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1.
It is generally accepted that the lifetime of molecular clouds does not exceed 3×107 yr due to disruption by stellar feedback. We put together some arguments giving evidence that a substantial fraction of molecular clouds (primarily in the outer regions of a disc) may avoid destruction process for at least 108 yr or even longer. A molecular cloud can live long if massive stars are rare or absent. Massive stars capable to destroy a cloud may not form for a long time if a cloud is low massive, or stellar initial mass function is top-light, or if there is a delay of the beginning of active star formation. A long duration of the inactive phase of clouds may be reconciled with the low amount of the observed starless giant molecular clouds if to propose that they were preceded by slowly contraction phase of the magnetized dark gas, non-detected in CO-lines.  相似文献   

2.
In order to investigate the differences between the molecular clouds which are associated with the massive star forming regions and those which are not, we have performed the single-dish simultaneous observations of 12CO J=2-1 and J=3-2 lines toward a sample of 59 Spitzer Extended Green Objects (EGOs) as the massive star forming regions in the northern sky. Combining our results with the data of the 12CO J=1-0 observations toward the same sample EGOs in the literature, we have made the statistical comparisons on the intensities and linewidths of multiple 12CO lines between the molecular clouds associated with EGOs (EGO molecular clouds, in brief) and other non-EGO molecular clouds. On this basis, we have discussed the effects of the gas temperature, density, and velocity field distributions on the statistical characteristics of the two kinds of molecular clouds. It is found that both the EGO molecular clouds and non-EGO molecular clouds have similar mass ranges, hence we conclude that for the formation of massive stars, the key-important factor is probably not the total mass of a giant molecular cloud (GMC), but the volume filling factor of the molecular clumps in the GMC (or the compression extent of the molecular gas in the cloud).  相似文献   

3.
We present a new survey for Hα emission objects in the Circinus cloud complex and introduce an efficient photometric method for detecting Hα emission via observations in a narrow‐band filter. The observed flux is compared to a blackbody fit of the continuum. Our search strategy reveals 20 stars with strong Hα emission (EW > 10 Å), eight of them being new detections. All Hα stars display infrared excess corroborating their youth. On the other hand, the region contains a number of infrared excess objects that do not show Hα emission. Our results support the picture that accretion – as witnessed by Hα emission – is a highly variable phenomenon. Therefore, Hα surveys can only trace the temporarily active objects. In contrast, infrared excess is a more robust tracer that reveals most of the population of young stellar objects in a star forming region. Our analysis of the general stellar content of the region yields a reliable distance of 450 pc for the Circinus cloud. Moreover, we find a ratio of total‐to‐selective extinction of RV = 2.8 suggesting that smaller‐than‐normal dust grains may be present. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)  相似文献   

4.
The near-infrared (NIR) extinction power-law index (β) and its uncertainty is derived from three different techniques based on star counts, colour excess and a combination of them. We have applied these methods to Two Micron All Sky Survey (2MASS) data to determine maps of β and NIR extinction of the small cloud IC 1396 W. The combination of star counts and colour excess results in the most reliable method to determine β. It is found that the use of the correct β map to transform colour excess values into extinction is fundamental for column density profile analysis of clouds. We describe how artificial photometric data, based on the model of stellar population synthesis of the Galaxy, can be used to estimate uncertainties and derive systematic effects of the extinction methods presented here. We find that all colour excess based extinction determination methods are subject to small but systematic offsets, which do not affect the star counting technique. These offsets occur since stars seen through a cloud do not represent the same population as stars in an extinction-free control field.  相似文献   

5.
We present a fully sampled C18O (1–0) map towards the southern giant molecular cloud (GMC) associated with the H  ii region RCW 106, and use it in combination with previous 13CO (1–0) mapping to estimate the gas column density as a function of position and velocity. We find localized regions of significant 13CO optical depth in the northern part of the cloud, with several of the high-opacity clouds in this region likely associated with a limb-brightened shell around the H  ii region G333.6−0.2. Optical depth corrections broaden the distribution of column densities in the cloud, yielding a lognormal distribution as predicted by simulations of turbulence. Decomposing the 13CO and C18O data cubes into clumps, we find relatively weak correlations between size and linewidth, and a more sensitive dependence of luminosity on size than would be predicted by a constant average column density. The clump mass spectrum has a slope near −1.7, consistent with previous studies. The most massive clumps appear to have gravitational binding energies well in excess of virial equilibrium; we discuss possible explanations, which include magnetic support and neglect of time-varying surface terms in the virial theorem. Unlike molecular clouds as a whole, the clumps within the RCW 106 GMC, while elongated, appear to show random orientations with respect to the Galactic plane.  相似文献   

6.
Gas to Dust Ratio (GDR) indicates the mass ratio of interstellar gas to dust. It is widely adopted that the GDR in our Galaxy is 100～150. We choose three typical star forming regions to study the GDR: the Orion molecular cloud — a massive star forming region, the Taurus molecular cloud — a low-mass star forming region, and the Polaris molecular cloud — a region with no or very few star formation activities. The mass of gas only takes account of the neutral gas, i.e. only the atomic and molecular hydrogen, because the amount of ionized gas is very small in a molecular cloud. The column density of atomic hydrogen is taken from the high-resolution and high-sensitivity all-sky survey EBHIS (Effelsberg-Bonn HI Survey). The CO J = 1 →0 line is used to trace the molecular hydrogen, since the spectral lines of molecular hydrogen which can be detected are rare. The intensity of CO J = 1 →0 line is taken from the Planck all-sky survey. The mass of dust is traced by the interstellar extinction based on the 2MASS (Two Micron All Sky Survey) photometric database in the direction of anti-Galactic center. Adopting a constant conversion coefficient from the integrated intensity of the CO line to the column density of molecular hydrogen, XCO = 2.0 × 1020 cm?2 · (K · km/s)?1, the gas to dust ratio N(H)/AV is calculated, which is 25, 38, and 55 (in units of 1020 cm?2 · mag?1) for the Orion, Taurus, and Polaris molecular clouds, respectively. These values are significantly higher than the previously obtained average value of the Galaxy. Adopting the WD01 interstellar dust model (when the V-band selective extinction ratio is RV = 3.1), the derived GDRs are 160, 243, and 354 for the Orion, Taurus, and Polaris molecular clouds, respectively, which are apparently higher than 100～150, the commonly accepted GDR of the diffuse interstellar medium. The high N(H)/AV values in the star forming regions may be explained by the growth of dust in the molecular clouds because of either the particle collision or accretion, which can lead to the reduction of extinction efficiency per unit mass in the V band, rather than the increase of the GDR itself.  相似文献   

7.
We present the first results of a submillimetre continuum survey of Lynds dark clouds. Submillimetre surveys of star-forming regions are an important tool with which to obtain representative samples of the very first phases of star formation. Maps of 24 small clouds were obtained with SCUBA, the bolometer array receiver at the James Clerk Maxwell Telescope, and 19 clouds were detected. The total dark cloud area surveyed was ∼130 arcmin2, and a total gas mass of 90 M⊙ was detected. The dust emission is in general in good agreement with the extinction of optical starlight. The observed clouds contain a newly discovered protostar in L944, and a previously known protostar IRAS 23228+4320 in L1246. Another eight starless cores, either gravitationally unbound or pre-stellar in nature, were also detected. All starless cores and protostars were detected in only seven clouds, and the remaining 17 clouds seem quiescent and do not show any signs of recent star formation activity. The 850-μm images of all detected clouds are presented, as well as 450-μm images of L328, L944, L1014 and L1262. The outflows of the protostars in L944 and L1246 were also discovered and were mapped in 12CO J =2→1. The detection of the young protostar in L944, which is not present in the IRAS Point Source Catalog, shows the capacity of submillimetre surveys to detect unknown protostars.  相似文献   

8.
We have mapped 16 molecular clouds toward a new OB association in the Pup-CMa region to derive their physical properties. The observations were carried out in the 12CO (J = 1 – 0) line with the Southern millimetre-wave Telescope at Cerro Tololo, Chile. Distances have been determined kinematically using the rotation curve of Brand with R⊙ = 8.5 kpc and V⊙ = 220 km/s. Masses have been derived adopting a CO luminosity to H2 conversion factor X = 3.8 . 1020 molecules cm-2 (K km/s)-1. The observed mean radial velocity of the clouds is comparable with the mean radial velocity of stars composing an OB association in Pup-CMa; it is in favor of the close connection of clouds with these stars. __________ Published in Astrofizika, Vol. 48, No. 4, pp. 491–501 (October–December, 2005).  相似文献   

9.
This paper is based on 2MASS photometry (J H Ks magnitudes) of 1172 Be stars. The observed mean intrinsic colours have been derived with aid of two‐colour diagrams for Be stars of luminosity classes Ie‐IIe, IIIe and IVe‐Ve. The obtained results are the first determinations of their intrinsic colours in the astronomical literature. The smoothed infrared colours are compared with those obtained for “normal” B stars. Several two‐colour diagrams and plots of observed and smoothed intrinsic colour versus spectral type of luminosity classes Ie‐IIe, IIIe and IVe‐Ve are presented. Generally the determined infrared intrinsic colours of Be stars (V – J)0, (V – H)0, and (V – Ks)o differ substantially from those of “normal” B stars. It is found that the intrinsic colours of B stars are generally bluer than Be stars of corresponding spectral type and luminosity class. The mean absolute visual magnitude Mv of 528 Be stars for luminosity classes Iae, Ibe‐Iabe, IIe, IIIe and IVe‐Ve is derived from HIPPARCOS parallaxes. The Mv calibration is compared with the existing ones. The Be stars are generally brighter than “normal” B stars of corresponding spectral types. (© 2015 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)  相似文献   

10.
As a result of feedback from massive stars, via their intense winds and/or supernova explosions, massive star‐forming regions are entirely filled with hot, X‐ray emitting plasmas, which escape into the ambient ISM. As shown recently by Townsley et al. for several “extreme” cases (Carina, M17, NGC 3576, NGC 3603, 30 Dor), by way of large Chandra ACIS mosaics, extra, non‐thermal emission lines are present on top of the standard lines emitted by hot plasmas. Some of them are very close to lines characteristic of charge‐exchange reactions between the hot plasma and the cold surrounding material, suggesting that this mechanism operates on large spatial scales (several 10 pc) in star‐forming regions in general. The connection with starburst galaxies is briefly mentioned, and it is pointed out that supernovae interacting with molecular clouds may also provide a good environment to look for charge exchange processes (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)  相似文献   

11.
Towards the high-latitude cloud MBM 40, we identify 3 dense molecular cores of M0.2–0.5 M, and sizes of 0.2 pc in diameter embedded in the H I cloud of 8 M which is observed to be extended along the northeast–southwest direction. The molecular cloud is located almost perpendicularly to the H I emission. We confirm the previous result of Magnani et al. that MBM 40 is not a site for new star formations. We found a very poor correlation between the H I and the IRAS 100 μm emissions, but the CO (1–0) and 100 μm emissions show a better correlation of WCO/I100=1±0.2 K km s−1 (MJy sr−1)−1. This ratio is larger by a factor of ≥5 than in dense dark clouds, which may indicate that the CO is less depleted in MBM 40 than in dense dark clouds.  相似文献   

12.
The dense molecular cloud cores that form stars, like other self-gravitating objects, undergo bulk oscillations. Just at the point of gravitational instability, their fundamental oscillation mode has zero frequency. We study, using perturbation theory, the evolution of a spherical cloud that possesses such a frozen mode. We find that the cloud undergoes a prolonged epoch of subsonic, accelerating contraction. This slow contraction occurs whether the cloud is initially inflated or compressed by the oscillation. The subsonic motion described here could underlie the spectral infall signature observed in many starless dense cores.  相似文献   

13.
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14.
The Spitzer Space Telescope allows for the .rst time to search systematically for very low luminosity (≲0.1 L⊙) objects (VeLLOs) associated with dense molecular cores. They may be the .rst candidate Class 0 sources with sub‐stellar masses. We describe such a source in the dense molecular core L1148. VeLLO natal cores show properties that are unusual for star‐forming cores. The low luminosity and in some cases the lack of prominent out.ow could be the result of the small gas supply near the VeLLO. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)  相似文献   

15.
In this paper we estimate the star formation efficiency using the assumption that star formation continues until the radiation pressure disrupts the cloud. The results that in the case of low/mediummass star formation the efficiency could be about five times higher than in the case of high-mass star formation.For a three-component star-forming system (low/medium-mass stars, high-mass stars, gas) we investigate the temporal behaviour and the final star formation efficiency. We can show that the efficiency in 104 M clouds is higher than in 106 M clouds. This supports our view that bound stellar systems form from medium-mass clouds, whereas OB associations form in the cores of giant molecular clouds. Furthermore, the effect of induced high-mass star formation may cause a change of the mass spectrum during the formation of an OB association.Paper presented at a Workshop on The Role of Dust in Dense Regions of Interstellar Matter, held at Georgenthal, G.D.R., in March 1986.  相似文献   

16.
通过N体数值模拟以不同粒子数对星系中巨分子云的形成的影响进行了研究。结果表明：在聚合形成机制下，当对不同数目的分子云取相同的平均密度时，基本分子云的有效半径与其相应质量的立方根成正比，巨分子云的碎裂率与模拟基本分子云的数目无关。  相似文献   

17.
In order to reveal the star-forming history of the molecular cloud complex we studied the intermediate mass stellar population in the Cepheus Flare region. (Paper I dealt with the distance and the young stellar object candidates of the region.) Correlating the IRAS Point Source Catalogue and Faint Source Catalogue positions with those of 1214 B8–A8 and 1760 F0–F5 type stars brighter than     and classified during an objective prism survey, we identified 19 stars showing far-infrared excess emission in the Cepheus Flare region. In addition to the 16 stars whose counterparts are given in the IRAS catalogues, we found three more stars with infrared excess not recognized before. In order to identify the young medium-mass stars associated with the Cepheus Flare molecular clouds we observed the optical spectra of the IR-excess stars, and using published optical photometry and the IRAS data we examined their spectral energy distributions (SEDs) and IRAS two-colour diagram. The observations resulted in the discovery of a new Herbig Ae/Be star, BD +68°1118, coinciding with IRAS 21169+6842. More evolved HAe/Be stars may be SAO 19953, BD +67°1314 and BD +69°1231, whose H α lines showed weak emission components. Possible β Pictoris- or Vega-type stars may be HD 203854, HD 212826 and HD 216486, whereas the far-infrared fluxes at the positions of BD +72°1018, HD 210806 and HD 217903 can be attributed to the heating of the interstellar environment. We used distances and radial velocities of the stars derived from the spectroscopy and published optical photometry as indicators of their relations to the clouds. Information on the environment of the observed stars deduced from the diffuse interstellar band at λ 6613 is briefly discussed.  相似文献   

18.
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   

19.
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20.
Low mass star formation may be triggered by the dynamical effects of radiation fields and winds from massive stars on nearby molecular material. The columns of neutral material observed at the edges of many Hii regions may be the tracers of this process. Magnetic fields are dynamically important in the molecular clouds from which new stars form, but their effect on the development of molecular columns has not been studied in detail. In this paper, I present initial MHD simulations of this process.  相似文献   

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