Star formation history in the central region of the barred galaxy NGC 7177

Using the method of two-dimensional spectroscopy, we have investigated the kinematics and distribution of the gas and stars at the center of the early-type spiral galaxy NGC 7177 with a mediumscale bar as well as the change in the mean age of the stellar population along the radius. A classical picture of radial gas inflow to the galactic center along the shock fronts delineated by dust concentration at the leading edges of the bar has been revealed. The gas inflow is observed down to a radius R = 1″.5−2″, where the gas flows at the inner Lindblad resonance concentrate in an azimuthally highly inhomogeneous nuclear star formation ring. The bar in NGC 7177 is shown to be thick in z coordinate—basically, it has already turned into a pseudo-bulge as a result of secular dynamical evolution. The mean stellar age inside the star formation ring, in the galactic nucleus, is old, ∼10 Gyr.Outside, at a distance R = 6″−8″ from the nucleus, the mean age of the stellar population is ∼2 Gyr. If we agree that the bar in NGC 7177 is old, then, obviously, the star formation ring has migrated radially inward in the last 1–2 Gyr, in accordance with the predictions of some dynamical models.     

Investigation of the conspicuous infrared star cluster and star-forming region “RCW 38 IR Cluster”

The infrared star cluster RCW 38 IR Cluster, which is also a massive star-forming region, is investigated. The results of observations with the SEST (Cerro La Silla, Chile) telescope on the 2.6-mm ¹²CO spectral line and with SIMBA on the 1.2-mm continuum are given. The ¹²CO observations revealed the existence of several molecular clouds, two of which (clouds 1 and 2) are connected with the object RCW 38 IR Cluster. Cloud 1 is a massive cloud, which has a depression in which the investigated object is embedded. It is not excluded that the depression was formed by the wind and/or emission from the young bright stars belonging to the star cluster. Rotation of cloud 2, around the axis having SE-NW direction, with an angular velocity ω = 4.6 · 10⁻¹⁴ s⁻¹ is also found. A red-shifted outflow with velocity ∼+5.6 km/s, in the SE direction and perpendicular to the elongation of cloud 2 has also been found. The investigated cluster is associated with an IR point source IRAS 08573-4718, which has IR colors typical for a non-evolved embedded (in the cloud) stellar object. The cluster is also connected with a water maser. The SIMBA image shows the existence of a central bright condensation, coinciding with the cluster itself, and two extensions. One of these extensions (the one with SW-NE direction) coincides, both in place and shape, with cloud 2, so that the possibility that this extension might also be rotating like cloud 2 is not excluded. In the vicinity of these extensions there are condensations resembling HH objects. Published in Astrofizika, Vol. 51, No. 1, pp. 29–40 (February 2008).     

Synchronous evolution of galaxies in groups: NGC524 group

With the means of panoramic spectroscopy at the SAO RAS BTA telescope, we investigated the properties of stellar populations in the central regions of five early-type galaxies—the NGC524 group members. The evolution of the central regions of galaxies looks synchronized: the average age of stars in the bulges of all the five galaxies lies in the range of 3–6Gyr.Four of the five galaxies revealed synchronized bursts of star formation in the nuclei 1–2 Gyr ago. The only galaxy, in which the ages of stellar population in the nucleus and in the bulge coincide (i.e. the nuclear burst of star formation did not take place) isNGC502, the farthest from the center of the group of all the galaxies studied.     

Investigation of flare stars in the pleiades by the method of stellar tracks. I

Observations of the Pleiades cluster by the method of stellar tracks, carried out on the 40-inch Schmidt telescope of Byurakan Astrophysical Observatory, have resulted in the detection of 49 flares from 38 stars, 17 of which were not previously known to be flare stars. It is shown that for bright stars (U ≤ 16.0) the detection of flares in observations by the method of stellar tracks is at least three times more efficient than for observations by the method of stellar chains. Another advantage of the first method is that one can detect brief flares that last less than 6 min. The visual stellar magnitude at the minimum for the brightest of the flare stars that we found is 11.92. This raised the upper luminosity limit of known flare stars in the Pleiades by 0.21 magnitude. A comparison of the expected number of bright flare stars in the Pleiades with the number of all bright members of the cluster (falling in the range from V ≈ 12.0 toV ≈16.0) suggested that all these stars evidently must be flare stars. Translated from Astrofizika, Vol. 42, No. 3, pp. 351–358, July–September, 1999.     

Globular Clusters in the Large Magellanic Cloud: An Impact from GAIA Photometry

The goal of this work is to assess the expected scientific output from the photometric studies of globular clusters in the Large Magellanic Cloud with ESA's astrometric space mission GAIA. For this purpose we simulate GAIA photometry of individual stars in synthetic cluster populations, covering a large range of cluster ages and metallicities. We find that accurate effective temperatures (Δ T _eff<10%) can be obtained from GAIA photometry down to V ∼ 18 for stars in populations within the studied metallicity range ([M/H] = -0.4 ... -1.7). GAIA will also provide photometric metallicities (Δ [M/H] ≲ 0.3 dex) for the cluster giants brighter than V ∼ 17.5. The knowledge of the effective temperature sand metallicities will allow to obtain accurate ages of stellar populations younger than about 1 Gyr using the usual procedure of main sequence turn-off point fitting. Ages of older stellar populations (≳ 1 Gyr) may be constrained from the isochrone fits to the giant branches in the observed CMDs. We conclude that GAIA will provide excellent opportunities for studying star formation histories far beyond the Milky Way, providing means for better understanding of stellar and galactic evolution in different astrophysical environments. This revised version was published online in July 2006 with corrections to the Cover Date.     

The SFR and IMF of the galactic disk

There is a long term dynamical heating of stellar populations with age observed in the age – velocity dispersion – relation (AVR). This effect allows a determination of the star formation history SFR(t) from local kinematical data of main sequence stars. Using a self-consistent disk model for the vertical structure of the disk, we find from the kinematics of the stars in the solar neighbourhood that the SFR shows a moderate star burst about 10 Gyr ago followed by a continuous decline to the present day value consistent with the observed number of OB stars. The gravitational potential of the gas component and of the Dark Matter Halo is included and the effect of chemical enrichment, finite lifetime of the stars and mass loss of the stellar component are taken into account. The scale heights for main sequence stars together with the SFR is then used to determine constistently the IMF from the observed local luminosity function. The main new result is that the power law break in the present day mass function (PDMF) around 1 M _⊙ is entirely due to evolutionary effects of the disk and does not appear in the IMF. This revised version was published online in August 2006 with corrections to the Cover Date.     

The Fossil Starburst in M82

We present high-resolution optical and near-infrared HST observations of two adjacent regions in the fossil starburst region inM82, M82 B1 and B2. The presence of both the active and the fossil starburst in M82 provides a unique physical environment to study the stellar and dynamical evolution of star cluster systems. The cluster population in B2 is more heavily affected by internal extinction than that in B1, amounting to an excess extinction in B2 of A_V,excess≃1.1±0.3 mag. Preliminary age estimates date the cluster population in the fossil starburst between ∼2× 10⁸ and ∼ 10⁹ years. The radial luminosity profiles of the brightest clusters are more closely approximated by power laws than by a Gaussian model, in particular in their wings, which favours a slow star formation scenario. This revised version was published online in July 2006 with corrections to the Cover Date.     

Spectral variability of LBV star V 532 (Romano’s star)

We present the results of studying the spectral and photometric variability of the luminous blue variable star V532 in M33. The photometric variations are traced from 1960 to 2010, spectral variations—from 1992 to 2009. The star has revealed an absolute maximum of visual brightness (1992–1994, high/cold state) and an absolute minimum (2007–2008, low/hot state) with a brightness difference of ΔB ≈ 2.3 ^m . The temperature estimates in the absolute maximum and absolute minimum were found to be T ∼ 22000 K and T ∼ 42000 K, respectively. The variability of the spectrum of V532 is fully consistent with the temperature variations in its photosphere, while both permitted and forbidden lines are formed in an extended stellar atmosphere. Broad components of the brightest lines were found, the broadening of these components is due to electron scattering in the wind parts closest to the photosphere. We measured the wind velocity as a difference between the emission and absorption peaks in the PCyg type profiles. The wind velocity clearly depends on the size of the stellar photosphere or on the visual brightness, when brightness declines, the wind velocity increases. In the absolute minimum a kinematic profile of the V532 atmosphere was detected. The wind velocity increases and its temperature declines with distance from the star. In the low/hot state, the spectral type of the star corresponds to WN8.5h, in the high/cold state—to WN11. We studied the evolution of V532 along with the evolution of AGCar and the massive WR binary HD5980 in SMC. During their visual minima, all the three stars perfectly fit with the WNL star sequence by Crowther and Smith (1997). However, when visual brightness increases, all the three stars form a separate sequence. It is possible that this reflects a new property of LBV stars, namely, in the high/cold states they do not pertain to the bona fide WNL stars.     

Trapezium type multiple stars — instable systems

Trapezium type multiple stars are centers of star formation in stellar associations. Theoretical and observational studies show that they are dynamically instable. Due to the special space configurations of the components, Trapezium type multiple stars disintegrate completely or partly after several rotations around their centers. These stars have a main role in the origin and evolution of stars on OB- and T-associations. According to observations of 15 Trapezium type multiple stars having the brightest star of OB-class 14 are in expansion. The age of the Trapezium type multiple stars is of the order of 21(P years. There are weighty arguments to assume that the majority of them are in a state of expansion with positive total energy. Translated from Abastumani Astrophysical Observatory, Academy of Sciences, Georgia. Published in Astrofizika, Vol. 41, No. 1, pp. 91–100, January-March, 1998.     

Two new LBV candidates in the M 33 galaxy

We present two new luminous blue variable (LBV) candidate stars discovered in the M33 galaxy. We identified these stars as massive star candidates at the final stages of evolution, presumably with a notable interstellar extinction. The candidates were selected from the Massey et al. catalog based on the following criteria: emission in H _α , V<18^./m 5 and 0_.^m 35 < (B - V) < 1_.^m 2. The spectra of both stars reveal a broad and strong H _α emission with extended wings (770 and 1000 kms⁻¹). Based on the spectra we estimated the main parameters of the stars. Object N45901 has a bolometric luminosity log(L/L_⊙) = 6.0–6.2 with the value of interstellar extinction A _V = 2.3 ± 0.1. The temperature of the star’s photosphere is estimated as T⋆ ∼ 13000–15000 K, its probable mass on the Zero Age Main Sequence is M∼ 60–80 M_⊙. The infrared excess in N 45901 corresponds to the emission of warm dust with the temperature Twarm ∼ 1000 K, and amounts to 0.1%of the bolometric luminosity. A comparison of stellar magnitude estimates from different catalogs points to the probable variability of the object N45901. Bolometric luminosity of the second object, N125093, is log(L/L_⊙) = 6.3 − 6.6, the value of interstellar extinction is A _V = 2.75 ± 0.15. We estimate its photosphere’s temperature as T⋆∼ 13000–16000K, the initial mass as M ∼ 90–120 M_⊙. The infrared excess in N125093 amounts to 5–6% of the bolometric luminosity. Its spectral energy distribution reveals two thermal components with the temperatures T_warm ∼ 1000K and T_cold ∼ 480 K. The [Ca II] λλ7291, 7323 lines, observed in LBV-like stars Var A and N93351 in M33 are also present in the spectrum of N 125093. These lines indicate relatively recent gas eruptions and dust activity linked with them. High bolometric luminosity of these stars and broad H α emissions allow classifying the studied objects as LBV candidates.     

Spectroscopy of B and Be stars in the diffuse stellar clusters NGC 6871 and NGC 6913

This is a study of the population of B and Be stars in the young, relatively poor, diffuse stellar clusters NGC 6871 and NGC 6913. High resolution spectra are used to study the Hα line of eleven stars in order to detect emission. Emission profiles were found for three stars in the cluster NGC 6871; one of these is a known WR-star and the Be-star BD +35°3956 demonstrates the transition from the B to the Be phase. Spectra of seven of the B stars revealed no traces of emission in the Hα line. During the time of our observations, the Be star V1322 Cyg in the cluster NGC 6913 had a strong emission Hα line profile with substantial variability in intensity and equivalent width. Moderate resolution spectra of seven stars in the cluster NGC 6871 over wavelengths of 4420-4960 ? and ten stars in the cluster NGC 6913 over wavelengths of 4050-5100 ? are used to classify the series of B and Be stars spectrally and to estimate their T _eff and log g. It was found that three of the stars are not members of the clusters NGC 6871 or NGC 6913. Translated from Astrofizika, Vol. 52, No. 2, pp. 257–274 (May 2009).     

Study of the galaxy DDO 68: New evidence for its youth

DDO 68 (UGC 5340) is the second most metal-poor star-forming galaxy (12 + log(O/H) = 7.14). Its peculiar optical morphology and its HI distribution and kinematics are indicative of a merger origin. We use the u, g, r, and i photometry based on the SDSS images of DDO 68 to estimate its stellar population ages. The Hα images of DDO 68 were used to select several representative regions without nebular emission. The derived colors were analyzed by comparison with the PEGASE2 evolutionary tracks for various star formation (SF) scenarios, including the two extreme cases: (i) an instantaneous starburst and (ii) continuous SF with a constant rate. The (u − g) and (g − r) colors for all of the selected regions are consistent with the scenario of several “instantaneous” SF episodes with ages between ∼0.05 and ∼1 Gyr. The total mass of the visible stars in DDO 68 was estimated by comparing the colors and fluxes of the observed stellar subsystems with PEGASE2 models to be ∼2.4 × 10⁷ M _⊙. This accounts for ∼6% of the total baryonic mass of the galaxy. All of the available data are consistent with the fact that DDO 68 is a very rare candidate for young galaxies. The bulk of its stars were formed during the recent (with the first encounter ∼1 Gyr ago) merger of two very gas-rich disks. DDO 68 is closest in its properties to cosmologically young low-mass galaxies. This article was submitted by the authors in English.     

Stellar Populations in the Magellanic Clouds

There seems to be no strong evidence that the young globular clusters in the MC have metallicities differing significantly from the metallicities of MC field stars of the same age. The old globular clusters in the LMC are of the same age as, or slightly younger than, those in the outer halo of our Galaxy. It is suggested that the increase in the SFR in the LMC ∼ 4 Gyr ago was related to the collapse of the system to a plane. Evidence for a spread in metallicities amongst young stars in either Cloud remains tentative. There is no strong evidence for bursts of star formation being triggered by LMC-SMC-Galaxy interactions but the possibility is raised that the SFR in the SMC has been strongly affected by this interaction. This revised version was published online in July 2006 with corrections to the Cover Date.     

Pulsations in subdwarf B stars

Subdwarf B stars play a significant role in close binary evolution and in the hot star content of old stellar populations, in particular in giant elliptical galaxies. While the question of their origin poses several problems for stellar evolution theory, one of their most fascinating properties is the presence of multi-periodic 2–3 minute oscillations. Interpreting these oscillations optimally requires the correct identification of the modes. Partial identifications can be obtained using high-speed observations of radial velocity and colour variations. We review some of the several attempts to make such observations, most recently with the Multi-Site Spectroscopic Telescope campaign and withULTRACAM.     

Circumstellar envelopes and Asymptotic Giant Branch stars

Summary. Red giants are sometimes surrounded by envelopes, the result of the ejection of stellar matter at a large rate (/yr) and at a low velocity (10 km/s). In this review the envelopes are discussed and the relation between stars and envelope: what stars combine with what envelopes? The envelope emits radiation by various processes and has been detected at all wavelengths between the visual and the microwave range. I review the observations of continuum radiation emitted by dust particles and of rotational transitions of molecules, where these molecules have been excited by thermal or by non–thermal (“maser”) processes. I discuss mainly the oxygen–rich stars, those of spectral type M, and only briefly the closely related carbon–rich stars. By and large the density in the envelope is well described by spherically symmetric outflow at a constant velocity; on the time scale needed to flow from stellar surface to the outermost layers, i.e. yr, the loss of mass is sometimes interrupted suddenly after which the envelope becomes “detached” from the star. The temperature decreases when moving outward; heat input is by friction between dust particles and gas and cooling occurs by line radiation by various molecules, especially by HO. The molecular composition is determined by formation in an equilibrium process deep in the atmosphere and by destruction in the outer parts of the outflow by interstellar UV radiation (H, CO, HO) or by depletion due to condensation on dust grains (SiO); dust particles of silicate material solidify where the radiation temperature is decreased to about 1000 K, and this is at a few stellar radii. The various continuum spectra produced by the dust particles in different stars are well modelled by a simple model of the density and dust temperature distribution plus the assumption that the particles consist of “dirty silicate”, i.e. silicate with Fe and Al ions added. A large range of optical depths, , is observed: from 0.01 to 10. In envelopes with large optical depth the star itself can no longer be detected directly. Model calculations also show that the momentum in the outflow, i.e. is provided by radiation pressure on the dust particles followed by the complete transfer of this momentum to the gas. The mass–loss rate itself, , is not determined by radiation pressure but by dynamic processes in the region below the dust condensation layer. When is sufficiently large its measurement, that of the stellar luminosity, and that of the outflow velocity, , permit the determination of , i.e. the total outflow rate, without making assumptions about the abundance of the dust particles or of the molecular gases. Detached envelopes have been seen in a few cases. Thermal molecular radiation is faint compared to the maser emission but has been measured in distant stars, e.g. in stars near the galactic center. Different molecules outline different “spheres” around the star. The largest sphere (a radius of 0.1 pc) is outlined by an emission line belonging to the CO() transition. Higher rotational transitions of CO give smaller diameters. A comparison of CO () and () fluxes in stars with very thick envelopes leads to the conclusion that an abrupt decrease in the mass–loss rate occurred some ten thousand years ago. Three molecules produce each several maser lines: SiO, HO and OH. Several new HO lines have recently been discovered; their exploration has hardly been started. The high intensity of the maser lines makes interferometry possible and hence detailed mapping. The SiO lines are formed deep in the envelope, below the dust condensation layer. OH maser lines are produced farthest out, HO lines in between. The excitation mechanisms for most maser lines is understood globally, but detailed models are lacking, largely because the problem is non–linear and the solution of the radiative transfer equation requires a highly anisotropic geometry. The geometrical and kinematical properties of the 1612 MHz OH maser, which in many objects is very strong, are explained by a thin shell of OH; because the angular diameter of the shell can be measured directly and the linear diameter can be determined from the difference in the time of maximum flux of blue and red maser peaks, the distance of the shell and of the star can be measured. The presence or absence of individual maser lines appears to depend on the value of and is well described by a sequence called “Lewis' chronology”. The central star is a long–period variable with a period of 300 days or longer and with a large luminosity amplitude (). Evidence is given that each star has the maximum luminosity it will reach during its evolution and that it is a thermally–pulsing Asymptotic–Giant–Branch star (TP–AGB) with a main–sequence mass between 1 and 6 . Stars of the same main–sequence mass, , have different mass–loss rates, in some cases by a factor of 10. The mass–loss rate probably increases with time, and the highest mass–loss rates are reached toward the end of the evolution. Stars with higher ultimately reach higher mass–loss rates. The calibration of the main–sequence mass is reviewed. Most Mira variables with mass loss have a mass between 1.0 and 1.2 . OH/IR stars with periods over 1000 days have no counterparts among the carbon stars and thus have . Stars as discussed in this review have been found only in the thin galactic disk and in the bulge. Finally I review several recently proposed scenarios for TP–AGB evolution in which mass loss is taken into account. These scenarios represent the observations quite well; their major short–coming is the lack of an explanation why the central stars are always large–amplitude, long–period variables and why such stars are the ones with high mass–loss rates. Received: 10 January 1996     

Star Formation in NGC 4038/4039 as seen in the NIR

We obtained various sets of near infrared observations of the prototypical merger, NGC 4038/4039 (‘the Antennae’). Integral field spectroscopy and broad- and narrow band imaging aimed at obtaining age and extinction estimates of the young star clusters seen in large numbers distributed throughout the disks of the interacting galaxies. High resolution spectroscopy led to estimates of the dynamical masses of the clusters. The clusters have ages ranging from 3.7 to ≈ 20 Myrs. Those in the ‘overlap region’ are very young (below 8 Myrs), while in the nothwestern loop ages are above that limit, and the nuclear starbursts are much older (∼ 100 Myrs). Some photometric cluster masses lie above 10⁶ M_⊙. The stellar velocity dispersions determined from the medium- to high resolution spectra yielded virial cluster masses again up to a few 10⁶ M_⊙. Large differences in the estimated photometric and virial masses suggest a variation of the IMF between the clusters. At least some of the clusters have masses, concentrations and IMFs that could allow them to evolve into globular clusters. This revised version was published online in September 2006 with corrections to the Cover Date.     

Infrared photometry for three RV Tau variable stars

We have analyzed our long-term (1995–2008) JHKLM photometry for three variable RV Tau supergiants. It follows from this analysis that: the amplitude of the JHK brightness variations is approximately the same, 0 ^m . 9–1 ^m , for the three stars; a negative linear trend is noticeable in the variations of the mean J brightness for AC Her; and the mean J brightness of R Sct and V Vul remained constant. The observed J brightness fluctuations in the supergiants are shown to be consistent either only with the temperature pulsations (AC Her and V Vul) or with the temperature and radial pulsations (R Sct). We have detected long-term pulsations in the J brightness and J-H color variations of the supergiants with the following characteristic time scales: ∼1500 days (AC Her), ∼3500 days (R Sct), and ∼800 and ∼1500 days (V Vul). Our search for a periodic component in the infrared brightness variations of the three stars has revealed periods that differ from those obtained from optical observations. We show that relatively hot dust shells (T _d ∼ 700–800 K) exist around the supergiants under study; the densest and most massive shell was observed for V Vul. The ranges of optical-depth variations for the dust shells were τ(1.25 μm) ≈ 0.04–0.10 (AC Her), ≈0.02–0.12 (R Sct), and ≈0.18–0.32 (V Vul). The spectral types in the infrared varied over narrower ranges than those that followed from optical observations of these stars.     

The UV Upturn in Elliptical Galaxies

The hot stellar component in elliptical galaxies offers clues to both stellar evolution and galaxy evolution. Current observations suggest that extreme horizontal branch (EHB) stars dominate the far-UV emission from galaxies with the strongest "UV upturns," while post asymptotic giant branch (PAGB) stars are probably significant contributors for weaker galaxies. Spectra near the Lyman limit indicate that a rather narrow range of temperature (and hence EHB star mass) is required. However, other arguments suggest that most of the helium-burning stars in elliptical galaxies are in the red clump. The HB star mass distribution therefore appears to be strongly bimodal. Such bimodality is qualitatively reproduced by two radically different stellar population models, (those of Lee and Bressan et al., 1994), both of which require that the galaxies be very old. However, the Galactic open cluster NGC 6791 also contains EHB stars and exhibits strong bimodality, indicating that old age may not necessarily be a requirement for the UV upturn phenomenon. This revised version was published online in July 2006 with corrections to the Cover Date.     

Recent advances in modeling stellar interiors

Advances in stellar interior modeling are being driven by new data from large-scale surveys and high-precision photometric and spectroscopic observations. Here we focus on single stars in normal evolutionary phases; we will not discuss the many advances in modeling star formation, interacting binaries, supernovae, or neutron stars. We review briefly: (1) updates to input physics of stellar models; (2) progress in two and three-dimensional evolution and hydrodynamic models; (3) insights from oscillation data used to infer stellar interior structure and validate model predictions (asteroseismology). We close by highlighting a few outstanding problems, e.g., the driving mechanisms for hybrid γ Dor/δ Sct star pulsations, the cause of giant eruptions seen in luminous blue variables such as η Car and P Cyg, and the solar abundance problem.     

Infrared companions to pre-main sequence stars

Summary We report 1.2, 1.6, 2.2 and 3.6 μm slit scan observations of the young low-luminosity stars Elias 22 (also known as GSS 31) in the Rho Ophiuchus dark cloud and Glass I in the Chamaeleon dark cloud. We discovered an infrared companion to Elias 22 at a projected separation of 2.0 arcsec (320 AU) and an optical/infrared companion to Glass I at a projected separation of 2.9 arcsec (410 AU). The companion of Elias 22 is a very cool optically almost invisible object (∼1000 K) less luminous than the primary, while the companion to Glass I is an optically visible late-type star with an infrared excess so large that it is actually bolometrically brighter than the primary. In both cases the infrared excess is likely to be due to hot circumstellar dust grains heated by a central young star (much more obscured in the case of Elias 22 than in the case of Glass I). We outline a new method to determine the mass ratio of such systems assuming that both components lie on an isochrone in the H-R diagram. If the companions are in a bound orbit, the estimated specific angular momentum exceeds or is of the order of 10²¹ cm² s⁻¹, only one order of magnitude larger than that of the Sun-Jupiter system. Based on observations collected at the European Southern Observatory at La Silla, Chile.