Stars In M31 Research Articles

The Metallicity Distribution Function of Field Stars in M31's Bulge

We have used Hubble Space Telescope Wide Field Planetary Camera 2 observations to construct a color-magnitude diagram (CMD) for the bulge of M31 at a location ∼1.6 kpc from the galaxy's center. Using scaled solar abundance theoretical red giant branches with a range of metallicities, we have translated the observed colors of the stars in the CMD to abundances and constructed a metallicity distribution function (MDF) for this region. The MDF shows a peak at [M/H] ∼ 0 with a steep decline at higher metallicities and a more gradual tail to lower metallicities. This is similar in shape to the MDF of the Milky Way bulge but shifted to higher metallicities by ∼0.1 dex. As is the case with the Milky Way bulge MDF, a pure closed-box model of chemical evolution, even with significant pre-enrichment, appears to be inconsistent with the M31 bulge MDF. However, a scenario in which an initial infall of gas enriched the bulge to an abundance of [M/H] ∼ -1.6 with subsequent evolution proceeding as a closed box provides a better fit to the observed MDF. The similarity between the MDF of the M31 bulge and that of the Milky Way stands in stark contrast to the significant differences in the MDFs of their halo populations. This suggests that the bulk of the stars in the bulges of both galaxies were in place before the accretion events that occurred in the halos could influence them.

Fluorine Abundance Variations in Red Giants of the Globular Cluster M4 and Early‐Cluster Chemical Pollution

We present fluorine abundances in seven red giant members of the globular cluster M4 (NGC 6121). These abundances were derived from the HF (1-0) R9 line at 2.3357 μm in high-resolution infrared spectra obtained with the Phoenix spectrograph on the Gemini South telescope. Many abundances in the target stars have been studied previously, so their overall abundance distributions are well mapped. The abundance of fluorine is found to vary by more than a factor of 6, with the 19F variations being correlated with the already established oxygen variations and anticorrelated with the sodium and aluminium variations. In this paper we thus add fluorine to the list of elements known to vary in globular cluster stars, and this provides further evidence that H burning is the root cause of the chemical inhomogeneities. The fact that 19F is found to decrease in the M4 stars, as the signature of H burning appears, indicates that the polluting stars must have masses greater than about 3.5 M☉, as less massive stars than this should produce, not destroy, fluorine.

Lithium survey in the solar age cluster M67

We present atmospheric parameters and Li abundances, obtained from a detailed spectroscopic analysis, for a sample of stars with different evolutionary stages (turn–off, subgiant and giant stars) in the solar age cluster M67. Observations were carried out with high resolution (R ∼ 47000) at high S/N using the UVES+FLAMES at VLT/UT2. From available photometry and computed synthetic spectra for the region around the Lithium line at 6707.78A, we derived atmospheric parameters (Teff, log g, [Fe/H], v sin i) and AL i for each star, in order to better understand the level of mixing and convective dilution of evolved stars in M67.

Hubble Space TelescopeWFPC2 Photometry of the Globular Clusters M3 and M13: Binaries and Intrinsic Broadening on the Main Sequence

We study the color-magnitude diagram (CMD) morphologies of M3 and M13 using high-precision Hubble Space Telescope photometry. Asymmetric broadening above and to the red of the main sequence is apparent. We use extensive artificial star experiments to synthetically reproduce chance superposition and photometric error and examine whether a main-sequence binary population is required to account for the observed asymmetric broadening. We find that the most probable binary fraction fb in the center of M3 lies between 6% and 22%. For those stars of M3 with distances from the cluster center between 1 and 2 core radii, the most probable value of fb lies between 1% and 3%. The similar radial distributions of binaries and blue stragglers (BSs) support the model that the BSs in the center of M3 are of collisional origin. A similar comparison between the color distributions of real stars and artificial stars in the CMD of M13 suggests that the real star distributions are consistently broader than the artificial ones. After corrections of real and artificial differences in color uncertainties, we find evidence for an intrinsic color dispersion among main-sequence stars of about 0.013–0.014 mag in both the center and outer regions of M13. This is larger than the expected value resulting from the spectroscopically observed [Fe/H] dispersion and might reflect differences in [C/Fe] and [N/Fe] that have been noted spectroscopically among upper main-sequence stars in M13.

Light Curve Analysis of Two New W Uma Stars in M15

Light Curve Analysis of Two New W Uma Stars in M15

Abundances in a Large Sample of Stars in M3 and M13

We have carried out a detailed abundance analysis for 21 elements in a sample of 25 stars with a wide range in luminosity from luminous giants to stars near the main-sequence turnoff in the globular cluster M13 ([Fe/H] = -1.50 dex) and in a sample of 13 stars distributed from the tip to the base of the red giant branch (RGB) in the globular cluster M3 ([Fe/H] = -1.39 dex). The analyzed spectra, obtained with HIRES at the Keck Observatory, are of high dispersion (R = ?/?? = 35,000). Most elements, including Fe, show no trend with Teff and low scatter around the mean between the top of the RGB and near the main-sequence turnoff, suggesting that at this metallicity, non-LTE effects and gravitationally induced heavy-element diffusion are not important for this set of elements over the range of stellar parameters spanned by our sample. We have detected an anticorrelation between O and Na abundances, observed previously among the most luminous RGB stars in both of these clusters, in both M3 and in M13 over the full range of luminosity of our samples, i.e., in the case of M13 to near the main-sequence turnoff. M13 shows a larger range in both O and Na abundance than does M3 at all luminosities, in particular having a few stars at its RGB tip with unusually strongly depleted O. We detect a correlation between Mg abundance and O abundance among the stars in the M13 sample. We also find a decrease in the mean Mg abundance as one moves toward lower luminosity, which we tentatively suggest is due to our ignoring non-LTE effects in Mg. Although CN burning must be occurring in both M3 and in M13, and ON burning is required for M13, we combine our new O abundances with published C and N abundances to confirm with quite high precision that the sum of C+N+O is constant near the tip of the giant branch, and we extend this down to the bump in the luminosity function. The same holds true for a smaller sample in M3, with somewhat larger variance. Star I-5 in M13 has large excesses of Y and of Ba, with no strong enhancement of Eu, suggesting that an s-process event contributed to its heavy-element abundances. The mean abundance ratios for M3 and for M13 are identical to within the errors. They show the typical pattern for metal-poor globular clusters of scatter among the light elements, with the odd atomic number elements appearing in the mean enhanced. The Fe-peak elements, where the odd atomic number elements are excessively depleted, do not show any detectable star-to-star variations in either cluster. The abundance ratios for 13 Galactic globular clusters with recent detailed abundance analyses, obtained by combining our samples with published data, are compared with those of published large surveys of metal-poor halo field stars. For most elements, the agreement is very good, suggesting a common chemical history for the halo field and cluster stars.

Chemical Abundances for Seven Giant Stars in M68 (NGC 4590): A Globular Cluster with Abnormal Silicon and Titanium Abundances

We present a detailed chemical abundance study of seven giant stars in M68, including six red giants and one post–asymptotic giant branch (AGB) star. We find significant differences in the gravities determined using photometry and those obtained from ionization balance, which suggests that non-LTE (NLTE) affects are important for these low-gravity, metal-poor stars. We adopt an iron abundance using photometric gravities and Fe II lines to minimize those effects, finding [Fe/H] = -2.16 ± 0.02 (σ = 0.04). For element-to-iron ratios, we rely on neutral lines versus Fe I and ionized lines versus Fe II (except for [O/Fe]) to also minimize NLTE effects. We find variations in the abundances of sodium among the program stars. However, there is no correlation (or anticorrelation) with the oxygen abundances. Furthermore, the post-AGB star has a normal (low) abundance of sodium. Both of these facts add further support to the idea that the variations seen among some light elements within individual globular clusters arise from primordial variations and not from deep mixing. M68, like M15, shows elevated abundances of silicon compared with other globular clusters and comparable-metallicity field stars. But M68 deviates even more in showing a relative underabundance of titanium. We speculate that in M68 titanium is behaving like an iron-peak element rather than its more commonly observed adherence to enhancements seen in the so-called α-elements such as magnesium, silicon, and calcium. We interpret this result as implying that the chemical enrichment seen in M68 may have arisen from contributions from supernovae with somewhat more massive progenitors than those that contribute to abundances normally seen in other globular clusters. The neutron capture elements barium and europium vary among the stars in M15, according to earlier work by Sneden et al., but [Ba/Eu] is relatively constant, suggesting that both elements arise in the same nucleosynthesis events. M68 shares the same [Ba/Eu] ratio as the stars in M15, but the average abundance ratio of these elements, as well as lanthanum, are lower in M68 relative to iron than in M15, implying a slightly weaker contribution of r-process nucleosynthesis in M68.

The Hot Interstellar Medium in M31 and M33

We report measurements from Far Ultraviolet Spectroscopic Explorer spectra of OB stars in M31 and M33, of O VI and C II interstellar absorptions. The stars are located in OB 78 in the outer region of M31 and in several locations across M33. The interstellar absorptions agree well between stars in nearby locations, and their principal radial velocities indicate that they arise in those galaxies and follow the disk rotation.

The Chandra Deep Field–North Survey. XVII. Evolution of Magnetic Activity in Old Late‐Type Stars

The extremely sensitive Chandra Deep Field–North (CDF-N) pencil-beam X-ray survey is used to identify and characterize the X-ray emission from old high-latitude main-sequence Galactic stars. Our principal goal is to investigate the expected long-term decay of magnetic activity of late-type stars due to the gradual spin-down of stellar rotation from a magnetized stellar wind. Thirteen X-ray sources are associated with late-type stars; 11 of these constitute a well-defined sample for statistical analysis. This sample consists of two G, two K0–K4, and seven M2–M5 stars with median V-band magnitude around 19 and median distance around 300 pc. X-ray luminosities are typically logLX ’ 27 ergs s � 1 but are substantially higher in two cases. The combination of large-amplitude variations on timescales of hours and plasma temperatures around 5–30 MK indicates that the observed X-ray emission is dominated by magnetic reconnection flares rather than quiescent coronal emission. These X-ray properties are quantitatively similar to those seen in the active contemporary Sun. The CDF-N stellar sample is compared to simulations based on convolution of X-ray luminosity functions (XLFs) with the known spatial distribution of old-disk stars. The model indicates that the CDF-N stars are the most magnetically active old-disk stars. A substantial decline in X-ray luminosities over the 1 Gyr < t < 11 Gyr age interval is required: 39 rather than 11 stars should have been detected if the XLF does not evolve over this time interval. This is a clear demonstration that the coronal and flaring components of stellar magnetic activity—and presumably the interior magnetic dynamos responsible for the reconnecting fields at the stellar surface—exhibit long-term decay over the age of the Galactic disk. The model that best fits the magnitudes, spectral types, and X-ray luminosities of the sample has LX / t � 2 ergs s � 1 , which is faster than the t � 1 decay rate predicted from widely accepted rotational spin-down rates and X-ray–activity relations. Subject headingg stars: activity — stars: coronae — stars: late-type — stars: magnetic fields — X-rays: stars

Dynamics and Stellar Content of the Giant Southern Stream in M31. I. Keck Spectroscopy of Red Giant Stars

This paper presents the first results from a large spectroscopic survey of red giant branch (RGB) stars in M31 using DEIMOS on the Keck 10 m telescope. A photometric prescreening method, based on the intermediate-width DDO51 band centered on the Mg b/MgH absorption feature, was used to select spectroscopic targets. RGB candidates were targeted in a small section of M31's giant southern tidal stream at a projected distance of 31 kpc from the galaxy's center. We isolate a clean sample of 68 RGB stars by removing contaminants (foreground Milky Way dwarf stars and background galaxies) using a combination of spectroscopic, imaging, and photometric methods: radial velocity and the surface gravity-sensitive Na I doublet are particularly useful in this regard. About 65% of the M31 stars are found to be members of the giant southern stream, while the rest appear to be members of the general spheroid population. The mean (heliocentric) radial velocity of the stream in our field is -458 km s-1, blueshifted by -158 km s-1 relative to M31's systemic velocity, in good agreement with recent velocity measurements at other points along the stream. The intrinsic velocity dispersion of the stream is found to be 15 km s-1 (90% confidence limit). A companion paper by Font and coworkers discusses possible orbits, implications of the coldness of the stream, and properties of the progenitor satellite galaxy. The kinematics, and possibly the metallicity distribution, of the general spheroid (i.e., nonstream) population in this region of M31 indicate that it is significantly different from samples drawn from other parts of the M31 spheroid; this is probably an indication of substructure in the bulge and halo. The stream appears to have a higher mean metallicity than the general spheroid, [Fe/H] ~ -0.54 versus -0.74, and a smaller metallicity spread. The relatively high metallicity of the stream implies that its progenitor must have been a luminous dwarf galaxy. The Ca II triplet line strengths of the M31 RGB stars are generally consistent with photometric estimates of their metallicity (derived by fitting RGB fiducials in the color-magnitude diagram). There is indirect evidence of a population of intermediate-age stars in the stream.

Hubble Space TelescopeObservations of the Main Sequence of M4

We present new results from a photometric study of the main-sequence stars in M4 (NGC 6121=C1620-264), the closest globular cluster to the Sun. Multifield, multiepoch observations at approximately 1, 2, and 6 core radii were obtained with WFPC2 on the Hubble Space Telescope through either the F606W filter or the F555W (V) and F814W (I) filters. The multiepoch observations allowed us to clean the data on the basis of proper motion and thus separate cluster from field stars or extragalactic objects. In all the fields the cluster main sequence can be traced to at least V = 27.0, but there remains a trail of stars to the limit of the data near V = 30 in the deepest outer field. There is no evidence that we have reached the end of the hydrogen-burning main sequence in any of our fields; however, there is some indication that very few stars remain to be detected in the deepest data. A study of the scatter about the cluster main sequence yields a surprisingly small and variable binary fraction: fb 2% in the inner parts of the cluster, falling to the 1% range outside. However, with one possible exception, no stars in the 6 core radius field exhibit photometric variability on timescales of a few hours through a few days. For the currently visible main-sequence stars, the cluster mass function (MF) is very flat (? = 0.1) in the outer field and flattens further in the inner fields, suggesting well-developed mass segregation. The observed variation in the MF is broadly consistent with isotropic, multimass Michie-King models. Because we have a large sample of white dwarfs in the outer field, we are able to show that the cluster IMF above 0.8 M? was considerably steeper than the present-day MF for low-mass stars. Two appendices are included in this contribution. The first is a detailed discussion of the techniques used to reduce the data, while the second provides a direct comparison between the cluster stars and those belonging to the inner spheroid of the Galaxy. This yields a relative distance between the cluster, dc, and the Galactic center, Ro, of Ro/dc = 4.36 ? 0.13. With our subdwarf-based estimate of dc = 1.73 ? 0.09 kpc to M4, we find Ro = 7.5 ? 0.6 kpc.

The Chemical Composition Contrast between M3 and M13 Revisited: New Abundances for 28 Giant Stars in M3

We report new chemical abundances of 23 bright red giants of the globular cluster M3, based on high-resolution spectra obtained with the Keck I telescope. Combining these data with a previously-reported small sample of M3 giants obtained with the Lick 3m telescope, we compare [X/Fe] ratios for 28 M3 giants with 35 M13 giants, and with halo field stars. All three groups exhibit C depletion with advancing evolutionary state beginning at the RGB bump region. but the overall depletion in the clusters is larger than that of the field stars. The behaviors of O, Na, Mg and Al are distinctively different among the three stellar samples. Both M3 and M13 show evidence of high-temperature proton capture synthesis from the ON, NeNa, and MgAl cycles, while there is no evidence for such synthesis among halo field stars. But the degree of such extreme proton-capture synthesis in M3 is smaller than it is in M13, and no indication that O depletions are a function of advancing evolutionary state as has been claimed for M13. We have also considered NGC 6752, for which Mg isotopic abundances have been reported by Yong et al. (2003). Giants in NGC 6752 and M13 satisfy the same anticorrelation of O abundances with the ratio (25Mg+26Mg)/24Mg. This suggests that these abundance ratios arose in the ejected material of 3-6 solar mass cluster stars. It also suggests that the low oxygen abundance seen among the most evolved M13 giants arose in hot-bottom O to N processing in these same intermediate-mass cluster stars. Thus mixing is required by the dependence of some abundance ratios on luminosity, but an earlier nucleosynthesis process in a hotter environment than giants or main-sequence stars is required by the variations previously seen in stars near the main sequence.

Evidence of Halo Microlensing in M31

We have completed an intensive monitoring program of two fields on either side of the center of M31 and report here on the results concerning microlensing of stars in M31. These results stem from a 3 yr study ( the Vatican Advanced Technology Telescope [VATT]/Columbia survey) of microlensing and variability of M31 stars, emphasizing microlensing events of 3 day to 2 month timescales and likely due to masses in M31. These observations were conducted intensively from 1997 to 1999, with baselines 1995 to present, at the VATT and the 1.3 m telescope at MDM Observatory, with additional data from the Isaac Newton Telescope, including about 200 epochs total. The two fields monitored cover 560 arcmin(2) total, positioned along the minor axis on either side of M31. Candidate microlensing events are subject to a number of tests discussed here with the purpose of distinguishing microlensing from variable star activity. A total of four probable microlensing events, when compared with carefully computed event rate and efficiency models, indicate a marginally significant microlensing activity above that expected for the stars alone in M31 ( and the Galaxy) acting as lenses. A maximum likelihood analysis of the distribution of events in timescale and across the face of M31 indicates a microlensing dark matter halo fraction consistent with that seen by MACHO in our Galaxy toward the Large Magellanic Cloud. Specifically, for a nearly singular isothermal sphere model, we find a microlensing halo mass fraction fb = 0.29(-0.13)(+0.30) of the total dark matter and a poorly constrained lensing component mass (0.02 - 1.5 M-.; 1 sigma limits). This study serves as the prototype for a larger study approaching completion; between the two there is significant evidence for an asymmetry in the distribution of microlensing events across the face of M31 and therefore a large population of halo microlensing dark matter objects.

Wolf-Rayet stars in M33 - I. Optical spectroscopy using CFHT-MOS

We have obtained spectroscopy of a large sample of Wolf-Rayet stars in M33 with the Canada-France-Hawaii Telescope Multi-Object Spectrograph (CFHT-MOS), including 26 WC stars, 15 WN stars and a WN/C star. In general, spectral types are merely refined, although the spectral type of X9 from Massey & Johnson is revised from WNL?+abs to WC4+abs, whilst their G1 and C21 candidates are not confirmed as Wolf-Rayet stars. We also re-examine the metallicity gradient of M33 from H ii regions and identify the present sample, lying in the inner disc, with 8.6 ≤ log(O/H) ≤ 8.9. Spectral types are in accord with similar regions in the Milky Way. Our large sample has allowed us to examine the claimed anticorrelation between WC linewidths and galactocentric distance by Schild et al. We find a much larger scatter, though there remains an absence of broad-line WC stars in the inner disc and narrow-line WC stars in the outer galaxy.

Variable stars in M31 from the MEGA survey

AbstractThe MEGA microlensing survey is introduced and several topics about variable stars in M31 are discussed. An adopted model population of semiregular (SR) variable stars is presented, and the efficiency to recover these stars in M31 in a specific example MEGA dataset is predicted. The period-flux amplitude relation of the SRs discovered by MEGA could be used to measure the inclination of the M31 disk.

Variable stars from two surveys of microlensing in M31

AbstractRecently we completed the VATT/Columbia survey monitoring a portion of M31 for microlensing and stellar variability, and report here on the microlensing result. Also, we have nearly completed a much larger survey (MEGA) covering most of the stars in M31, for which we have started cataloging many tens of thousands of Cepheids, Miras, eclipsing variables and other kinds of variable stars. The microlensing results from these two surveys hint that a significant fraction of the dark matter in M31 is composed of stellar-mass objects in the halo.

Evidence for Halo Microlensing from a Survey of M31

We report on the final microlensing result from one survey (VATT/Columbia) monitoring stars in M31, and the initial results of a larger study (MEGA), which together seem to indicate the presence of a microlensing halo component comprising a significant fraction of the dark matter in M31.

The distance to the Large Magellanic Cloud from first overtone RR Lyrae variables

AbstractFourier coefficients have been derived for the V and R light curves of 330 bona fide RR Lyrae first-overtone (RR1) pulsators in 16 MACHO fields near the bar of the LMC. We use the Fourier phase parameter ɸ31 and log P values to select a subsample of these stars which are similar to the RR1s in the Galactic globular cluster M5. Assuming that the M5-like stars in the LMC have absolute magnitudes comparable to the HB stars in M5, we use independent studies to derive their mean absolute V magnitude and compute a visual distance modulus of μLMC = 18.43 ± 0.06 (statistical) ±0.16 (systematic). By selecting stars on the basis of their light curve parameters, we are able to derive a distance modulus that does not depend on the somewhat poorly determined Mv – [Fe/H] relation for RR Lyr stars.

The effect of stellar evolution on the light curves’ shapes of RR Lyrae stars in M3

AbstractThe light curve characteristics of a homogeneous sample of variables in M3 are studied in detail. Accurate light curves and mean magnitudes of about 100 RRab and 50 RRc stars which do not show any type of modulation are analyzed. According to their mean magnitudes and Fourier parameters the variables can be sorted into four groups, representing different stages of the horizontal branch stellar evolution.

The Chemical Inhomogeneity of Faint M13 Stars: Carbon and Nitrogen Abundances

Building upon earlier observations that demonstrate substantial star-to-star differences in the carbon abundances of M13 subgiants, we present new Keck LRIS spectra reaching more that 1.5 mag below the M13 main-sequence turnoff (to V ≈ 20). Our analysis reveals a distribution of C abundances similar to that found among the subgiants, implying little change in the compositions of the M13 stars at least through the main-sequence turnoff. We presume these differences to be the result of some process operating early in the cluster history. Additional spectra of previously studied bright M13 giants have been obtained with the 5 m Hale Telescope. A comparison of C abundances derived using the present methods and those from the literature yield a mean difference of 0.03 ± 0.14 dex for four stars in common with the 1996 study by Smith et al. and 0.14 ± 0.07 dex for stars also observed in Suntzeff's 1981 survey (if one extreme case is removed). We conclude that the lower surface C abundances of these luminous giants as compared with the subgiants and main-sequence stars are likely the result of mixing rather than a difference in our abundance scales. NH band strengths have also been measured for a handful of the most luminous M13 turnoff stars. While molecular band formation in such stars is weak, significant star-to-star NH band strength differences are present. Moreover, for the stars with both C and N measurements, differences between stars in these two elements appear to be anticorrelated. Finally, the most recent C and N abundances for main-sequence, main-sequence turnoff, and subgiant stars in 47 Tuc, M71, M5, and the present M13 data are compared.