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A Hubble Space Telescope Survey of the Disk Cluster Population of M31. I. WFPC2 Pointings
As a follow-up to the automated cluster search carried out by Williams& Hodge, we have examined 39 Hubble Space Telescope Wide FieldPlanetary Camera 2 (WFPC2) pointings to locate and study a comprehensivecollection of disk clusters. The Williams technique was effective infinding young clusters, but not intermediate-age or old clusters. Oursearches have shown that M31 has large numbers of these intermediate andolder open clusters, most of them undetected by both the Williams surveyand other ground-based searches. We present a catalog of 343 clustersdetected on the WFPC2 images. Extrapolation from our data indicates thatthe entire disk of M31 contains approximately 80,000 star clusters. Wehave carried out integrated multicolor photometry of these clusters toascertain their properties and to compare their properties with clustersystems of other galaxies. We show the cluster luminosity function, thecolor-magnitude diagram, the formation function, and the sizedistribution. Cluster densities and colors show trends with diskposition. An age distribution is derived and, although the ages are veryuncertain for the fainter clusters, there is evidence for clusterdynamical destruction at about the same rate as in our Galaxy.Based on observations with the NASA/ESA Hubble Space Telescope obtainedat the Space Telescope Science Institute, which is operated by theAssociation of Universities for Research in Astronomy, Inc., under NASAcontract NAS 5-26555.
| Molecular gas in the Andromeda galaxy
Aims.We study the distribution of the molecular gas in the Andromedagalaxy (M 31) and compare this with the distributions of the atomic gasand the emission from cold dust at λ 175 μm.Methods.Weobtained a new 12CO(J = 1-0)-line survey of the Andromedagalaxy with the highest resolution to date (23 arcsec, or 85 pc alongthe major axis), observed On-the-Fly with the IRAM 30-m telescope. Wefully sampled an area of 2°× 0.5 ° with a velocityresolution of 2.6{ km s-1}. In several selected regions wealso observed the 12CO(2-1)-line.Results.Emission fromthe 12CO(1-0) line was detected from galactocentric radiusR=3 kpc to R=16 kpc with a maximum in intensity at R 10 kpc. Themolecular gas traced by the (velocity-integrated) (1-0)-line intensityis concentrated in narrow arm-like filaments, which often coincide withthe dark dust lanes visible at optical wavelengths. Between R=4 kpc andR=12 kpc the brightest CO filaments define a two-armed spiral patternthat is described well by two logarithmic spirals with a pitch angle of7°-8°. The arm-interarm brightness ratio averaged over a lengthof 15 kpc along the western arms reaches about 20 compared to 4 for H Iat an angular resolution of 45 arcsec. For a constant conversion factorX_CO, the molecular fraction of the neutral gas is enhanced in thespiral arms and decreases radially from 0.6 on the inner arms to 0.3 onthe arms at R≃ 10 kpc. The apparent gas-to-dust ratios N(HI)/I175 and (N(H I)+2N(H_2))/I175 increase by afactor of 20 between the centre and R≃ 14{ kpc}, whereas theratio 2N(H_2)/I175 only increases by a factor of 4.Conclusions.Either the atomic and total gas-to-dust ratios increase bya factor of 20 or the dust becomes colder towards larger radii. Astrong variation of X_CO with radius seems unlikely. The observedgradients affect the cross-correlations between gas and dust. In theradial range R=8-14 kpc total gas and cold dust are well correlated;molecular gas correlates better with cold dust than atomic gas. The massof the molecular gas in M 31 within a radius of 18 kpc is M(H2) = 3.6× 108 {M}ȯ at theadopted distance of 780 kpc. This is 7% of the total neutral gas mass inM 31.
| Spitzer MIPS Infrared Imaging of M31: Further Evidence for a Spiral-Ring Composite Structure
New images of M31 at 24, 70, and 160 μm taken with the MultibandImaging Photometer for Spitzer (MIPS) reveal the morphology of the dustin this galaxy. This morphology is well represented by a composite oftwo logarithmic spiral arms and a circular ring (radius ~10 kpc) of starformation offset from the nucleus. The two spiral arms appear to startat the ends of a bar in the nuclear region and extend beyond thestar-forming ring. As has been found in previous work, the spiral armsare not continuous, but composed of spiral segments. The star-formingring is very circular except for a region near M32 where it splits. Thelack of well-defined spiral arms and the prominence of the nearlycircular ring suggest that M31 has been distorted by interactions withits satellite galaxies. Using new dynamical simulations of M31interacting with M32 and NGC 205, we find that, qualitatively, suchinteractions can produce an offset, split ring like that seen in theMIPS images.
| DDO 43: A Prototypical Dwarf Irregular Galaxy?
We present sensitive and high-resolution 21 cm observations of the dwarfirregular (Im) galaxy DDO 43, in conjunction with optical broadband andnarrowband images in U, B, V, and Hα. The observations are used toexamine the relationship of its H I morphology and kinematics to pastand present star formation. Optically, it is a small (R25=990pc), faint (MB of -14.0) dwarf Im with a slightly boxy shape.In H I, DDO 43 has an extended (RHI/RH=2.8) gasenvelope. There is a high-density ridge associated with the optical bodyof the galaxy containing several higher density knots and lower densityholes. The largest hole is ~850×530 pc. No expansion is detected,so it must be relatively old. The largest and potentially oldest (7-70Myr) of the six identified star clusters is located at the western edgeof the hole. Four of the other clusters are located near high-densitypeaks. There are several H II regions, most (but not all) of which areassociated with peaks in the H I surface density. The overall starformation rate is average for its type. In many ways, DDO 43 is a verytypical dwarf Im galaxy. Its H I morphology is consistent with a historyof episodes of localized star formation that create holes and shells inthe interstellar medium, some of which can overlap. These features arelocated within the area of solid-body rotation in the galaxy; the lackof shear in these small systems allows such structures to persist forlong periods of time.
| Classical Cepheid Pulsation Models. X. The Period-Age Relation
We present new period-age (PA) and period-age-color (PAC) relations forfundamental and first-overtone classical Cepheids. Current predictionsrely on homogeneous sets of evolutionary and pulsation models covering abroad range of stellar masses and chemical compositions. We found thatPA and PAC relations present a mild dependence on metal content.Moreover, the use of different PA and PAC relations for fundamental andfirst-overtone Cepheids improves the accuracy of age estimates in theshort-period (logP<1) range (old Cepheids), because they presentsmaller intrinsic dispersions. At the same time, the use of the PACrelations improves the accuracy in the long-period (logP>=1) range(young Cepheids), since they account for the position of individualobjects inside the instability strip. We performed a detailed comparisonbetween evolutionary and pulsation ages for a sizable sample of LMC (15)and SMC (12) clusters which host at least two Cepheids. In order toavoid deceptive uncertainties in the photometric absolute zero point, weadopted the homogeneous set of B, V, and I data for clusters andCepheids collected by OGLE. We also adopted the same reddening scale.The different age estimates agree at the level of 20% for LMC clustersand of 10% for SMC clusters. We also performed the same comparison fortwo Galactic clusters (NGC 6067, NGC 7790), and the difference in age issmaller than 20%. These findings support the use of PA and PAC relationsto supply accurate estimates of individual stellar ages in the Galaxyand in external Galaxies. The main advantage of this approach is itsindependence from the distance.
| DDO 88: A Galaxy-sized Hole in the Interstellar Medium
We present an H I and optical study of the gas-rich dwarf irregulargalaxy DDO 88. Although the global optical and H I parameters of DDO 88are normal for its morphological type, it hosts a large (3 kpc diameter)and unusually complete ring of enhanced H I emission. The normalappearance of this galaxy in the optical and the outer regions of the HI give no hint of the presence of the striking H I ring in the innerregions. The gas ring is located at approximately one-third of the totalH I radius and one-half the optically defined Holmberg radius, andcontains 30% of the total H I of the galaxy. The ring surrounds acentral depression in the H I distribution. If the H I ring and centraldepression in the gas were formed by the energy input from winds andsupernova explosions of massive stars formed in a starburst, as isthought often to be the case, the star-forming event would have formed0.1%-1% of the total stellar mass of the galaxy. However, the UBV colorsin the H I hole are not bluer than the rest of the galaxy, as would beexpected if an unusual star-forming event had taken place thererecently, although there is an old (~1-3 Gyr), red cluster near thecenter of the hole that is massive enough to have produced the hole inthe H I. An age estimate for the ring is uncertain, however, because itis not observed to be expanding. An expansion model produces a lowerestimate of 0.5 Gyr, but the presence of faint star formation regionsassociated with the ring indicates a much younger age. We also estimatethat the ring could have dispersed by now if it is older than 0.5 Gyr.This implies that the ring is younger than 0.5 Gyr. A younger age wouldindicate that the red cluster did not produce the hole and ring.Therefore, uncertainties prevent us from concluding that the cluster andthe H I hole are definitely related. If this ring and the depression inthe gas that it surrounds were not formed by stellar winds andsupernovae, this would indicate that some other, currently unidentified,mechanism is operating.
| A Survey of Compact Star Clusters in the South-West Field of the M 31 Disk
A survey for compact clusters with a dimension of 10pc order wasconducted in an area of about 500 square arc-minutes of the south-westpart of the M31 disk, making use of the high-resolution capability ofSuprime-Cam. Photometry in the B, V, and R broad-bands, and in theR* medium-band centered around Hα with varyingapertures was carried out for about 1200 targets, which are related toabout 300 compact objects detected in the survey. The results for 101prominent compact objects are presented as photometric catalogues andmorphological atlases, separately for samples with and without strongHα emission. Many of the compact objects, which were previouslysuspected to be globular cluster candidates, are judged to be openclusters based upon their internal structures of sub-arc-second order.The majority of the 49 listed compact non-emission objects, which arerestricted to be brighter than MV ˜ -5, have colors of0 < B - V < 1.0, indicating their nature of massive evolvedclusters. In contrast, only about 10% of the 52 listed compact emissionobjects are brighter than MiV ˜ -5, probably reflectingthe short period of the emission phase and the substantial effects ofthe circum-stellar extinction. The detection of a few candidates ofbackground galaxies is also reported.
| The Hot Interstellar Medium in M31 and M33
We report measurements from Far Ultraviolet Spectroscopic Explorerspectra of OB stars in M31 and M33, of O VI and C II interstellarabsorptions. The stars are located in OB 78 in the outer region of M31and in several locations across M33. The interstellar absorptions agreewell between stars in nearby locations, and their principal radialvelocities indicate that they arise in those galaxies and follow thedisk rotation.
| Initial mass function in the South-Western part of M 31
We derive an average initial mass function (IMF) for 50 OB associationsin the South-Western part of Andromeda galaxy, combining ultravioletphotometry of Hill et al. (\cite{Hill95}, ApJ, 98, 595) and two datasetsHST photometry with JHKs photometry from the 2MASS + I bandphotometry from Magnier et al. (\cite{Magnier92}, A&A, 96, 379). Themean extinctions within the associations were estimated in twoindependent ways and the stellar masses were determined by use ofprobabilistic techniques. The derived slope of the IMF Γ=-1.59± 0.09 is close to the universal Salpeter's law and to recentestimates for massive stars' IMF in the Galaxy and in the MagellanicClouds.
| Chandra Observations and Models of the Mixed-Morphology Supernova Remnant W44: Global Trends
We report on the Chandra observations of the archetypicalmixed-morphology (or thermal composite) supernova remnant W44. As withother mixed-morphology remnants, W44's projected center is bright inthermal X-rays. It has an obvious radio shell but no discernible X-rayshell. In addition, X-ray-bright knots dot W44's image. The spectralanalysis of the Chandra data shows that the remnant's hot brightprojected center is metal-rich and that the bright knots are regions ofcomparatively elevated elemental abundances. Neon is among the affectedelements, suggesting that ejecta contributes to the abundance trends.Furthermore, some of the emitting iron atoms appear to be underionizedwith respect to the other ions, providing the first potential X-rayevidence for dust destruction in a supernova remnant. We use the Chandradata to test the following explanations for W44's X-ray-bright center:(1) entropy mixing due to bulk mixing or thermal conduction, (2)evaporation of swept-up clouds, and (3) a metallicity gradient, possiblydue to dust destruction and ejecta enrichment. In these tests, we assumethat the remnant has evolved beyond the adiabatic evolutionary stage,which explains the X-ray dimness of the shell. The entropy-mixed modelspectrum was tested against the Chandra spectrum for the remnant'sprojected center and found to be a good match. The evaporating-cloudmodel was constrained by the finding that the ionization parameters ofthe bright knots are similar to those of the surrounding regions. Whileboth the entropy-mixed and the evaporating-cloud models are known topredict centrally bright-X-ray morphologies, their predictions fallshort of the observed brightness gradient. The resulting brightness gapcan be largely filled in by emission from the extra metals in and nearthe remnant's projected center. The preponderance of evidence (includingthat drawn from other studies) suggests that W44's remarkable morphologycan be attributed to dust destruction and ejecta enrichment within anentropy-mixed, adiabatic-phase supernova remnant. The Chandra dataprompt a new question-by what astrophysical mechanisms are the metalsdistributed so inhomogeneously in the supernova remnant.
| Superassociations and Stellar Complexes in Galaxies
The basic characteristics of stellar complexes and superassociations, aswell as the differences between these kinds of gigantic groups of youngstars, are discussed. The main difference is that superassociations arethe result of induced (triggered) star formation, while the stars andclusters in stellar complexes develop as a result of the spontaneousprocesses typical of galactic gaseous disks.
| The Recent Star Formation History of the M31 Disk
The star formation history of the northern and southern M31 disk ismeasured using samples of BV photometry for4'×4' regions taken from the KPNO/CTIO LocalGroup Survey. The distances, mean reddening values, and agedistributions of the stars in these regions were measured using theroutines of Dolphin. Independent measurements of overlapping fields showthat the results are stable for most samples. A slight distance gradientis seen across the major axis of the southern disk, and a mean distanceof 24.47+/-0.03 is found by combining the results. Higher mean reddeningvalues are seen to follow the spiral structure. The stellar agedistributions are consistent with episodic star formation confinedmainly to the gas-rich arm regions. If these episodes were caused bypropagating density waves, the waves did not cause significant starformation episodes in the gas-poor interarm regions. A combination ofall of the results provides the total star formation rate for 1.4deg2 of the M31 disk for six epochs. These results suggestthat star formation in the disk declined by ~50% from ~250 to ~50 Myrago. The lowest star formation rate occurred ~25 Myr ago, followed by a~20% increase to the present. The mean star formation rate for thislarge portion of M31 over the past 60 Myr is 0.63+/-0.07Msolar yr-1, suggesting a total mean rate for thedisk of ~1 Msolar yr-1.
| Comparing young stellar populations across active regions in the M31 disc
I present preliminary BV resolved stellar photometry of the M31 discmeasured from the data set of the NOAO/CTIO Local Group Survey. I haveperformed detailed analyses of the star-formation histories in andaround three currently active regions in the M31 disc: OB 78, OB 102 andthe north-east spiral arm. The results suggest that low Hαemission from OB 78 compared with other active regions is directlyrelated to the details of the recent star formation histories of theregions. In addition, while every active region I analysed shows arecent increase in star formation activity, some active regions alsocontain overdensities of stars with ages <~100 Myr compared withadjacent regions. The adjacent regions show a relatively constant, lowstar formation rate over the past 100 Myr. There is no significantdifference between the stellar populations on either side of the activeregions. This symmetry provides no obvious signature of recentpropagation of star formation near these currently active regions of theM31 disc.
| Cepheid Variables and the Circum/Interstellar Matter
Various aspects of the relation of classical Cepheids and inter- andcircumstellar matter are summarized. Emphasis is given to the questionof mass loss from Cepheids and to the role of these pulsating variablesin revealing the recent star formation history in their neighbourhood.
| Space Telescope Imaging Spectrograph Ultraviolet Spectroscopy of Early B Supergiants in M31
We analyze Space Telescope Imaging Spectrograph (STIS) spectra in the1150-1700 Å wavelength range obtained for six early B supergiantsin the neighboring galaxy M31. Because of their likely high (nearlysolar) abundance, these stars were originally chosen to be directlycomparable to their Galactic counterparts and represent a much neededaddition to our current sample of B-type supergiants, in our efforts tostudy the dependence of the wind momentum-luminosity relationship onspectral type and metallicity. As a first step to determine wind momentawe fit the P Cygni profiles of the resonance lines of N V, Si IV, and CIV with standard methods and derive terminal velocities for all of theSTIS targets. From these lines we also derive ionic stellar wind columndensities. Our results are compared with those obtained previously inGalactic supergiants and confirm earlier claims of ``normal'' wind lineintensities and terminal velocities in M31. For one-half of the samplewe find evidence for an enhanced maximum turbulent velocity whencompared to Galactic counterparts. Based on observations with theNASA/ESA Hubble Space Telescope obtained at the Space Telescope ScienceInstitute, which is operated by the Association of Universities forResearch in Astronomy, Inc., under NASA contract NAS5-26555.
| Chemical composition of B-type supergiants in the OB 8, OB 10, OB 48, OB 78 associations of M 31
Absolute and differential chemical abundances are presented for thelargest group of massive stars in M 31 studied to date. These resultswere derived from intermediate resolution spectra of seven B-typesupergiants, lying within four OB associations covering a galactocentricdistance of 5-12 kpc. The results are mainly based on an LTE analysis,and we additionally present a full non-LTE, unified model atmosphereanalysis of one star (OB 78-277) to demonstrate the reliability of thedifferential LTE technique. A comparison of the stellar oxygen abundancewith that of previous nebular results shows that there is an offset ofbetween ~ 0.15-0.4 dex between the two methods which is criticallydependent on the empirical calibration adopted for the R_23 parameterwith [O/H]. However within the typical errors of the stellar and nebularanalyses (and given the strength of dependence of the nebular results onthe calibration used) the oxygen abundances determined in each methodare fairly consistent. We determine the radial oxygen abundance gradientfrom these stars, and do not detect any systematic gradient across thisgalactocentric range. We find that the inner regions of M 31 are not, aspreviously thought, very ``metal rich''. Our abundances of C, N, O, Mg,Si, Al, S and Fe in the M 31 supergiants are very similar to those ofmassive stars in the solar neighbourhood.
| The Star Formation History and Mass Function of the Double Cluster h and χ Persei
The h and χ Per ``double cluster'' is examined using wide-field(0.98d×0.98d) CCD UBV imaging supplemented by optical spectra ofseveral hundred of the brightest stars. Restricting our analysis to nearthe cluster nuclei, we find identical reddenings [E(B-V)=0.56+/-0.01],distance moduli (11.85+/-0.05), and ages (12.8+/-1.0 Myr) for the twoclusters. In addition, we find an initial mass function slope for eachof the cluster nuclei that is quite normal for high-mass stars,Γ=-1.3+/-0.2, indistinguishable from a Salpeter value. We derivemasses of 3700 Msolar (h) and 2800 Msolar (χ)integrating the present-day mass function from 1 to 120Msolar. There is evidence of mild mass segregation within thecluster cores. Our data are consistent with the stars having formed at asingle epoch; claims to the contrary are very likely due to theinclusion of the substantial population of early-type stars located atsimilar distances in the Perseus spiral arm, in addition tocontamination by G and K giants at various distances. We discuss theuniqueness of the double cluster, citing other examples of suchstructures in the literature but concluding that the nearly identicalnature of the two cluster cores is unusual. We fail to settle thelong-standing controversy regarding whether or not the double cluster isthe core of the Per OB1 association and argue that this may beunanswerable with current techniques. We also emphasize the need forfurther work on the pre-main-sequence population of this nearby andhighly interesting region.
| Environmental dependences for star formation triggered by expanding shell collapse
Criteria for gravitational collapse of expanding shells in rotating,shearing galaxy discs were determined using three-dimensional numericalsimulations in the thin shell approximation. The simulations were runover a grid of seven independent variables, and the resultantprobabilities for triggering and unstable masses were determined asfunctions of eight dimensionless parameters. When the ratio of themidplane gas density to the midplane total density is small, anexpanding shell reaches the disc scaleheight and vents to the halobefore it collapses. When the Toomre instability parameter Q, or asimilar shear parameter, QA, is large, Coriolis forces andshear stall or reverse the collapse before the shell accumulates enoughmass to be unstable. With large values ofcsh5/(GL), for rms velocity dispersioncsh in the swept-up matter and shell-driving luminosity L,the pressure in the accumulated gas is too large to allow collapseduring the expansion time. Considering ~5000 models covering a widerange of parameter space, the common properties of shell collapse as amechanism for triggered star formation are: (1) the time-scale is~4(csh/2πGρ[GL]0.2)0.5 forambient midplane density ρ, (2) the total fragment mass is ~2× 107 Msolar, of which only a small fractionis likely to be molecular, (3) the triggering radius is ~2 times thescaleheight, and the triggering probability is ~0.5 for large OBassociations. Star formation triggered by shell collapse should be mostcommon in gas-rich galaxies, such as young galaxies or those with lateHubble types.
| The Mass Function of Young Star Clusters in our Galaxy and Nearby Galaxies: Is It Universal?
Mass functions (MFs) derived from photometric observations of young starclusters of our Galaxy, the Magellanic Clouds (MCs), M31 and M33 havebeen used to investigate the question of universality of the initialmass function and presence of mass segregation in these systems.Observational determination of the MF slope of young star clusters havean inherent uncertainty of at least 1.0 dex in the Milky Way andof 0.4 dex in the MCs. There is no obvious dependence of the MFslope on either galactocentric distance or age of the young starclusters or on the spatial concentration of the stars formed or on thegalactic characteristics including metallicity. Effects of masssegregation have been observed in a good number of young stellar groupsof our Galaxy and MCs. As their ages are much smaller than theirdynamical evolution times, star formation processes seem to beresponsible for the observed mass segregation in them.
| The Stellar Population and Star Clusters in the Unusual Local Group Galaxy IC 10
We present an analysis of Hubble Space Telescope U, V, I, and Hαimages of the peculiar Local Group irregular galaxy IC 10. The imagesare used to determine the nature of the stellar products in a portion ofthe recent starburst in this galaxy. We identified 13 stellarassociations and clusters, two of which are probably old (>=350 Myr)and the rest of which are young (4-30 Myr) and presumably formed in thestarburst. We found the following: (1) The slope of the stellar initialmass function (IMF) for 6.3-18 Msolar stars formed in thestarburst lies between two limiting cases: a value of -1.9+/-0.4 underthe assumption of coevality over the past 13 Myr and of -0.9+/-0.3 underthe assumption of constant star formation over the past 40 Myr forZ=0.004 (-2.1+/-0.4 and -1.0+/-0.04, respectively, for Z=0.008). Thus,most likely, the IMF of the intermediate-mass stars is not very unusual.The slope of the IMF for the underlying galaxy population under theassumption of constant star formation is -2.6+/-0.3 for 4.8-18Msolar stars assuming Z=0.004 (-2.3+/-0.3 for Z=0.008) and isunusually steep. (2) The lower stellar mass limit in the starburst is<=6.3 Msolar. This constraint is less than somepredictions of what lower stellar mass limits might be in starbursts buthigher than others. (3) There are two modest-sized Hα shells (~50pc diameter) that easily could have been produced in the past few Myr bythe clusters they encircle. (4) The dominant mode of star formation inthe starburst has been that of a scaled-up OB association. This mode,with a few compact clusters sprinkled in, is similar to the starformation that took place in Constellation III in the LMC, as well asthat in the blue compact dwarfs I Zw 18 and VII Zw 403. The starburst inthis part of IC 10 has not produced a superstar cluster. We also comparethe high WC/WN ratio to evolutionary models and discuss possibleexplanations. The high ratio can be reproduced if there were small,well-synchronized (Δτ<=1 Myr), but widely scattered,pockets of secondary star formation 3-4 Myr ago. Based on observationswith the NASA/ESA Hubble Space Telescope, obtained at the SpaceTelescope Science Institute, which is operated by the Association ofUniversities for Research in Astronomy, Inc., under NASA contract NAS5-26555.
| The DIRECT Project: Catalogs of Stellar Objects in Nearby Galaxies. II. Eastern Arm and NGC 206 in M31
DIRECT is a project to directly obtain the distances to two importantgalaxies in the cosmological distance ladder, M31 and M33, usingdetached eclipsing binaries and Cepheids. As part of our search forthese variables, we have obtained photometry and positions for thousandsof stellar objects within the monitored fields, covering an area of557.8 arcmin2. Here we present the equatorial coordinates andBVI photometry for 26,712 stars in the M31 galaxy, along the eastern armand in the vicinity of the star-forming region NGC 206.
| WFPC2 Observations of Massive and Compact Young Star Clusters in M31
We present color-magnitude diagrams of four blue massive and compactstar clusters in M31: G38, G44, G94, and G293. The diagrams of the fourclusters reveal a well-populated upper main sequence and various numbersof supergiants. The U-B and B-V colors of the upper main sequence starsare used to determine reddening estimates of the different lines ofsight in the M31 disk. Reddening values range fromEB-V=0.20+/-0.10 to 0.31+/-0.11. We statistically removefield stars on the basis of completeness, magnitude, and color.Isochrone fits to the field-subtracted, reddening-corrected diagramsyield age estimates ranging from 63+/-15 to 160+/-60 Myr. Implicationsfor the recent evolution of the disk near NGC 206 are discussed. Basedon observations with the NASA/ESA Hubble Space Telescope obtained at theSpace Telescope Science Institute, which is operated by the Associationof Universities for Research in Astronomy, Inc., under NASA contractNAS5-26555.
| HST Observations of Massive Young Star Clusters in M31
We present color magnitude diagrams of four blue massive and compactstar clusters in M31: G38, G44, G94, and G293. The diagrams of the fourclusters reveal a well-populated upper main sequence and various numbersof supergiants. The U-B and B-V colors of the upper main sequence starsare used to determine reddening estimates of the different lines ofsight in the M31 disk. Reddening values range from EB-V =0.20+/- 0.10 to 0.31+/- 0.11. We statistically remove field stars on thebasis of completeness, magnitude and color. Isochrone fits to thefield-subtracted, reddening-corrected diagrams yield age estimatesranging from 63+/- 15 Myr to 160+/- 60 Myr. G38 and G44 are located tothe south south-west of the giant OB association NGC 206. We find theages of both G38 and G44 to be 100+/- 40 Myr. These clusters may havebeen formed by the same mechanism that has caused the formation of NGC206 if the density wave interaction located at NGC 206 has beenpropagating through the southern half of M31 at ~32 km/s with respect tothe stars in the disk for the past 100 Myr. This propagation velocity isalso consistent with the age distribution of the Cepheid population nearNGC 206 (Magnier et al. 1997). Support for this work was provided byNASA through grant number GO-06459.01-95A from the Space TelescopeScience Institute, which is operated by the Association of Universitiesfor Research in Astronomy, Incorporated, under NASA contract NAS5-26555.
| The DIRECT Project: Influence of Blending on the Cepheid Distance Scale. I. Cepheids in M31
We investigate the influence of blending on the Cepheid distance scale.Blending is the close association of a Cepheid with one or moreintrinsically luminous stars. High-resolution HST images are comparedwith our ground-based data, obtained as part of the DIRECT project, fora sample of 22 Cepheids in the M31 galaxy. The average (median) V-bandflux contribution from luminous companions that are not resolved on theground-based images is about 19% (12%) of the flux of the Cepheid. Thisis a large effect-at the 10% level for distances. The current Cepheiddistance estimates to M31 are all ground based and are thus affected(underestimated). We discuss indirect methods to find which Cepheids areblended, e.g., by the use of well-sampled light curves in at least twooptical bands. More generally, our ground-based resolution in M31corresponds to the HST resolution at about 10 Mpc. Blending leads tosystematically low distances in the observed galaxies and therefore tosystematically high estimates of H0; we discuss the issue andthe implications.
| Astrophysics in 1999
The year 1999 saw the arrival of a star with three planets, a universewith three parameters, and a solar corona that could be heated at leastthree ways. In addition, there were at least three papers on everyquestion that has ever been asked in astrophysics, from ``Will theUniverse expand forever?'' to ``Does mantle convection occur in one ortwo layers?'' The answers generally were, ``Yes,'' ``No,'' and ``None ofthe above,'' to each of the questions. The authors have done their bestto organize the richness around centers defined by objects, methods, andmadnesses.
| Distribution of stellar mass in young star clusters of our Galaxy and nearby galaxies
Stellar mass distribution in young star clusters of our Galaxy, theMagellanic Clouds and the nearby local groups of galaxies has been usedto investigate the universality of initial mass function and presence ofmass segregation in these systems. There is no obvious dependence of theMF slope on either galactocentric distance or age of the galactic openstar clusters. A comparison of initial mass function slopes that havebeen measured in star clusters and associations of our and nearbygalaxies indicates that the slope is independent of the spatialconcentration of the star formed, galactic characteristics includingmetallicity, and at least down to 0.85 M?, the stellar mass range.Effects of mass segregation have been observed in good number of youngstellar groups of our Galaxy and Magellanic Clouds. As their ages aremuch smaller than their dynamical evolution times, star formationprocesses seems to be responsible for the observed mass segregation inthem.
| DIRECT Distances to Nearby Galaxies Using Detached Eclipsing Binaries and Cepheids. V. Variables in the Field M31F
We undertook a long-term project, DIRECT, to obtain the direct distancesto two important galaxies in the cosmological distance ladder-M31 andM33-using detached eclipsing binaries (DEBs) and Cepheids. While rareand difficult to detect, DEBs provide us with the potential to determinethese distances with an accuracy better than 5%. The extensivephotometry obtained in order to detect DEBs provides us with good lightcurves for the Cepheid variables. These are essential to the parallelproject to derive direct Baade-Wesselink distances to Cepheids in M31and M33. For both Cepheids and eclipsing binaries, the distanceestimates will be free of any intermediate steps. As a first step in theDIRECT project, between September 1996 and October 1997 we obtained 95full/partial nights on the F. L. Whipple Observatory 1.2 m telescope and36 full nights on the Michigan-Dartmouth-MIT 1.3 m telescope to searchfor DEBs and new Cepheids in the M31 and M33 galaxies. In this paper,the fifth in the series, we present the catalog of variable stars foundin the field M31F [(α,δ)=(10.10d,40.72d), J2000.0]. We havefound 64 variable stars: four eclipsing binaries, 52 Cepheids and eightother periodic, possible long-period or nonperiodic variables. Thecatalog of variables, as well as their photometry and finding charts, isavailable via anonymous ftp and the World Wide Web. The complete set ofthe CCD frames is available upon request.
| A CO survey of the southwest half of M 31
A CO survey of most of the southwest half of M 31 at an effectiveangular resolution of 1' has been performed with the 14-m radiotelescopeof the Five College Radio Astronomy Observatory QUARRY receiver array.At the 200 pc linear resolution of this instrument at the distance of M31 (690 kpc), the CO emission, already known to peak in a broadPopulation I ring 10 kpc in radius, is found to be concentrated in twospiral arm segments, Baade's (1963) S3 and S4, both well defined inspace and velocity. Fainter CO emission features are also detectedfurther from the center, mainly in the outer arm S5, and closer to thecenter. The prominent molecular arms S3 and S4 are aligned with dust andatomic counterparts. At the intersection of S4 with the major axis,there is an offset between the arm defined by gas and dust and thatdefined by the classical tracers of Population I objects. The H IIregions are mostly concentrated along the outer edge of the gaseous arm,and the OB associations are found even further out. However, nosignificant streaming motions are detected across the arm. Althoughoverall M 31 is several times fainter in CO than the Milky Way, the COdistributions in the two galaxies are similar in certain respects. Theprominent molecular spiral arm S4 in M 31 is very much like the Carinaarm in our Galaxy at the same galactocentric radius, and beyond a radiusof about 8 kpc (the solar circle), M 31 and the Milky Way are verysimilar in their molecular content. The overall difference in COintensity between the two galaxies originates in the inner 7 kpc where M31 is very dim and the Milky Way very bright. However, even in thisinner region, common features are found. First, the ratio between the COintensity and the H I column density changes in the same way withgalactocentric radius in both systems, rising inwards to about 2-3 kpcfrom the center. Second, the molecular gas in the inner two kiloparsecsappears in both cases to follow a perturbed kinematics, perhaps as aresult of a stellar bar.
| Mid-infrared and far-ultraviolet observations of the star-forming ring of M 31
We present mid-IR images of a 15\arcminx15' field in the south-west partof the Andromeda galaxy M 31 obtained with the ISOCAM camera (6''pixels) on board ISO. These broad-band images complement spectro-imagingobservations of smaller fields (Cesarsky et al. \cite{Cesarsky98}). Wealso present a 20'' resolution far-UV image of a larger field at 200 nmobtained with the balloon-borne telescope FOCA 1000. These images areinter-compared and also compared with {H i} , CO(1-0) and Hα maps.The mid-IR emission as seen through wide-band filters centered at 7 and15 mu m is extremely well correlated with the distribution of neutralgas as shown by the {H i} and CO(1-0) maps, while the correlation ispoorer with the distribution of the ionized gas seen through itsHα emission. There is some correlation with the UV radiation, butit appears that the contribution of UV photons to the excitation of thecarriers of the mid-IR emission is not dominant in most of M 31. Thespectro-imaging observations of Cesarsky et al. (\cite{Cesarsky98}) showthat the mid-IR spectra of several regions of M 31, two of which are inthe presently studied area, are dominated by a strong emission band at11.3 mu m while emission in the other classical Aromatic Infrared Bands(AIBs) at 6.2, 7.7 and 8.6 mu m is faint or absent. This result isprecised, and we find that the mid-IR spectral variations are notclearly related to the UV radiation field. The present observations haveimportant consequences on our understanding of excitation of theinterstellar mid-IR emission in general. In particular, we conclude thatlike for M 31, excitation in the Galactic cirruses may not be dominatedby UV photons but rather by another mechanism which remains to beidentified (visible photons?). The UV excitation appears to becomeimportant when the UV radiation density is of the order of 2 times thatnear the Sun. Based on observations with ISO, an ESA project withinstruments funded by ESA member states (especially the PI countries:France, Germany, the Netherlands and the United Kingdom) and with theparticipation of ISAS and NASA.
| On the evolution of moving groups: an application to the Pleiades moving group
The disruption of stellar systems, such as open clusters or stellarcomplexes, stands out as one of the most reasonable physical processesaccounting for the young moving groups observed in the solarneighbourhood. In the present study we analyse some of the mechanismsthat are important in the kinematic evolution of a group of unboundstars, such as the focusing phenomenon and its ability to recover theobserved moving group's velocity dispersions, and the efficiency of discheating and galactic differential rotation in disrupting unbound stellarsystems. Our main tools used to perform this analysis are both theepicycle theory and the integration of the equations of motion using arealistic gravitational potential of the Galaxy. The study of thetrajectories followed by stars in each of the Pleiades moving groupsubstructures found by Asiain et al. _ote*{asiain2} allows usto determine their stellar spatial and velocity distribution evolution.The kinematic properties of these substructures are compared to those ofa simulated stellar complex which has evolved under the influence of thegalactic gravitational potential and the disc heating. We conclude thata constant diffusion coefficient compatible with the observationalheating law is able to explain the velocity and spatial dispersions ofthe Pleiades moving group substructures that are younger than sigmaim1.5 * 10(8) yr.
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