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Masses of Star Clusters in the Nuclei of Bulgeless Spiral Galaxies
In the last decade star clusters have been found in the centers ofspiral galaxies across all Hubble types. We here present a spectroscopicstudy of the exceptionally bright (106-108Lsolar) but compact (re~5 pc) nuclear starclusters in very late type spirals with the Ultraviolet and VisualEchelle Spectrograph at the VLT. We find that the velocity dispersionsof the nine clusters in our sample range from 13 to 34 kms-1. Using photometric data from the Hubble Space TelescopeWFPC2 and spherically symmetric dynamical models, we determine massesbetween 8×105 and 6×107Msolar. The mass-to-light ratios range from 0.2 to 1.5 in theI band. This indicates a young mean age for most clusters, in agreementwith previous studies. Given their high masses and small sizes, we findthat nuclear clusters are among the objects with the highest meansurface density known (up to 105 Msolarpc-2). From their dynamical properties we infer that, ratherthan small bulges, the closest structural kin of nuclear clusters appearto be massive compact star clusters. This includes such differentobjects as globular clusters, ``super star clusters,'' ultracompactdwarf galaxies (UCDs), and the nuclei of dwarf elliptical galaxies. Itis a challenge to explain why, despite the widely different currentenvironments, all different types of massive star clusters share verysimilar and structural properties. A possible explanation links UCDs andmassive globular clusters to nuclear star clusters through stripping ofnucleated dwarf galaxies in a merger event. The extreme properties ofthis type of cluster would then be a consequence of the clusters'location in the centers of their respective host galaxies.
| Rotational velocities of A-type stars in the northern hemisphere. II. Measurement of v sin i
This work is the second part of the set of measurements of v sin i forA-type stars, begun by Royer et al. (\cite{Ror_02a}). Spectra of 249 B8to F2-type stars brighter than V=7 have been collected at Observatoirede Haute-Provence (OHP). Fourier transforms of several line profiles inthe range 4200-4600 Å are used to derive v sin i from thefrequency of the first zero. Statistical analysis of the sampleindicates that measurement error mainly depends on v sin i and thisrelative error of the rotational velocity is found to be about 5% onaverage. The systematic shift with respect to standard values fromSlettebak et al. (\cite{Slk_75}), previously found in the first paper,is here confirmed. Comparisons with data from the literature agree withour findings: v sin i values from Slettebak et al. are underestimatedand the relation between both scales follows a linear law ensuremath vsin inew = 1.03 v sin iold+7.7. Finally, thesedata are combined with those from the previous paper (Royer et al.\cite{Ror_02a}), together with the catalogue of Abt & Morrell(\cite{AbtMol95}). The resulting sample includes some 2150 stars withhomogenized rotational velocities. Based on observations made atObservatoire de Haute Provence (CNRS), France. Tables \ref{results} and\ref{merging} are only available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.125.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/393/897
| Physical parameters of lambda Bootis stars
This is the first of two papers whose main goal is to update and improvethe information available on the physical properties of the lambdaBootis stars. The determination of the stellar parameters is offundamental importance to shed light into the different theoriesproposed to explain the lambda Bootis phenomenon. With this aim,projected rotational velocities, effective temperatures, surfacegravities and chemical abundances of a sample of suspected lambda Bootisstars have been calculated. Five objects showing composite spectratypical of binary systems were found in our analysis. The abundancedistribution of the program stars does not resemble the chemicalcomposition of the class prototype, lambda Boo, which poses someconcerns regarding the idea of a well-defined, chemically homogeneousgroup of stars. A possible relation between rotational velocities andthe lambda Bootis phenomenon has been found. This result would be inagreement with the accretion scenario proposed by Turcotte &Charbonneau (\cite{Turcotte93}). Figure 3 is only available inelectronic form at http://www.edpsciences.org
| A spectroscopic survey for lambda Bootis stars. II. The observational data
lambda Bootis stars comprise only a small number of all A-type stars andare characterized as nonmagnetic, Population i, late B to early F-typedwarfs which show significant underabundances of metals whereas thelight elements (C, N, O and S) are almost normal abundant compared tothe Sun. In the second paper on a spectroscopic survey for lambda Bootisstars, we present the spectral classifications of all program starsobserved. These stars were selected on the basis of their Strömgrenuvbybeta colors as lambda Bootis candidates. In total, 708 objects insix open clusters, the Orion OB1 association and the Galactic field wereclassified. In addition, 9 serendipity non-candidates in the vicinity ofour program stars as well as 15 Guide Star Catalogue stars were observedresulting in a total of 732 classified stars. The 15 objects from theGuide Star Catalogue are part of a program for the classification ofapparent variable stars from the Fine Guidance Sensors of the HubbleSpace Telescope. A grid of 105 MK standard as well as ``pathological''stars guarantees a precise classification. A comparison of our spectralclassification with the extensive work of Abt & Morrell(\cite{Abt95}) shows no significant differences. The derived types are0.23 +/- 0.09 (rms error per measurement) subclasses later and 0.30 +/-0.08 luminosity classes more luminous than those of Abt & Morrell(\cite{Abt95}) based on a sample of 160 objects in common. The estimatederrors of the means are +/- 0.1 subclasses. The characteristics of oursample are discussed in respect to the distribution on the sky, apparentvisual magnitudes and Strömgren uvbybeta colors. Based onobservations from the Observatoire de Haute-Provence, OsservatorioAstronomico di Padova-Asiago, Observatório do Pico dosDias-LNA/CNPq/MCT, Chews Ridge Observatory (MIRA) and University ofToronto Southern Observatory (Las Campanas).
| The Relation between Rotational Velocities and Spectral Peculiarities among A-Type Stars
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1995ApJS...99..135A&db_key=AST
| The A0 stars
A photometric grid, standardized on MK spectral standards, has been usedto compare spectral types and luminosity classes obtainedphotometrically with those in two extensive spectral surveys coveringthe entire sky. Major discrepancies include the spectroscopicclassification of B9.5, which may indicate an otherwise unrecognizedspectral peculiarity, a different A0/A1 spectral type boundary in thetwo samples involved, the well-known misclassification of weak heliumstars, and an appreciable percentage of stars which are called dwarfsspectroscopically but are of higher photometric luminosity. The spacemotion vectors of these stars for which radial velocities are available,and excluding the minimum of 25 percent that are spectroscopic binarieswithout orbital elements, show structure in their distribution in the(U, V)-plane, with members of the Local Association and the Hyades andSirius superclusters forming obvious concentrations. The members of theLocal Association in the samples are mainly old (more than 200 millionyears) mode A stars, although a few much younger stars are included. Themembers of the Hyades and Sirius superclusters contain many bluestragglers, including several peculiar stars of the Hg, Mn, and Sivarieties.
| New photoelectric sequences in PUP (L)=250,(B)"0.
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1977MNRAS.178..467H&db_key=AST
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