O i line emission in cool stars: calculations using partial redistribution

Radiative transfer calculations have been performed for five cool stars: α Tau, β Gem, Procyon, ε Eri and the Sun, for the purpose of investigating the behaviour of the O  i emission over a wide range of stellar types, and its dependence on coherent photon scattering. These stars span a range of spectral types from F5 iv – v to K5 iii and surface gravities  1.25 < log  g * < 4.75  . Particular attention has been paid to the calculation of the flux in the resonance triplet around 1305 Å which is pumped by H Lyβ, including the effects of partial redistribution (PRD) and cross-redistribution of photons. These are the first calculations for the resonance triplet in giant stars using a full PRD treatment. Calculations of the predominantly collisionally excited intersystem doublet at 1355, 1358 Å are included, and it is found that the ratio of these fluxes shows the effects of opacity. The flux in the forbidden line at 1641 Å is calculated for the giant stars and the effects of coherent scattering on this line are investigated. The discrepancy between the calculated and observed fluxes in the O  i lines is used to infer the inadequacy of single-component chromospheric models.     

Self-regulated gravitational accretion in protostellar discs

We present a numerical model for the evolution of a protostellar disc that has formed self-consistently from the collapse of a molecular cloud core. The global evolution of the disc is followed for several million years after its formation. The capture of a wide range of spatial and temporal scales is made possible by use of the thin-disc approximation. We focus on the role of gravitational torques in transporting mass inward and angular momentum outward during different evolutionary phases of a protostellar disc with disc-to-star mass ratio of order 0.1. In the early phase, when the infall of matter from the surrounding envelope is substantial, mass is transported inward by the gravitational torques from spiral arms that are a manifestation of the envelope-induced gravitational instability in the disc. In the late phase, when the gas reservoir of the envelope is depleted, the distinct spiral structure is replaced by ongoing irregular non-axisymmetric density perturbations. The amplitude of these density perturbations decreases with time, though this process is moderated by swing amplification aided by the existence of the disc's sharp outer edge. Our global modelling of the protostellar disc reveals that there is typically a residual non-zero gravitational torque from these density perturbations, i.e. their effects do not exactly cancel out in each region. In particular, the net gravitational torque in the inner disc tends to be negative during first several million years of the evolution, while the outer disc has a net positive gravitational torque. Our global model of a self-consistently formed disc shows that it is also self-regulated in the late phase, so that it is near the Toomre stability limit, with a near-uniform Toomre parameter Q ≈ 1.5–2.0. Since the disc also has near-Keplerian rotation, and comparatively weak temperature variation, it maintains a near-power-law surface density profile proportional to r ^−3/2.     

Evidence for Co-Propagation of 4765- and 1720-MHz OH Masers in Star-Forming Regions

Two star-forming regions Cepheus A and W75N, were searched for the 4765-MHz OH maser emission using the multi-element radio linked interferometer network (MERLIN). The excited OH emission has an arc-like structure of 40 mas in Cep A and a linear structure of size 45 mas in W75N. We also found the 1720-MHz line in Cep A and Hutawarakorn [MNRAS 330 (2002) 349] reported the 1720-MHz emission in W75N. The 1720- and 4765-MHz OH spots coincided in space within 60 mas and in velocity within 0.3 km s^–1 in both targets implying that both maser transitions arise from the same region. According to the modelling by Gray [MNRAS 252 (1991) 30] the 1720/4765-MHz co-propagation requires a low density, warm environment. The masers lie at the edges of H II regions where such conditions are expected.     

A MERLIN survey of 4.7-GHz excited OH masers in star-forming regions

Phase-referenced observations of 13 star-forming regions in the  ²Π_1/2, J = 1/2  transition of rotationally excited OH at 4765 MHz have been carried out using MERLIN. Two of the regions were also observed at 4750 MHz and one at 4660 MHz. There were 10 maser detections at 4765 MHz and three non-detections. There were no detections at 4750 and 4660 MHz. The 4765-MHz masers have brightness temperatures of  ∼10⁷ K  at MERLIN resolution (∼50 mas). Several cases of 4765-MHz masers overlapping in position and velocity with 1720- and 1665-MHz masers are reported. There are also isolated 4765-MHz masers with peak flux densities ≥30 times that of any ground-state counterpart. Most of the 4.7-GHz maser spots are unresolved at 50-mas angular resolution, but in four of the nearest sources the maser spots are resolved, indicating a characteristic size for 4765-MHz maser regions of ∼100 au. In W3(OH) we discovered that 20 per cent of the 4765-MHz emission comes from a narrow low-brightness filament that stretches north–south for ∼1.0 arcec (∼2200 au) between two previously known 4765-MHz maser spots. The filament appears in projection against the H  ii region and has a brightness temperature of  ∼4 × 10⁵ K  . There are matching absorption features in mainline transitions of highly excited OH. The filament may trace a shock front in a rotating disc.     

Contrasting the chemical evolution of the Milky Way and Andromeda

The chemical evolution history of a galaxy hides clues about how it formed and has been changing through time. We have studied the chemical evolution history of the Milky Way (MW) and Andromeda (M31) to find which are common features in the chemical evolution of disc galaxies as well as which are galaxy-dependent. We use a semi-analytic multizone chemical evolution model. Such models have succeeded in explaining the mean trends of the observed chemical properties in these two Local Group spiral galaxies with similar mass and morphology. Our results suggest that while the evolution of the MW and M31 shares general similarities, differences in the formation history are required to explain the observations in detail. In particular, we found that the observed higher metallicity in the M31 halo can be explained by either (i) a higher halo star formation efficiency (SFE), or (ii) a larger reservoir of infalling halo gas with a longer halo formation phase. These two different pictures would lead to (i) a higher [O/Fe] at low metallicities, or (ii) younger stellar populations in the M31 halo, respectively. Both pictures result in a more massive stellar halo in M31, which suggests a possible correlation between the halo metallicity and its stellar mass.     

Period changes in the pulsating extreme helium stars V652 Her and BX Cir

A new statement of the eigenvalue problem of studying small perturbations in arbitrary integrable self-gravitating systems is presented. An example of such a system, a 2D stellar disc, is considered in detail. The theory, based on the general equation for disc eigenmodes, reveals mechanisms for the formation and growth of global galactic structures. This new point of view specifies the limits of the unified theory of bar-like and spiral modes that was based on the assumption that global galactic structures could be understood in terms of low-frequency disc modes.     

Stellar contents of two young open clusters: NGC 663 and 654

UBVRI CCD photometry in a wide field around two young open clusters, NGC 663 and 654, has been carried out. Hα and polarimetric observations for the cluster NGC 654 have also been obtained. We use the photometric data to construct colour–colour and colour–magnitude diagrams, from which we can investigate the reddening, age, mass and evolutionary states of the stellar contents of the these clusters. The reddening across the cluster regions is found to be variable. There is evidence for anomalous reddening law in both clusters; however, more infrared and polarimetric data are needed to conclude about the reddening law. Both clusters are situated at about a distance of 2.4 kpc. Star formation in both clusters is found to be a continuous process. In the case of NGC 663, star formation seems to have taken place sequentially, in the sense that formation of low-mass stars precedes the formation of most massive stars. Whereas, in the case of NGC 654, formation of low-mass stars did not cease after the formation of most massive stars in the cluster.