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Near-IR spectroscopy of OH/IR stars in the Galactic centre
Context.Based on the expansion velocities of their circumstellar shells,Galactic centre (GC) OH/IR stars can be divided into two groups that arekinematically different and therefore believed to have evolved fromdifferent stellar populations.Aims.We studied the metallicitydistribution of the OH/IR stars population in the GC on the basis of atheoretical relation between EW(Na), EW(Ca), and EW(CO) and themetallicity.Methods.For 70 OH/IR stars in the GC, we obtainednear-IR spectra. The equivalent line-widths of Na I, Ca I,12CO(2, 0), and the curvature of the spectrum around 1.6μm due to water absorption were determined.Results.The near-IRspectrum of OH/IR stars is influenced by several physical processes.OH/IR stars are variable stars suffering high mass-loss rates. The dustthat is formed around the stars strongly influences the near-IR spectraand reduces the equivalent line-widths of Na I and Ca I. A similareffect is caused by the water content in the outer atmosphere of theOH/IR star. Because of these effects, it is not possible to determinethe metallicities of these stars with our low-resolution near-infraredspectroscopy.
| MCG-6-30-15: long time-scale X-ray variability, black hole mass and active galactic nuclei high states
We present a detailed study of the long time-scale X-ray variability ofthe Seyfert 1 Galaxy MCG-6-30-15, based on eight years of frequentmonitoring observations with the Rossi X-ray Timing Explorer. Whencombined with the published short-time-scale XMM-Newton observations, wederive the power-spectral density (PSD) covering six decades offrequency from ~10-8 to ~10-2 Hz. As with NGC4051, another narrow-line Seyfert 1 galaxy (NLS1), we find that the PSDof MCG-6-30-15 is a close analogue of the PSD of a galactic black holeX-ray binary system (GBH) in a `high' rather than a `low' state. As withNGC 4051 and the GBH Cygnus X-1 in its high state, a smoothly bendingmodel is a better fit to the PSD of MCG-6-30-15, giving a derived breakfrequency of 7.6+10-3× 10-5 Hz.Assuming linear scaling of the break frequency with black hole mass, weestimate the black hole mass in MCG-6-30-15 to be~2.9+1.8-1.6×106Msolar.Although, in the X-ray band, it is one of the best observed Seyfertgalaxies, there has as yet been no accurate determination of the mass ofthe black hole in MCG-6-30-15. Here we present a mass determinationusing the velocity dispersion (MBH-σ*)technique and compare it with estimates based on the width of theHα line. Depending on the calibration relationship assumed for theMBH-σ* relationship, we derive a mass ofbetween 3.6 and 6 × 106Msolar, consistentwith the mass derived from the PSD.Using the newly derived mass and break time-scale, and revisedreverberation masses for other active galactic nuclei (AGN) fromPeterson et al., we update the black hole mass-break-time-scale diagram.The observations are still generally consistent with narrow-line Seyfert1 galaxies having shorter break time-scales, for a given mass, thanbroad-line AGN, probably reflecting a higher accretion rate. However,the revised, generally higher, masses (but unchanged break time-scales)are also consistent with perhaps all of the X-ray bright AGN studied sofar being high-state objects. This result may simply be a selectioneffect, based on their selection from high-flux X-ray all-skycatalogues, and their consequent typically high X-ray/radio ratios,which indicate high-state systems.
| Luminosity and Temperature from Near-Infrared Spectra of Late-Type Giant Stars
We present moderate resolution (lambda / Delta lambda ~ 1380 and lambda/ Delta lambda ~ 4830) spectra of 43 K0 to M6 III stars covering 2.19 -2.34 mu m and measure equivalent widths of the strongest absorptionfeatures - Na I, Ca I, and (12) CO(2,0) - present on the spectra. Thehigh resolution Wallace & Hinkle (1996) spectral atlas shows thatour moderate resolution measurements of the atomic features havesignificant contributions from other species, such as Sc, S, Fe, Ti, Si,and V. We also find that our measured equivalent widths are affected byCN absorption present in the continuum bands. In spite of this, theequivalent widths of Na I and Ca I features at moderate resolution stillshow a strong dependence on effective temperature. The CO equivalentwidth at moderate resolution is less affected by other lines andcontinuum placement than the atomic features, because of its relativelygreater strength. We compare our data to similar data taken forlate-type dwarf stars (Ali et al. 1995) and find that a two dimensionalspectral classification can be constructed based on the near-IR spectra.The quantity log[EW(CO)/(EW(Na)+EW(Ca))] is a strong luminosityindicator independent of effective temperature, while the equivalentwidth of (12) CO(2,0) has a well-defined dependence on effectivetemperature for a given luminosity. This two dimensional spectralclassification is ideal for cool stars obscured by dust in, for example,the central part of the Galactic bulge and regions of star formation.
| SANTIAGO 91, a right ascension catalogue of 3387 stars (equinox J2000).
The positions in right ascension of 3387 stars belonging to the Santiago67 Catalogue, observed with the Repsold Meridian Circle at Cerro Calan,National Astronomical Observatory, during the period 1989 to 1994, aregiven. The average mean square error of a position, for the wholeCatalogue, is +/-0.009 s. The mean epoch of the catalogue is 1991.84.
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Osservazione e dati astrometrici
| Costellazione: | Centauro |
| Ascensione retta: | 13h34m15.32s |
| Declinazione: | -34°23'15.2" |
| Magnitudine apparente: | 7.556 |
| Distanza: | 179.856 parsec |
| Moto proprio RA: | -8.3 |
| Moto proprio Dec: | -1.7 |
| B-T magnitude: | 8.919 |
| V-T magnitude: | 7.669 |
Cataloghi e designazioni:
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