Planetary system formation through binary star merging

A scenario is considered for the formation of a planetary system through the merging of a binary star comprised of low-mass (0.5–1 M _⊙) stars in the stage of contracting towards the main sequence. According to our previous computations (Sirotkin and Karetnikov, 2006), under certain conditions, the destruction of the more massive component can result in the formation of a central star, an accretion disk, and an extended arm. The extended arm is fragmented to form clouds of planetary masses (<5M _J). The formed disk and clouds rotate in the same direction as the central star. The clouds are in elongated orbits (e > 0.3) lying in the orbital plane of the initial binary system. To test these earlier results, we repeated computations for the same system parameters but with higher accuracy. The new computations confirmed the earlier results and gave new information about the cloud and disk structure.     

A Collimated Stellar Wind Emanating from a Massive Protostar

We report multi-wavelength observations towards IRAS 16547–4247, a luminous infrared source with a bolometric luminosity of 6.2 × 10⁴ L _⊙. Dust continuum observations at 1.2-mm indicate that this object is associated with a dust cloud with a size of about 0.4 pc in diameter and a mass of about 1.3 × 10³ M _⊙. Radio continuum observations show the presence of a triple radio source consisting of a compact central object and two outer lobes, separated by about 0.3 pc, symmetrically located from the central source. Molecular hydrogen line observations show a chain of knots that trace a collimated flow extending over 1.5 pc. We suggest that IRAS 16547–4247 corresponds to a dense massive core which hosts near its central region a high-mass star in an early stage of evolution. This massive YSO is undergoing the ejection of a collimated stellar wind which drives the H₂ flow. The radio emission from the lobes arises in shocks resulting from the interaction of the collimated wind with the surrounding medium. We conclude that the thermal jets found in the formation of low-mass stars are also produced in high-mass stars.     

Stellar dynamics in young clusters: the formation of massive runaways and very massive runaway mergers

In the present paper we combine an N-body code that simulates the dynamics of young dense stellar systems with a massive star evolution handler that accounts in a realistic way for the effects of stellar wind mass loss. We discuss two topics.

1. The formation and the evolution of very massive stars (with masses >120 M_⊙) is followed in detail. These very massive stars are formed in the cluster core as a consequence of the successive (physical) collisions of the 10–20 most massive stars in the cluster (this process is known as ‘runaway merging’). The further evolution is governed by stellar wind mass loss during core hydrogen and core helium burning (the WR phase of very massive stars). Our simulations reveal that, as a consequence of runaway merging in clusters with solar and supersolar values, massive black holes can be formed, but with a maximum mass ≈70 M_⊙. In low-metallicity clusters, however, it cannot be excluded that the runaway-merging process is responsible for pair-instability supernovae or for the formation of intermediate-mass black holes with a mass of several 100 M_⊙.
2. Massive runaways can be formed via the supernova explosion of one of the components in a binary system (the Blaauw scenario), or via dynamical interaction of a single star and a binary or between two binaries in a star cluster. We explore the possibility that the most massive runaways (e.g. ζ Pup, λ Cep, BD+43°3654) are the product of the collision and merger of two or three massive stars.

     

The H2O maser toward IRAS 06308+0402

We present the monitoring results for the H₂O maser toward the infrared source IRAS 06308+0402 associated with a dense cold molecular cloud. The observations were carried out with the 22-m radio telescope at the Pushchino Radio Astronomy Observatory (Russia) during 1992–2003. The H₂O maser was discovered in May 1992 (Pashchenko 1992) during a survey of IRAS sources associated with dense cold clouds with bipolar molecular outflows. The H₂O spectrum contains many emission features, suggesting the fragmentation of the envelope around a young star. The star has a low peculiar velocity relative to the CO molecular cloud (～2.2 km s^?1). We found a cyclic variability of the total maser flux with a period from 1.8 to 3.1 yr.     

Observations of the bright-rimmed molecular clouds near the Cepheus OB2 association

Massive stars have significant influence on the evolution of the interstellar medium. Bright rims, cometary morphology of clouds, as well as their motion are some examples of the influence of massive stars on nearby molecular clouds. The cometary clouds in the Gum-Vela region are very good examples. In an attempt to understand the kinematics of the clouds in such regions we have carried out CO line observations towards bright-rimmed clouds near the OB Association Cep OB2. The radial velocities of the clouds are consistent with an expansion of the system at ≈ 4kms^?1 away from the dominant O6.5V star in the association, HD206267. We find the rocket mechanism to be the most likely cause for expansion as found for both the Gum-Vela and the Rosette globules. We conclude that such expanding motions are quite common in regions near massive stars and make a brief comparison of the Cepheus system with the Gum-Vela system.     

X-ray Observations of Eight Young Open Star Clusters: I. Membership and X-ray Luminosity

We present a detailed investigation of X-ray source contents of eight young open clusters with ages between 4 to 46 Myr using archival X-ray data from XMM-Newton. The probable cluster memberships of the X-ray sources have been established on the basis of multi-wavelength archival data, and samples of 152 pre-main sequence (PMS) low mass (<2M _⊙), 36 intermediate mass (2–10M _⊙) and 16 massive (>10M _⊙) stars have been generated. X-ray spectral analyses of high mass stars reveal the presence of high temperature plasma with temperature <2 keV, and mean L _X/L _bol of 10^???6.9. In the case of PMS low mass stars, the plasma temperatures have been found to be in the range of 0.2 keV to 3 keV with a median value of ~1.3 keV, with no significant difference in plasma temperatures during their evolution from 4 to 46 Myr. The X-ray luminosity distributions of the PMS low mass stars have been found to be similar in the young star clusters under study. This may suggest a nearly uniform X-ray activity in the PMS low mass stars of ages ~4–14 Myr. These observed values of L _X/L _bol are found to have a mean value of 10^??3.6±0.4, which is below the X-ray saturation level. The L _X/L _bol values for the PMS low mass stars are well correlated with their bolometric luminosities, that implies its dependence on the internal structure of the low mass stars. The difference between the X-ray luminosity distributions of the intermediate mass stars and the PMS low mass stars has not been found to be statistically significant. Their L _X/L _bol values, however have been found to be significantly different from each other with a confidence level greater than 99.999% and the strength of X-ray activity in the intermediate mass stars is found to be lower compared to the low mass stars. However, the possibility of X-ray emission from the intermediate mass stars due to a low mass star in close proximity of the intermediate mass star can not be ruled out.     

Masses and luminosities of O‐ and B‐type stars and red supergiants

Massive stars are of interest as progenitors of supernovae, i.e. neutron stars and black holes, which can be sources of gravitational waves. Recent population synthesis models can predict neutron star and gravitational wave observations but deal with a fixed supernova rate or an assumed initial mass function for the population of massive stars. Here we investigate those massive stars, which are supernova progenitors, i.e. with O‐ and early B‐type stars, and also all supergiants within 3 kpc. We restrict our sample to those massive stars detected both in 2MASS and observed by Hipparcos, i.e. only those stars with parallax and precise photometry. To determine the luminosities we calculated the extinctions from published multi‐colour photometry, spectral types, luminosity class, all corrected for multiplicity and recently revised Hipparcos distances. We use luminosities and temperatures to estimate the masses and ages of these stars using different models from different authors. Having estimated the luminosities of all our stars within 3 kpc, in particular for all O‐ and early B‐type stars, we have determined the median and mean luminosities for all spectral types for luminosity classes I, III, and V. Our luminosity values for supergiants deviate from earlier results: Previous work generally overestimates distances and luminosities compared to our data, this is likely due to Hipparcos parallaxes (generally more accurate and larger than previous ground‐based data) and the fact that many massive stars have recently been resolved into multiples of lower masses and luminosities. From luminosities and effective temperatures we derived masses and ages using mass tracks and isochrones from different authors. From masses and ages we estimated lifetimes and derived a lower limit for the supernova rate of ≈20 events/Myr averaged over the next 10 Myr within 600 pc from the sun. These data are then used to search for areas in the sky with higher likelihood for a supernova or gravitational wave event (like OB associations) (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Can molecular clouds live long?

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.     

Stellar Mass—Halo Mass Relation and Star Formation Efficiency in High-Mass Halos

We study relation between stellar mass and halo mass for high-mass halos using a sample of galaxy clusters with accurate measurements of stellar masses from optical and ifrared data and total masses from X-ray observations. We find that stellar mass of the brightest cluster galaxies (BCGs) scales as M_*,BCG ∝ M ₅₀₀ ^αBCG with the best fit slope of α_BCG ≈ 0.4 ± 0.1. We measure scatter of M_*,BCG at a fixed M₅₀₀ of ≈0.2 dex. We show that stellar mass-halo mass relations from abundance matching or halo modelling reported in recent studies underestimate masses of BCGs by a factor of ～2?4. We argue that this is because these studies used stellar mass functions (SMF) based on photometry that severely underestimates the outer surface brightness profiles of massive galaxies. We show that M_*?M relation derived using abundance matching with the recent SMF calibration by Bernardi et al. (2013) based on improved photometry is in a much better agreement with the relation we derive via direct calibration for observed clusters. The total stellar mass of galaxies correlates with total mass M₅₀₀ with the slope of ≈0.6 ± 0.1 and scatter of 0.1 dex. This indicates that efficiency with which baryons are converted into stars decreases with increasing cluster mass. The low scatter is due to large contribution of satellite galaxies: the stellar mass in satellite galaxies correlates with M₅₀₀ with scatter of ≈0.1 dex and best fit slope of α_sat ≈ 0.8 ± 0.1. We show that for a fixed choice of the initial mass function (IMF) total stellar fraction in clusters is only a factor of 3?5 lower than the peak stellar fraction reached in M ≈ 10¹²M_⊙ halos. The difference is only a factor of ～1.5?3 if the IMF becomes progressively more bottom heavy with increasing mass in early type galaxies, as indicated by recent observational analyses. This means that the overall efficiency of star formation in massive halos is only moderately suppressed compared to L_* galaxies and is considerably less suppressed than previously thought. The larger normalization and slope of the M_*?M relation derived in this study shows that feedback and associated suppression of star formation in massive halos should be weaker than assumed in most of the current semi-analytic models and simulations.     

Dynamical study of wide pairs of stars based on data from the WDS catalog

Using the method of apparent motion parameters, we have studied the relative motion of the components in 561 pairs of wide (ρ > 2″) and relatively nearby (Hipparcos parallaxes > 0.01″) visual double stars based on data from the WDS catalog. The minimum masses of the double stars have been calculated at given parallaxes. We have identified 358 optical pairs. For 11 stellar pairs, we have found the minimum mass to exceed the estimate corresponding to their spectral types and luminosities. This excess is 5–7 M _⊙ for two stars, ADS 7446 and 9701.     

Formaldehyde and H110α observations towards 6.7 GHz methanol maser sources

Intriguing work on observations of 4.83 GHz formaldehyde (H₂CO) absorptions and 4.87 GHz H110α radio recombination lines (RRLs) towards 6.7 GHz methanol (CH₃OH) maser sources is presented. Methanol masers provide ideal sites to probe the earliest stages of massive star formation, while 4.8 GHz formaldehyde absorptions are accurate probes of physical conditions in dense (10³–10⁵ cm^?3) and low temperature molecular clouds towards massive star forming regions. The work is aimed at studying feature similarities between the formaldehyde absorptions and the methanol masers so as to expand knowledge of events and physical conditions in massive star forming regions. A total of 176 methanol maser sources were observed for formaldehyde absorptions, and formaldehyde absorptions were detected 138 of them. 53 of the formaldehyde absorptions were newly detected. We noted a poor correlation between the methanol and formaldehyde intensities, an indication that the signals (though arise from about the same regions) are enhanced by different mechanisms. Our results show higher detection rates of the formaldehyde lines for sources with stronger methanol signals. The strongest formaldehyde absorptions were associated with IRAS sources and IRDCs that have developed HII regions, and that do not have EGOs.     

Mass-luminosity relation for massive stars

A catalog of massive (⩾10 M _⊙) stars in binary and multiple systems with well-known masses and luminosities has been compiled. The catalog is analyzed using a theoretical mass-luminosity relation. This relation allows both normal main-sequence stars and stars with peculiarities: with clear manifestations of mass transfer, mass accretion, and axial rotation, to be identified. Least-squares fitting of the observational data in the range of stellar masses 10M _⊙ ⩽ M ≲ 50 M _⊙ yields the relation L ∼ M ^2.76. An erratum to this article is available at .     

Determining the minimum sum of the component masses for a binary with a known parallax from observations of a short arc of its apparent motion

An inequality that allows the minimum sum of the component masses for a binary, M₀, to be determined was derived from simple geometric considerations. This quantity satisfies the observed orbital motion according to Newton's law with a known parallax. The M₀ value can be calculated if the apparent-motion parameters for the components, including the curvature of the observed short orbital arc, were determined from observations. We estimated M₀ for 14 Pulkovo program stars for which the apparent orbital arc covered with observations was no less than 10°. We compare M₀ with the masses estimated from the mass-luminosity relation. A significant mass excess was found for the star ADS 10329.     

Pulsations of intermediate–mass stars on the asymptotic giant branch

Evolutionary tracks from the zero age main sequence to the asymptotic giant branch were computed for stars with initial masses 2 M _⊙ ≤ M _ZAMS ≤ 5 M _⊙ and metallicity Z = 0.02. Some models of evolutionary sequences were used as initial conditions for equations of radiation hydrodynamics and turbulent convection describing radial stellar pulsations. The early asymptotic giant branch stars are shown to pulsate in the fundamental mode with periods 30 day ? Π ? 400day. The rate of period change gradually increases as the star evolves but is too small to be detected (Π?/Π < 10^?5 yr^?1). Pulsation properties of thermally pulsing AGB stars are investigated on time intervals comprising 17 thermal pulses for evolutionary sequences with initial masses M _ZAMS = 2 M _⊙ and 3 M _⊙ and 6 thermal pulses for M _ZAMS = 4 M _⊙ and 5 M _⊙. Stars with initial masses M _ZAMS ≤ 3 M _⊙ pulsate either in the fundamental mode or in the first overtone, whereas more massive red giants (M _ZAMS ≥ 4 M _⊙) pulsate in the fundamental mode with periods Π ? 10³ day. Most rapid pulsation period change with rate ?0.02 yr^?1 ? Π?/Π ? ?0.01 yr^?1 occurs during decrease of the surface luminosity after the maximum of the luminosity in the helium shell source. The rate of subsequent increase of the period is Π?/Π ? 5 × 10^?3 yr^?1.     

Embedded star clusters and the formation of the Oort cloud: III. Evolution of the inner cloud during the Galactic phase

In a previous publication [Brasser, R., Duncan, M.J., Levison, H.F., 2006. Icarus 184, 59-82], models of the inner Oort cloud were built which included the effect of an embedded star cluster on cometary orbits about the Sun. The main conclusions of that paper were that the formation efficiency is about 10% and the median distance of the cloud to the Sun only depends on the mean density of gas and stars the Sun encountered. Here we report on the results of simulations which followed the ensuing dynamical evolution of these comet clouds in the current Galactic environment once the Sun left the embedded star cluster. The goal is to determine whether or not the dynamical influence of passing Galactic field stars and the Galactic tidal field is sufficient to replenish the current outer cloud (semi-major axis a>20,000 AU) with enough material from the inner cloud (a<20,000 AU). Since visible new comets come directly from the outer cloud, a mass estimate only exists for the latter, with a lower limit of 1 M_⊕ [Francis, P.J., 2005. Astrophys. J. 635, 1348-1361]. Knowing the amount of expansion of the inner cloud may therefore yield an estimate of the mass of said (unseen) inner cloud. Our results indicate that typically only 10% of the comets from the inner cloud land in the outer cloud and are bound after 4.5 Gyr. If one assumes that in the extreme case all or the majority of the current population of the outer cloud has come from the inner cloud, then a typical value of the mass of the inner cloud is about 10 M_⊕. The results of [Brasser, R., Duncan, M.J., Levison, H.F., 2006. Icarus 184, 59-82] showed that ∼10% of comets from the Jupiter-Saturn region were implanted in the inner Oort cloud, which implies an uncomfortably large value of about 100 M_⊕ for the mass of solids in the primordial Jupiter-Saturn region. This extreme case might be remedied in two says: either the effect of Giant Molecular Cloud complexes on the inner Oort cloud must be much more severe than originally thought, or there was a two-stage formation process for the Oort cloud, in which the outer cloud was largely populated by comets scattered once the Sun had left its primordial birth cluster.     

Very massive stars: Evolution with mass loss

The evolution of mass-losing very massive stars in the 500–10000M _⊙ range has been investigated for two different initial compositions, (X, Z)=(0.8,0.0) and (X, Z)=(1.0,0.0). The evolutionary tracks are governed by two opposing factors which are the increase in the mean molecular weight in the convective core and the effect of mass loss. Conservative evolution of stars with massM?10000M _⊙ is similar to that of massive stars (20–100M _⊙), always moving to lower effective temperatures. For low values of the standard mass loss parameterN (50?N?200) the two opposing factors are almost in balance and the star is forced to move in a series of loops. For higher mass loss rates the loops disappear. In the 10000M _⊙ case no loops are observed and the tracks always move to higher effective temperatures. For a given mass loss rate the transition between right and left moving tracks occurs at higher masses the lower is the mass loss rate.     

Properties of Wolf-Rayet stars in eclipsing binary systems

The results of investigations of a number of eclipsing Wolf-Rayet binaries are presented. The ‘core’ radiuses, the ‘core’ temperatures and masses of WR stars in the eclipsing WR+OB binary systems V 444 Cyg, CX Cep, CQ Cep, and CV Ser are obtained (see Table I). The results obtained from the light curves analysis of the V 444 Cyg in the range λλ2460 Å-3.5μ give strong evidence for the Beals (1944) model of WR phenomenon. The chromospheric-coronal effects in the WN5 extended atmosphere are not observed up to a distance ofr?20R _⊙. In the Hertzsprung—Russell diagram all the WR stars lie on the left side from the main sequence between the main sequence and the sequence of uniform helium stars (see Figure 9). Their locations are close to those of the helium remnants formed as a result of mass exchange in massive close binary systems. The period variations in the systems V 444 Cyg and CQ Cep have been discovered and a reliable value of the mass loss rateM=10^?5 M _⊙yr^?1 is obtained, for the two WR stars. The results of the photometric and spectroscopic investigations of the WR stars with low mass companions (post X-ray binary stage?) are presented too (see Table II). The masses of the companions are (1–2)M _⊙, their optical luminosity is ～10³⁶, erg s^?1 which implies that these companions cannot be the normal stars. It is possible that these companions are neutron stars accreting from the stellar wind of the WR stars. Low values of the X-ray luminosities of such WR stars with low mass companions imply that the accretion of matter in such systems is distinct from the accretion process in classical X-ray binary systems. It is noted also that the parameters of low massive companions coupled with WR stars are close to those of helium stars.     

Star-forming region BBW 36 in puppis

Recent studies of the star formation region BBW 36 and associated molecular clouds are presented. The ¹²CO (1-0) observations, carried out with the 15-m SEST (Swedish-ESO) telescope (Cerro La Silla, Chile), revealed the existence of cloud a, connected with BBW 36 and of cloud b, having elongation in SE-NW direction. A red-shifted molecular outflow with velocity ∼+5 km/s (with respect to cloud a), having a direction parallel to the line of sight, was also observed. VLA observations showed the presence of a source VLA 2 at 3.6 cm with an elongation in the N-S direction. It is suggested that the VLA 2 source coincides with a dust disc (surrounding the object BBW 36). The star 3, which is one of the YSOs in the star-forming region BBW 36 and is connected with a bright comma-like nebula, can be the source of the molecular outflow. The star 3 has very high IR colors and is associated with an IRAS point source IRAS 07280-1829, which has IR colors, typical for an IRAS point source, connected with a water maser. On the 2MASS K image of BBW 36 we can see the existence of a bright nebula; a group of stars is embedded in that nebula, and among these stars there are stars with dust discs (or envelopes). On the 2MASS K image several spiral jets are also present, some of them with a condensation at the end. Published in Astrofizika, Vol. 51, No. 3, pp. 469–477 (August 2008).     

Molecular Cores and Young Clusters of Star Forming Regions

By using the 13.7 m millimeter wave telescope of the Qinghai Station of Purple Mountain Observatory at Delingha, we have performed the mapping observations simultaneously at the (J = 1-0) lines of ¹²CO, ¹³CO and C¹⁸O towards respectively the 17 star forming regions associated with clusters. All of them show rather strong C¹⁸O emission, except IRAS 04547+4753. Because of the different sizes of molecular clouds, there are 13 regions being observed to the half maximum of ¹³CO integrated intensity, and the large-area mapping observation has not been made for the other 4 regions with rather large extents. Based on the observed data, the physical properties of molecular cores are calculated, such as the line width, brightness temperature, size, density and mass. The averaged ratios of the virial mass M_vir and local thermodynamic equilibrium mass M_LTE of the ¹³CO and C¹⁸O cores are 0.66 and 0.74, respectively, suggesting that these cores are nearly at the virial equilibrium state. In order to compare the cores and clusters in morphologies, the contour maps of the integrated intensities of ¹³CO and C¹⁸O are overlaid on the K-band images of 2MASS. At the same time, the sizes and masses of the clusters associated with cores are calculated by adopting the photometric results of the near-infrared point sources in 2MASS database. Based on the derived masses of the molecular cores and clusters, the star formation efficiency (SFE) is calculated for the molecular clouds, and we find that it varies in the range from 10% to 30%.     

The relation between molecular clouds and stellar kinematics

The relation between molecular clouds, star clusters, and the stellar component of the galactic disk is investigated. According to Elmegreen (1985) bound stellar systems, e.g., open star clusters, can be formed from molecular cloud of mass 10⁴ M . A close encounter with a giant molecular cloud or massive black hole disrupts such stellar systems and forms superclusters. This explains why some open star clusters are so mass-deficient. Unbound stellar systems, e.g., expanding OB associations, are formed from molecular clouds of mass 10⁵ M . When disruptive O-type stars appear the star formation is halted and the cloud is destroyed. An example of the relict of GMC disruption in the solar vicinity is Gould's belt. The velocity dispersion-versus-age relation is also investigated and explained as a consequence of gravitational scattering of stars on GMC, or massive black holes, or as due to recurrent transient spirals.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.