Unusual activity of outflow mass motion connected with infall events was recorded for the B0.5 IVe star FY CMa in February 1987 from both archival IUE spectrograms of circumstellar N v resonance lines and optical spec...Unusual activity of outflow mass motion connected with infall events was recorded for the B0.5 IVe star FY CMa in February 1987 from both archival IUE spectrograms of circumstellar N v resonance lines and optical spectra of Ha and HeI γ5876 A emission lines which showed inverse P Cygni-type profiles. We estimate the mass loss rate using ultraviolet Si IVresonance lines with expanding atmosphere modeling calculations, and describe how the radial pressure performs the dominant role in accelerating the stellar wind. We attempt to give a qualitative explanation for the activity observed for FY CMa in terms of circumstellar matter raining down to the star.展开更多
Debris disks are optically thin, almost gas-free dusty disks observed arounda significant fraction of main-sequence stars older than about 10 Myr. Since the circumstellar dust is short-lived, the very existence of the...Debris disks are optically thin, almost gas-free dusty disks observed arounda significant fraction of main-sequence stars older than about 10 Myr. Since the circumstellar dust is short-lived, the very existence of these disks is considered as evi-dence that dust-producing planetesimals are still present in mature systems, in whichplanets have formed – or failed to form – a long time ago. It is inferred that theseplanetesimals orbit their host stars at asteroid to Kuiper-belt distances and continuallysupply fresh dust through mutual collisions. This review outlines observational techniques and results on debris disks, summarizes their essential physics and theoreticalmodels, and then places them into the general context of planetary systems, uncovering interrelations between the disks, dust parent bodies, and planets. It is shownthat debris disks can serve as tracers of planetesimals and planets and shed light onthe planetesimal and planet formation processes that operated in these systems in thepast.展开更多
By means of a population synthesis code and by constructing a simple toy model of dust produced by asymptotic giant branch（AGB） stars, common envelope（CE） ejecta and binary merger ejecta, we estimate the dust prod...By means of a population synthesis code and by constructing a simple toy model of dust produced by asymptotic giant branch（AGB） stars, common envelope（CE） ejecta and binary merger ejecta, we estimate the dust product rates（DPRs） of these processes in the Milky Way. The total DPR from AGB stars is～ 6.7 × 10-4M yr-1, in which about 73% of dust grains are carbon, 24% are silicates and 3% are iron. The total DPR from CE ejecta is ～ 4.2 × 10-4M yr-1, in which about 83% of dust grains are silicates, about 12% are carbon and 5% are iron.The DPR from binary merger ejecta is less than 1/3 that from AGB stars or CE ejecta,and it could even be negligible under certain circumstances. Therefore, compared with AGB stars and CE ejecta, the contribution of dust produced by binary merger ejecta to total dust grains in the Milky Way is smaller or can be negligible.展开更多
A massive star is defined as one with mass greater than - 8-10.M⊙. Central to the on-going debate on how these objects [massive stars] come into being is the so-called Radiation Problem. For nearly forty years, it ha...A massive star is defined as one with mass greater than - 8-10.M⊙. Central to the on-going debate on how these objects [massive stars] come into being is the so-called Radiation Problem. For nearly forty years, it has been argued that the radiation field emanating from massive stars is high enough to cause a global re- versal of direct radial in-fall of material onto the nascent star. We argue that only in the case of a non-spinning isolated star does the gravitational field of the nascent star overcome the radiation field. An isolated non-spinning star is a non-spinning star without any circumstellar material around it, and the gravitational field beyond its surface is described exactly by Newton＇s inverse square law. The supposed fact that massive stars have a gravitational field that is much stronger than their radiation field is drawn from the analysis of an isolated massive star. In this case the gravitational field is much stronger than the radiation field. This conclusion has been erroneously extended to the case of massive stars enshrouded in gas and dust. We find that, for the case of a non- spinning gravitating body where we take into consideration the circumstellar material, at ,- 8-10M⊙, the radiation field will not reverse the radial in-fall of matter, but rather a stalemate between the radiation and gravitational field will be achieved, i.e. the infall is halted but not reversed. This picture is very different from the common picture that is projected and accepted in the popular literature where at -8-10 M⊙, all the circumstellar material, from the surface of the star right up to the edge of the molec- ular core, is expected to be swept away by the radiation field. We argue that massive stars should be able to start their normal stellar processes if the molecular core from which they form has some rotation, because a rotating core exhibits an Azimuthally Symmetric Gravitational Field which causes there to be an accretion disk and along this equatorial disk. The radiation field cannot be much stronger than the gravitational field, hence this equatorial accretion disk becomes the channel via which the nascent massive star accretes all of its material.展开更多
Atacama Large Millimetre/sub-millimetre Array(ALMA) observations of CO(1–0) and CO(2–1) emissions from the circumstellar envelope of the asymptotic giant branch(AGB) star EP Aqr have been made with four times better...Atacama Large Millimetre/sub-millimetre Array(ALMA) observations of CO(1–0) and CO(2–1) emissions from the circumstellar envelope of the asymptotic giant branch(AGB) star EP Aqr have been made with four times better spatial resolution than previously available. They are analysed with emphasis on the de-projection in space of the effective emissivity and flux of matter using as input a prescribed configuration of the velocity field, assumed to be radial. The data are found to display an intrinsic axisymmetry with respect to an axis making a small angle with respect to the line of sight. A broad range of wind configurations, from prolate(bipolar) to oblate(equatorial) has been studied and found to be accompanied by significant equatorial emission. Qualitatively, the effective emissivity is enhanced near the equator to produce the central narrow component observed in the Doppler velocity spectra and its dependence on star latitude generally follows that of the wind velocity with the exception of an omni-present depression near the poles. In particular, large equatorial expansion velocities produce a flared disc or a ring of effective emissivity and mass loss. The effect on the determination of the orientation of the star axis of radial velocity gradients, and possibly competing rotation and expansion in the equatorial disc, is discussed. In general,the flux of matter is found to reach a broad maximum at distances of the order of 500 AU from the star.Arguments are given that may be used to favour one wind velocity distribution over another. As a result of the improved quality of the data, a deeper understanding of the constraints imposed on morphology and kinematics has been obtained.展开更多
We present a statistical analysis of 482 6.7 GHz methanol maser sources from the available literature, on their maser emission and the characteristics of their associated infrared sources. On the color-color diagram, ...We present a statistical analysis of 482 6.7 GHz methanol maser sources from the available literature, on their maser emission and the characteristics of their associated infrared sources. On the color-color diagram, more than 70% of the objects fall within a very small region (0.57 ≤ [25-12] ≤ 1.30 and 1.30 ≤ [60-12] ≤ 2.50). This suggests that 6.7 GHz methanol maser emission occurs only within a very short evolutionary phase during the earliest stage of star formation. The velocity ranges of the masers belong to two main groups: one from 1 to 10 km s<SUP>?1</SUP>, and one from about 11 to 20 km s<SUP>?1</SUP>. These velocity ranges indicate that the masers are probably associated with both disks and outflows. The correlations between the maser and infrared flux densities, and between the maser and infrared luminosities, suggest that far-infrared radiation is a possible pumping mechanism for the masers which most probably originate from some outer molecular envelopes or disks.展开更多
基金This work was performed under the auspices of Max-Planck Society of Germanythe National NaturaI Science Foundation of China
文摘Unusual activity of outflow mass motion connected with infall events was recorded for the B0.5 IVe star FY CMa in February 1987 from both archival IUE spectrograms of circumstellar N v resonance lines and optical spectra of Ha and HeI γ5876 A emission lines which showed inverse P Cygni-type profiles. We estimate the mass loss rate using ultraviolet Si IVresonance lines with expanding atmosphere modeling calculations, and describe how the radial pressure performs the dominant role in accelerating the stellar wind. We attempt to give a qualitative explanation for the activity observed for FY CMa in terms of circumstellar matter raining down to the star.
基金Support by the German Research Foundation (Deutsche Forschungs-gemeinschaft, DFG), project number Kr 2164/8–1by the Deutscher Akademischer Austauschdienst(DAAD), project D/0707543by the International Space Science Institute in Bern, Switzerland("Exozodiacal Dust Disks and Darwin" working group
文摘Debris disks are optically thin, almost gas-free dusty disks observed arounda significant fraction of main-sequence stars older than about 10 Myr. Since the circumstellar dust is short-lived, the very existence of these disks is considered as evi-dence that dust-producing planetesimals are still present in mature systems, in whichplanets have formed – or failed to form – a long time ago. It is inferred that theseplanetesimals orbit their host stars at asteroid to Kuiper-belt distances and continuallysupply fresh dust through mutual collisions. This review outlines observational techniques and results on debris disks, summarizes their essential physics and theoreticalmodels, and then places them into the general context of planetary systems, uncovering interrelations between the disks, dust parent bodies, and planets. It is shownthat debris disks can serve as tracers of planetesimals and planets and shed light onthe planetesimal and planet formation processes that operated in these systems in thepast.
基金Supported by the National Natural Science Foundation of China
文摘By means of a population synthesis code and by constructing a simple toy model of dust produced by asymptotic giant branch（AGB） stars, common envelope（CE） ejecta and binary merger ejecta, we estimate the dust product rates（DPRs） of these processes in the Milky Way. The total DPR from AGB stars is～ 6.7 × 10-4M yr-1, in which about 73% of dust grains are carbon, 24% are silicates and 3% are iron. The total DPR from CE ejecta is ～ 4.2 × 10-4M yr-1, in which about 83% of dust grains are silicates, about 12% are carbon and 5% are iron.The DPR from binary merger ejecta is less than 1/3 that from AGB stars or CE ejecta,and it could even be negligible under certain circumstances. Therefore, compared with AGB stars and CE ejecta, the contribution of dust produced by binary merger ejecta to total dust grains in the Milky Way is smaller or can be negligible.
基金supported by the Republic of South Africa's National Research Foundation
文摘A massive star is defined as one with mass greater than - 8-10.M⊙. Central to the on-going debate on how these objects [massive stars] come into being is the so-called Radiation Problem. For nearly forty years, it has been argued that the radiation field emanating from massive stars is high enough to cause a global re- versal of direct radial in-fall of material onto the nascent star. We argue that only in the case of a non-spinning isolated star does the gravitational field of the nascent star overcome the radiation field. An isolated non-spinning star is a non-spinning star without any circumstellar material around it, and the gravitational field beyond its surface is described exactly by Newton＇s inverse square law. The supposed fact that massive stars have a gravitational field that is much stronger than their radiation field is drawn from the analysis of an isolated massive star. In this case the gravitational field is much stronger than the radiation field. This conclusion has been erroneously extended to the case of massive stars enshrouded in gas and dust. We find that, for the case of a non- spinning gravitating body where we take into consideration the circumstellar material, at ,- 8-10M⊙, the radiation field will not reverse the radial in-fall of matter, but rather a stalemate between the radiation and gravitational field will be achieved, i.e. the infall is halted but not reversed. This picture is very different from the common picture that is projected and accepted in the popular literature where at -8-10 M⊙, all the circumstellar material, from the surface of the star right up to the edge of the molec- ular core, is expected to be swept away by the radiation field. We argue that massive stars should be able to start their normal stellar processes if the molecular core from which they form has some rotation, because a rotating core exhibits an Azimuthally Symmetric Gravitational Field which causes there to be an accretion disk and along this equatorial disk. The radiation field cannot be much stronger than the gravitational field, hence this equatorial accretion disk becomes the channel via which the nascent massive star accretes all of its material.
基金supported by the Programme National "Physique et Chimie du MilieuInterstellaire" (PCMI) of CNRS/INSU with INC/INP co-funded by CEA and CNESfinancial support from VNSC/VAST+4 种基金the NAFOSTED funding agency under grant number 103.99–2015.39the World Laboratorythe Odon Vallet Foundationthe Rencontres du Viet Namfunded by Graduate University of Science and Technology undergrant number GUST.STS.DT 2017-VL01
文摘Atacama Large Millimetre/sub-millimetre Array(ALMA) observations of CO(1–0) and CO(2–1) emissions from the circumstellar envelope of the asymptotic giant branch(AGB) star EP Aqr have been made with four times better spatial resolution than previously available. They are analysed with emphasis on the de-projection in space of the effective emissivity and flux of matter using as input a prescribed configuration of the velocity field, assumed to be radial. The data are found to display an intrinsic axisymmetry with respect to an axis making a small angle with respect to the line of sight. A broad range of wind configurations, from prolate(bipolar) to oblate(equatorial) has been studied and found to be accompanied by significant equatorial emission. Qualitatively, the effective emissivity is enhanced near the equator to produce the central narrow component observed in the Doppler velocity spectra and its dependence on star latitude generally follows that of the wind velocity with the exception of an omni-present depression near the poles. In particular, large equatorial expansion velocities produce a flared disc or a ring of effective emissivity and mass loss. The effect on the determination of the orientation of the star axis of radial velocity gradients, and possibly competing rotation and expansion in the equatorial disc, is discussed. In general,the flux of matter is found to reach a broad maximum at distances of the order of 500 AU from the star.Arguments are given that may be used to favour one wind velocity distribution over another. As a result of the improved quality of the data, a deeper understanding of the constraints imposed on morphology and kinematics has been obtained.
基金Supported by the National Natural Science Foundation of China.
文摘We present a statistical analysis of 482 6.7 GHz methanol maser sources from the available literature, on their maser emission and the characteristics of their associated infrared sources. On the color-color diagram, more than 70% of the objects fall within a very small region (0.57 ≤ [25-12] ≤ 1.30 and 1.30 ≤ [60-12] ≤ 2.50). This suggests that 6.7 GHz methanol maser emission occurs only within a very short evolutionary phase during the earliest stage of star formation. The velocity ranges of the masers belong to two main groups: one from 1 to 10 km s<SUP>?1</SUP>, and one from about 11 to 20 km s<SUP>?1</SUP>. These velocity ranges indicate that the masers are probably associated with both disks and outflows. The correlations between the maser and infrared flux densities, and between the maser and infrared luminosities, suggest that far-infrared radiation is a possible pumping mechanism for the masers which most probably originate from some outer molecular envelopes or disks.
