9Be abundances in dwarfs of intermediate metal deficiency: Impications for galactic evolution

We present observations of the 3130 Å Beii resonance doublet in stars of intermediate metal deficiency, –0.6[Fe/H]–1.1 obtained with the Intermediate Dispersion Spectrograph and IPCS detector at the 2.5 m Isaac Newton Telescope on La Palma. The derived beryllium abundances range from 5.6×10^–12 (one hafl solar) to 2×10^–12 (one sixth solar). These values, interpolated between the sets of previous beryllium measurements at higher and at lower metallicities, serve to confirm the monotonic variation of the Be abundance with Fe during the evolution of the galactic disc. We find that there was no extreme burst of Be production in the halo. To circumvent the effects of depletion, a statistical set of data is needed, for which the upper envelope can be used to trace galactic Be evolution. We note that no observations with sufficient sensitivity to detect Be in the extremely metal deficient stars of the halo have been reported to date.     

The system of old clusters in the magellanic clouds

IntegratedUBV colours have been computed for synthetic clusters older than one billion years and for two chemical composition: (a)Y=0.30;Z=10^–4 and (b)Y=0.30;Z=10^–2, taking into account the contribution to the integrated light of Main Sequence, subgiant, red giant and horizontal branch stars. It has been found that integrated colours depend onZ and allow an estimate of the metal content, however not generally. Horizontal branch stars contribute to the integrated colours of clusters not significantly and the contribution of stars in more advanced phases (e.g., asymptotic branch stars) is almost negligible.Old clusters in LMC and SMC have been studied in terms of colour calibrations and this analysis has been supplemented, when possible, by photometric and spectroscopic data of individual stars. It was found that in the LMC clusters withZ=10^–2 andt>5×10⁹ yr are lacking, clusters with relatively blue colours are similar, both in age and chemical composition, to the halo galactic globular clusters. Moreover, there is a group of clusters with 1×10⁹t5×10⁹. In the SMC clusters withZ=10^–2 andt>5×10⁹ yr are lacking and clusters with 1×10⁹t5×10⁹ are rare. Clusters with relatively blue colours are interpreted with the following parameters:t=5×10⁹ yr, 10^–4Z10^–3 andY=0.20.The implication of these results on the chemical history of the two galaxies is discussed.     

The probability of detecting first generation stars of the Galaxy

First generation stars are the oldest stars that were formed in post-big bang, primitive gas, containing no elements heavier than carbon, with ages greater than 14 Gyr and having undergone no evolution so far. Observations over a long time have confirmed that, up to now, no stars with zero metallicity ([Fe/H]) or with [Fe/H]≤ −6 have been found in the Galaxy. To explain this absence, we shall make a theoretical calculation of the probability of detecting first generation stars using Tsuiimoto et al.'s model of chemical evolution of the galactic halo and assuming an initial mass function of the Miller-Scalo form. We use all the observational data on the halo stars to constrain the parameters. Our result is that, if the mass of the cloud that formed the stars is 10⁶–10⁷ M, then the probability of detecting first generation stars is 6.14×10⁻⁴–6.14×10⁻⁵.     

Stellar population of the blue compact dwarf galaxy SBS 1415+437

The stellar population of the blue compact dwarf galaxy SBS 1415+437 is investigated using the archive database of the Hubble space telescope. The color index-magnitude diagram for stars reaches a magnitude of 29 ^m in the V and I bands. It comprises young main-sequence stars, blue and red supergiants, and the old population of red giant branch and asymptotic giant branch. The tip of the red giant branch αTRGB) was used to calculate the distance modulus, which turned out to be m ? M = 30.65 ± 0.08 ^m . The corresponding distance to the galaxy is D = 13.5 ± 1.0 Mpc. The youngest stars are distributed irregularly near the bright H II region in the southwest part of SBS 1415+437. The old population occupies a larger area, it is distributed more evenly and forms the galactic halo. The spatial distribution of young stars shows that the star formation in the galaxy spread in the direction from northeast to southwest over the last 5 × 10⁷ yr with an average rate of 60 km/s. The TRGB of SBS 1415+437 was found to be appreciably shifted to the blue range: (V ? I) _TRGB ≈ 1.30. The galaxy age turns out to be not smaller than the age of Galactic globular clusters (about 10¹⁰ yr), provided that the galaxy originally had a very low metallicity (our photometric estimate is [Fe/H] = ?2.4). If the metallicity of SBS 1415+437 changed almost not at all in the course of evolution and was equal to [Fe/H] = ?1.3 (as estimated from the emission lines of ionized gas), the galaxy age is no more than 2 × 10⁹ yr.     

Bursts of gravitational radiation from active galactic nuclei and globular clusters

The characteristics of gravitational bursts from active galactic nuclei, and globular clusters are obtained for three astrophysical situations:(i) scattering of stars by massive black holes residued at the centers of galaxies and globular clusters; (ii) the close encounters of stars in the nuclear regions of these objects; (iii) scattering of stars by black holes of stellar mass containing in the stellar population of galactic nuclei and clusters. The most effective source of gravitational bursts appears to be a scattering of stars by the massive central black holes which produces the bursts with dimensionless amplitudeh10^–19–10^–21 and frequencies from 10^–1 to 10^–5 Hz. The characteristics obtained correspond to the possiblities of a future gravitational-wave experiment with use of laser Doppler tracking of interplanetary spacecrafts.     

A galactic model with a pulsating active nucleus

The chemical evolution of the Galaxy with a pulsating active nucleus is investigated. The surface densities of gas, stellar remnants, stars and chemical species such as helium and heavy elements inZ6 are calculated as functions of the position in the Galaxy and of the evolutional time of the Galaxy. According to this model, the entire luminosity of the galactic disk becomes almost constant at some 2×10⁹ yr after the galactic formation, but the nuclear bulge, whose dimensions gradually diminishes, becomes more and more luminous with time. On the other hand, the abundance depletion of helium and heavy elements appears in the inner region of the disk after some 6×10⁹ yr of the galactic formation. It also becomes clear that the activity for the nucleosynthesis in the nucleus is limited only in the early history of the Galaxy and has been reduced rapidly with time. Using this model, we can account for the observed phenomena such as the smooth dependence of the elemental abundance in the halo population on the distance from the galactic center, the high abundance of heavy elements in quasar spectra and etc.     

The metallicity of galactic populations and the history of star formation

The metal abundance distribution (the metallicity function, MF) of stars and globular clusters is studied. It is found to have three gaps, near [Fe/H]=–1.0, –0.5 and –0.1. The gaps are shown to be statistically significant practically at the 100% confidence level. They divide the galactic population into four metallicity groups with the average [Fe/H] of about –1.5, –0.8, –0.25 and +0.10 (groups I, II, III and IV, respectively). The main contribution into the scatter of metallicity within the groups (apart from group I) comes from random errors in abundance measurements. So we infer that the actual MF must be essentially discrete. These results substantiate the ones obtained in our previous Papers I–IV; they support our idea on active phases in the evolution of the Galaxy, which imply the intermittent enrichment and star formation.We find that the kinematics and metallicity of groups III and IV of F and G dwarfs show a paradox: the metal-rich group (group IV) of G dwarfs turns out to be kinematically older than the group III of F dwarfs with half the metal abundance. The implication of this result for star formation is discussed. Also we show that the portion of metal poor disk population F dwarfs (group III) is the same or even larger than that of G dwarfs. This fact disagrees with the conventional idea that the young kinematics of F dwarfs owes to the absence of old F dwarfs, which are supposed to be evolved into red giants.     

Quasi-stellar objects as rotating magnetic superstars

In this paper the magnetic superstar model is used to discuss QSO luminosity and density evolution. Our main hypotheses are that (i) mass loss from old stars in massive galaxies cools and then falls into the centre to form a nuclear disc (Bailey, 1980); and (ii) magnetic superstars in galactic nuclei condense out of gaseous material at the centre of a supermassive-magnetised disc (Kundt, 1979). On this generalised model we find that the non-thermal (synchrotron) optical luminosity scales asL _opt L ³ t ^–7/3, whereL is the total blue luminosity of old stars in the galaxy and t is cosmic time. In addition we show that QSO co-moving density follows the lawD(t)exp-(t/t _Evol)^16/15 with an evolution timescalet _Evol = 1.95 × 10⁹ yr. The model as a whole is in good agreement with observations.     

The galactic evolution of lithium

Measurements of lithium in stars of different galactic populations such as young open clusters ( Per, Pleiades, Praesepe, Coma, Hyades), very young stellar associations (Taurus-Auriga, Chamaeleon, Ophiuchus clouds), intermediate and old open clusters (NGC 752, M 67, NGC 188), old disc stars and halo stars give us the observational framework from which the galactic evolution of lithium has to be inferred. This element is produced mainly via three mechanisms: primordial nucleosynthesis, spallation reactions in the interstellar medium and thermonuclear reactions in some particular stellar evolutionary stages (novae, red giants). The complicated nucleosynthesis and the fact that astration of lithium in stars is not well understood, makes a direct interpretation of the lithium evolutionary abundance curve difficult. The constraints set by recent lithium measurements in very old open clusters and metal-deficient stars on galactic lithium production mechanisms are discussed. Current problems in the determination of the primordial lithium abundance are briefly reviewed.     

Chemical evolution of the galaxy: Abundances of the light elements (sodium to calcium)

The abundances of the light (Na to Ca) elements in disc and halo stars are reviewed. New analyses are emphasized. Elements considered are the α-nuclei (Mg, Si, and Ca), and the odd-even nuclei (Na and Al, also²⁵Mg and²⁶Mg). The α-nuclei are overabundant (relative to Fe) in the old disc and halo stars. Halo stars ([Fe/H] < —1.2) have [α/Fe] ∼0.3 with extreme halo ([Fe/H] ≲ −2.0) stars showing possibly higher overabundances. The scatter in [α/Fe] at a given [Fe/H] is small. To within the observational errors, the abundance patterns for Mg, Si, and Ca are identical. For disc stars, the Na and Al abundances relative to Mg are almost independent of the [Fe/H]. Halo stars ([Fe/H] < −1) show [Na/Mg] < 0 and [AI/Mg] < 0, but the form of the mean relation and the scatter about the relation between [odd-even/Mg] and [Fe/H] remains uncertain.     

The stellar content of binary star clusters in the LMC

The bright stellar content for fifteen binary star clusters and their adjoining fields in the Large Magellanic Cloud (LMC) were studied here. Film copies of plates taken with the 1.2 U.K. Schmidt telescope were used for deriving the spectral types of the stars in the studied regions. All classified stars are brighter thanV=17.5 mag and situated in large areas around each pair and in a neighbouring field. Seven of the pairs, the brightest and most populous are young clusters (located mainly at the north part of the parent galaxy). The derived distributions of spectral types of their stars give strong evidence that each pair consists of similar stellar content with ages 0.6–8×10⁷ yr.Eight more binary star clusters were studied as well, selected among the rest of the binaries in the LMC. It is found that their stars were faint for our limit of detection so the poor statistics did not allow a comparison among the two cluster members of each pair. However the bright limit of their stars implies ages >6×10⁸ yr. Considering that these objects were randomly selected it is unlikely that all are projected pairs. So it seems that binarity in star clusters is a phenomenon (favourable in the LMC) which did not happen only once in the life of this galaxy.     

Chromospherically active binaries,kinematics and age connections

Space velocities of 146 chromospherically active binary stars have been calculated. Containing F to M spectral types on the Main Sequence together with G and K giants, this very heterogeneous sample has been divided into subsamples in order to segregate stars so that they have similar kinematics and ages. Dispersions of space velocity components and other kinematical quantities (velocity averages and |Z| distributions) of these groups imply their ages as: Both old and young Main-Sequence systems (<10×10⁹ yr and 1×10⁹ yr, respectively) exist in the sample. Systems containing subgiants (single- and double-lined) or double-lined giants as their companions have ages about 2–3×10⁹ yr. Single-lined giants appear to be older than intermediate disk population stars (>5×10⁹ yr). The possible existence of white dwarfs as invisible companions of some of the single lined giants is suggested in order to explain why these systems are older than double-lined giants.Paper presented at the 11th European Regional Astronomical Meetings of the IAU on New Windows to the Universe, held 3–8 July, 1989, Tenerife, Canary Islands, Spain.     

The formation and evolution of low-mass close binaries with compact components

We discuss the formation and evolution of interacting low-mass close binaries with a He-1CO- or ONe-dwarf neutron star or a black hole as a compact component. Mass exchange leads to cataclysmic events in such systems. The rate of semidetached low-mass close binary formation is 5×10^–3 yr^–1 if the accreting component is a He degenerate dwarf, 5×10^–3 yr^–1 if it is a CO-dwarf and 3×10^–8 yr^–1 if it is a neutron star. Systems with compact accretors arise as the result of the common envelope phase of close binary evolution or due to collisions of single neutron stars or dwarfs with low-mass single stars in dense stellar clusters. Evolution of LMCB to the contact phase in semi-detached stages is determined mainly by the angular momentum losses by a magnetic stellar wind and radiation of gravitational waves. Numerical computations of evolution with momentum loss explain observed mass exchange rates in such systems, the absence of cataclysmic variables with orbital periods 2^h–3^h, the low number and the evolutionary status of systems with orbital periods shorter than 80^m. In conclusion we list unsolved problems related to magnetic stellar wind, the distribution of young close binaries over main initial parameters, stability of mass exchange.Paper presented at the IAU Colloquium No. 93 on Cataclysmic Variables. Recent Multi-Frequency Observations and Theoretical Developments, held at Dr. Remeis-Sternwarte Bamberg, F.R.G., 16–19 June, 1986.     

Some considerations on the non-detection of interstellar Li and B towards the SN1987a

A very low upper limit of 0.15 mÅ for the interstellar 6707 Å Lii line has been recently derived towards the SN1987a by Baade and Magain (1988). This value corresponds toN(Li)<1.4×10¹¹ cm^–2 and gives [Li/H]<5.4×10^–11 assumingN(Hi)=2.6×10²¹ cm^–2 for the hydrogen column density in the LMC towards SN1987a. This value is lower than the Li abundance found in the Population II stars and lower than the minimum abundance allowed in the framework of the standard Big-Bang theory. We indirectly estimate the Li depletion usingKi observations and show that a depletion of 1.2 dex is plausible. Therefore, an interstellar abundance [Li/H] as high as 0.8×10^–9 cannot be excluded. Any improvement in the above-mentioned upper limit will place important constraints on current theories for lithium nucleosynthesis.High-resolution IUE spectra of the SN1987a have been analysed in search for IS 1362 ÅBii resonance lines. A minimum detectable equivalent width of 22 mÅ has been found, that impliesN(B)<1.2×10^–12 cm^–2 and [B/H]<4.7×10^–10 cm^–2, i.e., comparable to the solar value of [B/H]=4×10^–10. This limit is the most stringent derived so far for an external galaxy, and suggests that the rate of spallation processes in the LMC has not been higher than in our own Galaxy.     

Spectroscopy of high proper motion stars in the ground-based UV

Based on high quality spectral data (spectral resolution R≈60000) within the wavelength range of 3550–5000 Å we determined main parameters (effective temperature, surface gravity, microturbulent velocity, and content of chemical elements including heavy metals from Sr to Dy) for 14 metal-deficient G–K stars with large proper motions. The stars studied have a high range of metallicity: [Fe/H]=?0.3÷?2.9. Abundances of Mg, Al, Sr and Ba were calculated with non-LTE line-formation effects accounted for. The abundance both of radioactive element Th and the r-process element Eu were determined through synthetic spectrum calculations. We selected stars that belong to different galactic populations according to the kinematical criterion and parameters determined by us. We found that the studied stars with large proper motions refer to different components of the Galaxy: thin, thick disks and halo. The chemical composition of the star BD+80°?245 far from the galactic plane agrees with its belonging to the accreted halo. For the giant HD?115444 we obtained [Fe/H]=?2.91, an underabundance of Mn, an overabundance of heavy metals from Ba to Dy, and especially a high excess of the r-process element europium: [Eu/Fe]=+1.26. Contrary to its chemical composition typical for halo stars, HD?115444 belongs to the disc population according to its kinematic parameters.     

Vacuum energy in cosmic dynamics

The analysis of the Th/U ratio in meteorites and the evolutionary ages of globular clusters favour values of the cosmic age of (19±5)×10⁹ yr. This evidence together with a Hubble parameterH ₀>70 km s^–1 Mpc^–1=(14×10⁹ yr)^–1 cannot be reconciled in a Friedmann model with =0. It requires a cosmological constant in the order of 10^–56 cm^–2, equivalent to a vacuum density _v =10^–29 g cm^–3 The Friedmann-Lemaître models (>0) with a hot big-bang have been calculated. They are based on a present value of the baryonic matter density of ₀=0.5×10^–30 g cm^–3 as derived from the primordial⁴He and²H abundances.For a Hubble parameter ofH ₀=75 km s^–1 Mpc^–1, our analysis favours a set of models which can be represented by a model with Euclidean metric (density parameter ₀=1.0, deceleration parameterq ₀=–0.93, aget ₀=19.7×10⁹ yr) and by a closed model with perpetual expansion (₀=1.072,q ₀=–1.0, aget ₀=21.4×10⁹ yr). A present density parameter close to one can indeed be expected if the conjecture of an exponential inflation of the very early universe is correct.The possible behaviour of the vacuum density is demonstrated with the help of Streeruwitz' formula in the context of the closed model with an inflationary phase at very early times.     

Physical properties of the outflow sources in taurus

We have searched for CO outflows in eight embedded IRAS sources located in the Taurus molecular cloud using the 45m telescope of Nobeyama Radio Observatory. We have detected CO wing emission in four of these sources. CO outflow associated with TMC1A (04365+2535) is strong and spatially compact (radius 0.04 pc). The dynamical timescale of 2.5 × 10³ yr suggests this outflow is the youngest one in Taurus.We have combined our data with previously published survey data and have analyzed the physical properties of the outflow sources. We found that 12 out of 16 embedded sources ( 75 %) have CO outflows associated with them; this indicates that almost all stars experience a phase of molecular outflow in their embedded stage. The IRAS color of the outflow sources suggests that the outflows appear in considerably early phase of the evolution of YSOs, that is, as early as YSOs became observable with IRAS and that visible outflow sources are in a transient phase of evolution between embedded sources and visible T Tauri stars without outflow. Visible outflow sources are systematically more luminous than visible no-outflow sources, while embedded outflow sources have comparable luminosities with visible no-outflow sources. Such luminosity function suggests that the YSOs with outflow undergo mass accretion and increase their stellar mass as they progress from embedded sources to visible outflow sources. Typical mass accretion rate derived from the bolometric luminosity is 2 ×10^–6 M _yr ^–1. The timescale for mass accretion to acquire typical stellar mass, 0.5 – 0.8M , is 2.5 – 4 × 10⁵ yr.     

A possible observational constraint on the production of lithium by galactic novae

Recent computations (D'Antona and Mazzitelli, 1982), together with the general scheme of evolution of cataclysmic binaries (CBs), lead to conclude that the secondaries in those CBs having periods shorter than 4.5 hr have a large³He content in the envelope, ranging from 10^–4 (P=4.5 hr) to >2×10^–3 (P=3 hr) in mass fraction, if the nova systems have an age of some billion years. The consequence on the frequency of novae outbursts is shortly examined. If lithium is produced by galactic novae, the⁷Li content of old disk stars should be very close to the Population II content.Presented at the 5th European Workshop on White Dwarfs, Kiel, 1984.     

Abundance analysis of selected cepheids and the galactic distribution of metallicity

We have determined the atmospheric abundances of selected Cepheids in order to study the large-scale chemical inhomogeneities across the galactic disk. The classical Cepheids were selected as probes to study the variation of metallicity in the galactic disk, because of their high intrinsic luminosity, small age and the existence of period-luminosity and period-age relationships. High dispersion spectra of programme stars WZ Sgr, X Sgr, ? Gem, T Mon and S V Mon were obtained using the 102-cm reflector of Kavalur Observatory. The atmospheric abundances were determined by theoretically synthesizing the selected portions of the stellar spectrum and comparing with the observed spectra. In order to compute the theoretical spectrum, the formal solution of the equation of radiative transfer was numerically evaluated with the simplifying assumptions of local thermodynamical equilibrium, plane-parallel geometry and hydrostatic equilibrium. These assumptions are reasonably good for the metallic lines of F-G supergiants and hence the observations were confined to the phases where Cepheids behave like nonvariable F-G supergiants. The atmospheric abundances of iron-peak elements, Fe, Cr, Ti, Ca and heavier s-process elements Y, Ba, La, Ce, Sm were obtained by synthesizing a selected spectral region in the range 4330 Å — 4650 Å. We derive a radial abundance gradient for iron \(\frac{{d(Fe/H)}}{{dr_{gc} }} = - 0.056 \pm 0.08\) for the region of galactic disk between 6.7 and 10.9 kpc from the galactic centre (assuming r_gc = 8.5 kpc for the Sun). This value agrees with the one obtained from the general sample of Cepheids for which spectroscopic abundances are available, and also with the existing photometric determinations, but is shallower than the one derived by Luck (1982). Abundances of the elements derived in the present investigation do not show any significant correlation with atomic number. Also the abundance ratio of s-process elements does not show any correlation with Fe. This lack of correlation for disk population stars shows the inadequacy of simple models of galactic chemical evolution and favours the infall models. Alternately, the evolution of [s/Fe] may be determined by the ratio of intermediate-mass stars (which contribute s-process nuclei) to high-mass stars (which contribute Fe peak nuclei). Thus the different behaviour of halo and disk population may indicate a difference in the mass spectrum of star formation.     

The possible origin of the spiral-arm instability

Physical arguments suggest the spiral arms may be manifestations of the galaxy not being in dynamical equilibrium — in the sense that the kinetic energy of tis stars and gas is less relative to its binding energy than that dictated by the virial theorem. Without constant cooling of the galactic disk (i.e., a progressive increase in the binding energy of the galaxy) such a departure from dynamical equilibrium would be corrected and the spiral arms destroyed in about 10⁹ yr due to an increase in the velocity dispersion of the stars in the disk resulting from their interacting with the spiral arms. The rate of cooling required to maintain the spiral arms, about 6×10⁴ L , may be provided by mass loss from stars in the disk population. The cooling arises from the average scale-heights and velocities of these stars being larger than that of the gas in the disk, so that there is a net loss of kinetic energy and an increase in the binding energy of the galaxy due to the ejected gas settling down to a lower terminal velocity and scale-height in the galactic disk.