteria of Hickson, selected from the Sixth Data Release of the Sloan Digital Sky Survey (SDSS DR6). We identify 2297 CGs down to a limiting magnitude of   r = 18 (∼0.24 groups  deg−2), and 74 791 CGs down to a limiting magnitude of   r = 21 (∼6.7 groups  deg−2). This represents 0.9 per cent of all galaxies in the SDSS DR6 at these magnitude levels. Contamination due to gross photometric errors has been removed from the bright sample of groups, and we estimate it is present in the large sample at the 14 per cent level. Spectroscopic information is available for 4131 galaxies in the bright catalogue (43 per cent completeness), and we find that the median redshift of these groups is   z med= 0.09  . The median line-of-sight (LOS) velocity dispersion within the CGs from the bright catalogue is  σLOS≃ 230 km s−1  , and their typical intergalactic separations are of the order of 50–100 kpc. We show that the fraction of groups with interloping galaxies identified as members is in good agreement with the predictions from our previous study of a mock galaxy catalogue, and we demonstrate how to select CGs such that the interloper fraction is well defined and minimized. This observational data set is ideal for large statistical studies of CGs, the role of environment on galaxy evolution and the effect of galaxy interactions in determining galaxy morphology.  相似文献   

14.
A statistical study of 233 pulsar proper motions   总被引：2，自引：0，他引：2  
We present and analyse a catalogue of 233 pulsars with proper motion measurements. The sample contains a wide variety of pulsars including recycled objects and those associated with globular clusters or supernova remnants. After taking the most precise proper motions for those pulsars for which multiple measurements are available, the majority of the proper motions (58 per cent) are derived from pulsar timing methods, 41 per cent using interferometers and the remaining 1 per cent using optical telescopes. Many of the one-dimensional (1D) and two-dimensional (2D) speeds (referring to speeds measured in one coordinate only and the magnitudes of the transverse velocities, respectively) derived from these measurements are somewhat lower than earlier estimates because of the use of the most recent electron density model in determining pulsar distances. The mean 1D speeds for the normal and recycled pulsars are 152(10) and 54(6) km s−1, respectively. The corresponding mean 2D speeds are 246(22) and 87(13) km s−1. PSRs B2011+38 and B2224+64 have the highest inferred 2D speeds of  ∼1600 km s−1  . We study the mean speeds for different subsamples and find that, in general, they agree with previous results. Applying a novel deconvolution technique to the sample of 73 pulsars with characteristic ages less than 3 Myr, we find the mean three-dimensional (3D) pulsar birth velocity to be 400(40) km s−1. The distribution of velocities is well described by a Maxwellian distribution with  1D rms σ= 265 km s−1  . There is no evidence for a bimodal velocity distribution. The proper motions for PSRs B1830−08 and B2334+61 are consistent with their proposed associations with the supernova remnants W41 and G114.3+0.3, respectively.  相似文献   

15.
The results of a near-infrared ( J H K L P) imaging linear polarimetry survey of 20 young stellar objects (YSOs) in ρ Ophiuchi are presented. The majority of the sources are unresolved, with K -band polarizations, P K < 6 per cent. Several objects are associated with extended reflection nebulae. These objects have centrosymmetric vector patterns with polarization discs over their cores; maximum polarizations of P K > 20 per cent are seen over their envelopes. Correlations are observed between the degree of core polarization and the evolutionary status inferred from the spectral energy distribution. K -band core polarizations >6 per cent are only observed in Class I YSOs.
A 3D Monte Carlo model with oblate grains aligned with a magnetic field is used to investigate the flux distributions and polarization structures of three of the ρ Oph YSOs with extended nebulae. A ρ∝ r −1.5 power law for the density is applied throughout the envelopes. The large-scale centrosymmetric polarization structures are due to scattering. However, the polarization structure in the bright core of the nebula appears to require dichroic extinction by aligned non-spherical dust grains. The position angle indicates a toroidal magnetic field in the inner part of the envelope. Since the measured polarizations attributed to dichroic extinction are usually ≤10 per cent, the grains must either be nearly spherical or very weakly aligned. The higher polarizations observed in the outer parts of the reflection nebulae require that the dust grains responsible for scattering have maximum grain sizes 1.05 μm.  相似文献   

16.
Greaves (2006) proposed that three red, high proper motion stars within 10° of 51 Peg (NLTT 54007, 54064, and 55547) are co‐moving companions to this famous exoplanet host star. While the stars clearly have proper motions similar to 51 Peg, the inferred kinematic parallaxes for these stars produce extremely inconsistent color‐magnitude positions 2 to 4 magnitudes below the main sequence. All three stars are likely to be background stars unrelated to 51 Peg (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)  相似文献   

17.
Jets and outflows are ubiquitous in the process of formation of stars since outflow is intimately associated with accretion. Free–free (thermal) radio continuum emission in the centimeter domain is associated with these jets. The emission is relatively weak and compact, and sensitive radio interferometers of high angular resolution are required to detect and study it. One of the key problems in the study of outflows is to determine how they are accelerated and collimated. Observations in the cm range are most useful to trace the base of the ionized jets, close to the young central object and the inner parts of its accretion disk, where optical or near-IR imaging is made difficult by the high extinction present. Radio recombination lines in jets (in combination with proper motions) should provide their 3D kinematics at very small scale (near their origin). Future instruments such as the Square Kilometre Array (SKA) and the Next Generation Very Large Array (ngVLA) will be crucial to perform this kind of sensitive observations. Thermal jets are associated with both high and low mass protostars and possibly even with objects in the substellar domain. The ionizing mechanism of these radio jets appears to be related to shocks in the associated outflows, as suggested by the observed correlation between the centimeter luminosity and the outflow momentum rate. From this correlation and that of the centimeter luminosity with the bolometric luminosity of the system it will be possible to discriminate between unresolved HII regions and jets, and to infer additional physical properties of the embedded objects. Some jets associated with young stellar objects (YSOs) show indications of non-thermal emission (negative spectral indices) in part of their lobes. Linearly polarized synchrotron emission has been found in the jet of HH 80–81, allowing one to measure the direction and intensity of the jet magnetic field, a key ingredient to determine the collimation and ejection mechanisms. As only a fraction of the emission is polarized, very sensitive observations such as those that will be feasible with the interferometers previously mentioned are required to perform studies in a large sample of sources. Jets are present in many kinds of astrophysical scenarios. Characterizing radio jets in YSOs, where thermal emission allows one to determine their physical conditions in a reliable way, would also be useful in understanding acceleration and collimation mechanisms in all kinds of astrophysical jets, such as those associated with stellar and supermassive black holes and planetary nebulae.  相似文献   

18.
We present high-resolution echelle spectroscopy of 20 stars in 16 systems catalogued as members of the TW Hydrae association, and 16 stars identified as possible new members. We have calibrated the range of coronal and chromospheric activity expected for such young stars as a function of spectral type by combining our observations with literature data for field and open cluster stars. We also compute space motions for TWA members and candidate members with proper motion measurements, using two techniques to estimate distances to stars lacking direct trigonometric parallax measurements. The mean space motion of the four TWA members with known parallaxes is  ( U , V , W : −10.0, −17.8, −4.6) km s−1  . 14 of the candidates have properties inconsistent with cluster membership; the remaining two are potential new members, although further observations are required to confirm this possibility.  相似文献   

19.
The excellent spatial resolution of the Chandra observatory offers the unprecedented possibility to measure proper motions at X-ray wavelength with relatively high accuracy using as reference the background of extragalactic or remote galactic X-ray sources. We took advantage of this capability to constrain the proper motion of RX J0806.4-4123 and RX J0420.0-5022, two X-ray bright and radio quiet isolated neutron stars (INSs) discovered by ROSAT and lacking an optical counterpart. In this paper, we present results from a preliminary analysis from which we derive 2σ upper limits of 76 mas/yr and 138 mas/yr on the proper motions of RX J0806.4-4123 and RX J0420.0-5022 respectively. We use these values together with those of other ROSAT discovered INSs to constrain the origin, distance and evolutionary status of this particular group of objects. We find that the tangential velocities of radio quiet ROSAT neutron stars are probably consistent with those of ‘normal’ pulsars. Their distribution on the sky and, for those having accurate proper motion vectors, their possible birth places, all point to a local population, probably created in the part of the Gould Belt nearest to the earth.   相似文献   

20.
The chemical and thermal structure of dense gas irradiated by FUV and X-ray photons is explored. The relevance of these so-called PDRs (FUV) and XDRs (X-ray) for AGN and YSOs is discussed. Observational line diagnostics and chemical tracers are summarized.  相似文献   

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