Bright Asymptotic Giant Branch Stars Research Articles

Distance estimates for AGB stars from parallax measurements

Context. Estimating the distances to asymptotic giant branch (AGB) stars using optical measurements of their parallaxes is not straightforward because of the large uncertainties introduced by their dusty envelopes, their large angular sizes, and their surface brightness variability. Aims. This paper aims to assess the reliability of the distances derived with Gaia DR3 parallaxes for AGB stars, and provide a new distance catalogue for a sample of ∼200 nearby AGB stars. Methods. We compared the parallaxes from Gaia DR3 with parallaxes measured with maser observations with very long baseline interferometry (VLBI) to determine a statistical correction factor for the DR3 parallaxes using a sub-sample of 33 maser-emitting oxygen-rich nearby AGB stars. We then calculated the distances of a total of ∼200 AGB stars in the DEATHSTAR project using a Bayesian statistical approach on the corrected DR3 parallaxes and a prior based on the previously determined Galactic distribution of AGB stars. We performed radiative transfer modelling of the stellar and dust emission to determine the luminosity of the sources in the VLBI sub-sample based on the distances derived from maser parallaxes, and derived a new bolometric period-luminosity relation for Galactic oxygen-rich Mira variables. Results. We find that the errors on the Gaia DR3 parallaxes given in the Gaia DR3 catalogue are underestimated by a factor of 5.44 for the brightest sources (G < 8 mag). Fainter sources (8 ≤ G < 12) require a lower parallax error inflation factor of 2.74. We obtain a Gaia DR3 parallax zero-point offset of −0.077 mas for bright AGB stars. The offset becomes more negative for fainter AGB stars. After correcting the DR3 parallaxes, we find that the derived distances are associated with significant, asymmetrical errors for more than 40% of the sources in our sample. We obtain a PL relation of the form Mbol = (− 3.31 ± 0.24) [log P − 2.5]+(−4.317 ± 0.060) for the oxygen-rich Mira variables in the Milky Way. A new distance catalogue based on these results is provided for the sources in the DEATHSTAR sample. Conclusions. The corrected Gaia DR3 parallaxes can be used to estimate distances for AGB stars using the AGB prior, but we confirm that one needs to be careful when the uncertainties on parallax measurements are larger than 20%, which can result in model-dependent distances and source-dependent offsets. We find that a RUWE (re-normalised unit weight error) below 1.4 does not guarantee reliable distance estimates and we advise against the use of only the RUWE to measure the quality of Gaia DR3 astrometric data for individual AGB stars.

Infrared Properties of Asymptotic Giant Branch Stars in Our Galaxy and the Magellanic Clouds

Abstract We investigate the infrared properties of asymptotic giant branch (AGB) stars in our Galaxy and the Magellanic Clouds using various infrared observational data and theoretical models. We use catalogs for the sample of 4996 AGB stars in our Galaxy and about 39,000 AGB stars in the Magellanic Clouds from the available literature. For each object in the sample, we cross-identify the 2MASS, Wide-field Infrared Survey Explorer, and Spitzer counterparts. To compare the physical properties of O- and C-rich AGB stars in our Galaxy and the Magellanic Clouds, we present IR two-color diagrams (2CDs) using various photometric data. We perform radiative transfer model calculations for AGB stars using various possible parameters of central stars and dust shells. Using the dust opacity functions of amorphous silicate and carbon, the theoretical dust shell models can roughly reproduce the observations of AGB stars on various IR 2CDs. Compared with our Galaxy, we find that the Magellanic Clouds are deficient in AGB stars with thick dust shells. Compared with the Large Magellanic Cloud (LMC), the Small Magellanic Cloud (SMC) is more deficient in AGB stars with thick dust shells. This could be because the Magellanic Clouds are more metal-poor than our Galaxy and the LMC is more metal-rich than the SMC. We also present the IR properties of known pulsating variables. Investigating the magnitude distributions at mid-IR (MIR) bands for AGB stars in the Magellanic Clouds, we find that the SMC is more deficient in bright AGB stars at MIR bands compared with the LMC.

THE DEEPESTHUBBLE SPACE TELESCOPECOLOR-MAGNITUDE DIAGRAM OF M32. EVIDENCE FOR INTERMEDIATE-AGE POPULATIONS

We present the deepest optical color-magnitude diagram (CMD) to date of the local elliptical galaxy M32. We have obtained F435W and F555W photometry based on HST ACS/HRC images for a region 110" from the center of M32 and a background field about 320" away from M32 center. Due to the high resolution of our Nyquist-sampled images, the small photometric errors, and the depth of our data we obtain the most detailed resolved photometric study of M32 yet. Deconvolution of HST images proves to be superior than other standard methods to derive stellar photometry on extremely crowded HST images. The location of the strong red clump in the CMD suggests a mean age between 8 and 10 Gyr for [Fe/H] = -0.2 in M32. We detect for the first time a red giant branch bump and an asymptotic giant branch bump in M32 which indicate that the mean age of M32's dominant population at ~2' from its center is between 5 and 10 Gyr. We see evidence of an intermediate-age population in M32 mainly due to the presence of bright asymptotic giant branch stars. Our detection of a blue component of stars (blue plume) may indicate for the first time the presence of a young stellar population, with ages of the order of 0.5 Gyr, in our M32 field. However, it is likely that the brighter stars of this blue plume belong to the disk of M31 rather than to M32. The fainter stars populating the blue plume indicate the presence of stars not younger than 1 Gyr and/or blue straggler stars in M32. M32's dominant population of 8--10 Gyr implies a formation redshift of 1 < z_f < 2, precisely when observations of the specific star formation rates and models of "downsizing" imply galaxies of M32's mass ought to be forming their stars. Our CMD therefore provides a "ground-truth" of downsizing scenarios at z=0. Our background field data represent the deepest optical observations yet of the inner disk and bulge of M31. [Abridged]

THE YOUNG OUTER DISK OF M83

Deep near-infrared images recorded with NICI on Gemini South are used to investigate the evolved stellar content in the outer regions of the south east quadrant of the spiral galaxy M83. A diffuse population of asymptotic giant branch (AGB) stars is detected outside of the previously identified young and intermediate age star clusters in the outer disk. The brightest AGB stars have M_K > -8, and the AGB luminosity function (LF) is well-matched by model LFs that assume ages < 1 Gyr. The specific star formation rate (SFR) during the past few Gyr estimated from AGB star counts is consistent with that computed from mid-infrared observations of star clusters at similar radii, and it is concluded that the disruption timescale for star clusters in the outer disk is << 1 Gyr. The luminosity function and specific frequency of AGB stars varies with radius, in a manner that is indicative of lower luminosity-weighted ages at larger radii. Modest numbers of red supergiants are also found, indicating that there has been star formation during the past 100 Myr, while the ratio of C stars to M giants is consistent with that expected for a solar metallicity system that has experienced a constant SFR for the past few Gyrs.

The Stellar Populations of M32: Resolving the nearest elliptical with HST ACS/HRC

AbstractWe present the deepest colour-magnitude diagram (CMD) of M32 to date, obtained from deep (F435W, F555W) photometry of HST ACS/HRC images. Due to the high resolution of our images, the small photometric errors, and the completeness level of our data we obtain the most detailed resolved photometric study of M32 to date. The CMD of M32 displays a wide colour distribution of red giant branch stars, mainly due to a metallicity spread, a strong red clump and bright asymptotic giant branch stars. The detection of a “blue plume” in M32 indicates the presence of a very young stellar population. There is not a noticeable presence of blue horizontal branch stars, suggesting that an old population with [Fe/H] &lt; −1.5 does not significantly contribute to the light or mass of M32 in our observed fields.

AND THE REST: THE STELLAR ARCHEOLOGICAL RECORD OF M82 OUTSIDE THE CENTRAL STARBURST

Deep images obtained with MegaCam and WIRCam on the Canada-France-Hawaii Telescope are used to probe the stellar content outside of the central star-forming regions of M82. Stars evolving on the asymptotic giant branch (AGB) are traced along the major axis out to projected distances of 12 kpc, which corresponds to 13 disk scale lengths. The numbers of red supergiants (RSGs) and AGB stars normalized to local surface brightness (the specific frequency or SF) is constant when R GC > 4 kpc, indicating that RSGs and AGB stars are well mixed throughout the disk. Moreover, the SF of bright AGB stars in the outer disks of M82 and the Sc galaxy NGC 2403 are identical, suggesting that the specific star-formation rates (SFRs) in these galaxies during intermediate epochs were similar. This similarity in stellar content, coupled with the presence of an extended stellar disk, is consistent with M82 having been a late-type disk galaxy prior to interacting with M81. Still, there is a paucity of RSGs in the outer disk of M82 when compared with NGC 2403, indicating that the SFR in the outer regions of M82 during the past ~0.1 Gyr has declined dramatically with respect to that in isolated late-type galaxies. The stellar content of the M82 disk plane is also investigated. A mixture of bright main-sequence stars, RSGs, and AGB stars is detected out to minor-axis distances of 7 kpc. These stars, which span a range of ages, are concentrated along the outflow. The brightest extraplanar AGB stars define a system with an exponential scale height of 1.8 ± 0.2 kpc, as measured along the minor axis. It is suggested that the young- and intermediate-aged stars in the extraplanar regions formed in structures similar to M82 South, and that these were subsequently disrupted by the tidal action of M82.

THE STELLAR CONTENT OF THE POST-STARBURST S0 GALAXY NGC 5102

The stellar content of the S0 galaxy NGC 5102 is investigated using deep r' and i' images obtained with GMOS-S. A modest population of bright main-sequence stars and red supergiants is detected throughout the western portion of the disk. Based on the numbers of main-sequence stars, the star-formation rate (SFR) in NGC 5102 during the past ten million years is estimated to have been 0.02 M ? year. The majority of red giant branch (RGB) stars in the disk of NGC 5102 have [M/H] between ?0.9 and ?0.1, and the metallicity distribution of RGB stars at intermediate galactocentric radii peaks near [M/H] ~?0.6. RGB stars are traced out to galactocentric distances of 10 kpc, which corresponds to ~14 disk scale lengths. A large population of bright asymptotic giant branch stars is seen throughout the western portion of the disk, and the youngest of these have log(t yr) ~ 8.1. It is concluded that (1) stars that formed within the past Gyr comprise ~20% of the total stellar disk mass and (2) the SFR during intermediate epochs in the disk of NGC 5102 was at least 1.4 M ? year. Thus, large-scale star formation occurred throughout the disk of NGC 5102 at approximately the same time that similar elevated levels of star formation occurred in the bulge. It is suggested that NGC 5102 was a spiral galaxy that experienced a galaxy-wide episode of star formation that terminated a few hundred Myr in the past, and that much of its interstellar medium was ejected in an outflow.

The Brightest Stars in M32: Comparing Predictions from Spectra with the Resolved Stellar Content

Broad- and narrow-band images covering the 1 - 4um wavelength interval are used to investigate the properties of the brightest AGB stars in the Local Group galaxy M32. The brightest AGB stars near the center of M32 have peak M_L' brightnesses and K-L' colors that are similar to those of luminous AGB stars in the Galactic disk, while the density of bright AGB stars per unit visible and near-infrared surface brightness is found to be constant out to projected major axis distances of 1 kpc. The J-K color distribution of bright AGB stars throughout much of the galaxy is consistent with that of a single population of AGB stars, the majority of which are long period variables, having a common metallicity and age. Thus, these data do not support spectroscopic studies that find an age gradient in M32. The well-mixed AGB content of M32 is consistent with tidal stirring. The stellar content of M32 is compared with that of the M31 bulge. While the peak K-band brightnesses of AGB stars in the two systems agree to within a few tenths of a magnitude, M32 contains more bright AGB stars per unit integrated brightness than the outer bulge of M31.

A Large Stellar Evolution Database for Population Synthesis Studies. III. Inclusion of the Full Asymptotic Giant Branch Phase and Web Tools for Stellar Population Analyses

Stellar evolution tracks and isochrones are key inputs for a wide range of astrophysical studies; in particular, they are essential to the interpretation of photometric and spectroscopic observations of resolved and unresolved stellar populations. We have made available to the astrophysical community a large, homogenous database of up-to-date stellar tracks and isochrones and a set of programs useful in population synthesis studies. In this paper we first summarize the main properties of our stellar model database (BaSTI ) already introduced by Pietrinferni and coworkers. We then discuss an important update of the database, i.e., the extension of all stellar models and isochrones until the end of the thermal pulses along the asymptotic giant branch. This extension of the library is particularly relevant for stellar population analyses in the near-infrared or longer wavelengths, where the contribution to the integrated photometric properties by cool and bright asymptotic giant branch stars is significant. A few comparisons with empirical data are also presented and briefly discussed. We then present three Web tools that allow an interactive access to the database and make it possible to compute user-specified evolutionary tracks, isochrones, stellar luminosity functions, and synthetic color-magnitude diagrams and integrated magnitudes for arbitrary star formation histories. All these Web tools are available at the BaSTI database official Web site.

Near-IR photometry of asymptotic giant branch stars in the dwarf elliptical galaxy NGC 147

Near-infrared J, H and K' images were used to investigate the stellar contents of the asymptotic giant branch (AGB) population in the nearby dwarf elliptical galaxy NGC 147. The obtained (K, J - K) and (K, H - K) color-magnitude diagrams contain stars of AGB and red giant branch populations, where the former consists of a group of bright blue stars, a dominant population of M giant and a red C star population. We identified 91 AGB C stars in NGC 147 with the mean absolute magnitude and colors of = -7.56, = 1.81 and = 0.74. The estimated number ratio of C stars to M giant stars (C/M) is 0.16 ± 0.02. The estimated local C/M ratios of 0.14 ± 0.02 for the inner region (r 70) indicate a weak radial gradient. The mean bolometric magnitude of 91 C stars in NGC 147 is = -4.32 ± 0.49. The bolometric luminosity function of M giant stars in NGC 147 extends up to M bol = -5.8 mag, and that of only C stars spans -5.6 < M bol < -3.5. The color histograms of AGB stars in (J - K) and (H - K) show a main peak containing M giant stars, a red tail containing C stars with a weak excess, and a blue excess possibly due to bright blue foreground stars and AGB stars younger than M giants. The comparison of the theoretical isochrone models with the color distribution and the brightness of AGB stars indicates that most of the bright M giants in NGC 147 were formed at log(t yr ) ∼ 8.4 with a range of 8.2 ∼ 8.6. Ages of the majority of M giant stars in NGC 147 are similar to those of younger M giant stars in its companion NGC 185, while their formation occurred more recently than the older M giant stars in NGC 185 by ∼2 Gyr.

AHSTStudy of the Stellar Populations in the Cometary Dwarf Irregular Galaxy NGC 2366

We present V and I photometry of the resolved stars in the cometary dwarf irregular galaxy NGC 2366, using Wide Field Planetary Camera 2 images obtained with the Hubble Space Telescope. The resulting color-magnitude diagram reaches down to I ~ 26.0 mag. It reveals not only a young population of blue main-sequence stars (age 30 Myr) but also an intermediate-age population of blue and red supergiants (20 Myr age 100 Myr) and older evolved populations of asymptotic giant branch (AGB) stars (age 100 Myr) and red giant branch (RGB) stars (age 1 Gyr). The measured magnitude I = 23.65 ± 0.10 mag of the RGB tip results in a distance modulus m - M = 27.67 ± 0.10, which corresponds to a distance of 3.42 ± 0.15 Mpc, in agreement with previous distance determinations. The youngest stars are associated with the bright complex of H II regions NGC 2363 (=Mrk 71) in the southwest extremity of the galaxy. As a consequence of the diffusion and relaxation processes of stellar ensembles, the older the stellar population is, the smoother and more extended is its spatial distribution. An underlying population of older stars is found throughout the body of NGC 2366. The most notable feature of this older population is the presence of numerous relatively bright AGB stars. The number ratio of AGB to RGB stars and the average absolute brightness of AGB stars in NGC 2366 are appreciably higher than in the BCD VII Zw 403, indicating a younger age of the AGB stars in NGC 2366. In addition to the present burst of age 100 Myr, there has been strong star formation activity in the past of NGC 2366, from ~100 Myr to 3 Gyr ago.

The Disk and Extraplanar Regions of NGC 55

The stellar content of the nearby SB(s)m galaxy NGC 55 is investigated using images obtained with the Gemini South and Canada-France-Hawaii telescopes. The (K, H - K) and (K, J - K) color-magnitude diagrams (CMDs) of stars near the plane of the disk reveal signatures of large-scale star formation during recent and intermediate epochs in the form of red supergiants (RSGs) with MK = -11.5, and an asymptotic giant branch (AGB) that peaks near MK = -10. Comparisons with stellar evolution models suggest that the brightest RSGs have an age near 8 Myr. A well-defined plume, which stellar evolution models suggest contains stars with masses near the RSG-AGB transition, is detected in CMDs constructed both from infrared and visible wavelength observations. It is concluded that star formation in the thin disk of NGC 55 has occurred at a significant rate for at least the past 0.1-0.2 Gyr, and this is consistent with other indicators. The near-infrared spectral energy distribution of the integrated light near the center of the galaxy is consistent with that in other Magellanic irregular galaxies, indicating that the star-forming history of NGC 55, when averaged over timescales of 0.1-1 Gyr, has likely not been peculiar when compared with other late-type systems. Evidence is also presented that the disk contains a large population of old [log(tyr) ~ 10] stars, and it is argued that a stable disk has been in place in NGC 55 for a significant fraction of the age of the universe. At projected distances in excess of 2 kpc off of the disk plane, the brightest AGB stars have ages 10 Gyr. Thus, despite indications that dust and gas are present in the envelope surrounding the NGC 55 disk, the AGB content suggests that recently formed stars do not occur in large numbers in the extraplanar region. The (r' - i') colors of the RGB in the extraplanar region are consistent with [Fe/H] between -2.2 and -0.7, with the majority of stars having [Fe/H] > -1.2, and the mean metallicity inferred from the RGB color does not change with distance above the disk plane. Thus, the stellar component in the extraplanar envelope is well mixed, at least in terms of metallicity. The mean metallicity of RGB stars is in excellent agreement with that measured in the extraplanar H II regions EHR 1 and 2, suggesting that the age-metallicity relation in this part of NGC 55 has been flat for at least a few Gyr. Finally, the RGB tip occurs near i' = 23.1 in the extraplanar region, and a distance modulus of 26.5 is computed from this feature.

Photometric Variability among the Brightest Asymptotic Giant Branch Stars near the Center of M32

Deep K' images with 01 angular resolution, obtained with Altair+NIRI on Gemini North, are used to investigate photometric variability among the brightest asymptotic giant branch (AGB) stars in the central regions of M32. On the basis of a comparison with brightnesses obtained from the K-band data discussed by Davidge et al., it is concluded that (1) at least 60% of bright AGB stars near the center of M32 are photometrically variable, and (2) the amplitudes of the light variations are similar to those of long-period variables in the Galactic bulge. We do not find evidence for a population of large-amplitude variables, such as those detected by IRAS in the Galactic bulge. The technique discussed here may prove useful for conducting an initial reconnaisance of photometric variability among AGB stars in spheroids in the Virgo Cluster and beyond, where the required long exposure times may restrict observations to only a few epochs.

Deep Near‐Infrared Imaging of a Field in the Outer Disk of M82 with the Altair Adaptive Optics System on Gemini‐North

ABSTRACTDeep H and K′ images recorded with the Altair adaptive optics system and Near‐Infrared Imager on Gemini‐North are used to probe the red stellar content in a field with a projected distance of 1 kpc above the disk plane of the starburst galaxy M82. The data have an angular resolution of 008 FWHM, and individual asymptotic giant branch (AGB) and red giant branch (RGB) stars are resolved. The AGB extends to at least 1.7 mag in K above the RGB tip, which occurs at K = 21.7. The relative numbers of bright AGB stars and RGB stars are consistent with stellar evolution models, and one of the brightest AGB stars has an H−K color and K brightness that is consistent with it being a C star. The brightnesses of the AGB stars suggest that they formed during intermediate epochs, possibly after the last major interaction with M81. Therefore, star formation in M82 during intermediate epochs may not have been restricted to the plane of the disk.

The Outer Regions of the Nearby Sc Galaxies NGC 2403 and M33: Evidence for an Intermediate-Age Population at Large Radii

Deep g'r'i'z' images obtained with the Gemini Multiobject Spectrograph (GMOS) on Gemini North are used to investigate the stellar content in the outer regions of the nearby Sc galaxies NGC 2403 and M33. The field observed in NGC 2403 covers galactocentric distances between 5 and 11 kpc perpendicular to the line of sight (R) and 7 and 19 kpc along the plane of the disk (R). The red giant branch (RGB) tip occurs at i' = 23.6 ? 0.1, and the Cepheid and RGB-tip distance scales for NGC 2403 are in good agreement. The number density of bright main-sequence stars in this field experiences a steep cutoff at R ~ 10 kpc, which is consistent with the expected truncation radius of the disk predicted from studies of edge-on spiral galaxies. While very young stars are restricted to R < 10 kpc, a population of bright asymptotic giant branch (AGB) stars is present throughout the entire GMOS field, indicating that star formation occurred outside of the present-day star-forming disk of NGC 2403 during intermediate epochs. The AGB stars are not in a tidal stream; in fact, the ratio of AGB stars above the RGB tip to those below the RGB tip does not change with radius, indicating that the bright AGB stars are uniformly mixed with the fainter stellar content throughout the field. The AGB luminosity function (LF) scales with r-band surface brightness over a wide range of radii throughout the main body of NGC 2403, indicating that the age distribution of stars in the outer regions of the present-day star-forming disk is not skewed to younger values than in the inner disk. Based on the color of stars on the upper portions of the RGB, it is concluded that metallicity changes across the field, with [Fe/H] = -0.8 ? 0.1(random) ? 0.3(systematic) at R = 5 kpc, and [Fe/H] = -2.2 ? 0.2(random) ? 0.8(systematic) at R = 11 kpc. The M33 field samples R between 8 and 10 kpc and R between 14 and 17 kpc. Bright AGB stars are detected in this field, and the ratio of bright AGB stars to stars on the upper RGB is at least as large as that measured in the outer regions of NGC 2403; thus, an intermediate-age population occurs well outside of the young star-forming disk of M33. The color of stars on the upper RGB suggests that [Fe/H] = -1.0 ? 0.3(random) ? 0.3(systematic) in this field. The globular cluster systems of both NGC 2403 and M33 contain objects that formed during intermediate epochs, and it is suggested that the luminous AGB stars detected in the current study are the field counterparts of these clusters. The detection of intermediate-age stars in the outer regions of these galaxies is consistent with models in which late-type spiral galaxies formed more slowly than earlier-type systems.

The Upper Asymptotic Giant Branch of the Elliptical Galaxy Maffei 1 and Comparisons with M32 and NGC 5128

Deep near-infrared images obtained with adaptive optics systems on the Gemini North and Canada-France-Hawaii telescopes are used to investigate the bright stellar content and central regions of the nearby elliptical galaxy Maffei 1. Stars evolving on the upper asymptotic giant branch (AGB) are resolved in a field 3 arcmin from the center of the galaxy. The locus of bright giants on the (K, H-K) color-magnitude diagram is consistent with a population of stars like those in Baade's Window reddened by E(H-K) = 0.28 +/- 0.05 mag. This corresponds to A_V = 4.5 +/- 0.8 mag, and is consistent with previous estimates of the line of sight extinction computed from the integrated properties of Maffei 1. The AGB-tip occurs at K = 20.0, which correponds to M_K = -8.7; hence, the AGB-tip brightness in Maffei 1 is comparable to that in M32, NGC 5128, and the bulges of M31 and the Milky-Way. The near-infrared luminosity functions (LFs) of bright AGB stars in Maffei 1, M32, and NGC 5128 are also in excellent agreement, both in terms of overall shape and the relative density of infrared-bright stars with respect to the fainter stars that dominate the light at visible and red wavelengths. It is concluded that the brightest AGB stars in Maffei 1, NGC 5128, M32, and the bulge of M31 trace an old, metal-rich population, rather than an intermediate age population. It is also demonstrated that Maffei 1 contains a distinct red nucleus, and this is likely the optical signature of low-level nuclear activity and/or a distinct central stellar population. Finally, there is an absence of globular clusters brighter than the peak of the globular cluster LF in the central 700 x 700 parsecs of Maffei 1.

The brightest asymptotic giant branch stars in the Leo I dwarf spheroidal galaxy

The first results of a study of the dwarf spheroidal galaxy Leo I using the new Nagoya‐South African Infrared Survey Facility (IRSF) are presented. J, H and Ks observations show that most, if not all, of at least the top magnitude of the asymptotic giant branch (AGB) in Ks is populated by carbon stars. In addition there are five very red objects which are believed to be dust-enshrouded AGB stars. One of these is, remarkably, well outside the main body of the galaxy. Three of these obscured stars and five known carbon stars show variability in

THE BRIGHTEST ASYMPTOTIC GIANT BRANCH STARS IN THE INNER BULGE OF M31

JHK images with angular resolutions approaching the diffraction limit of the 3.6 m Canada-France-Hawaii Telescope are used to investigate the bright asymptotic giant branch (AGB) content of the M31 bulge. The AGB tip in a field 26 from the galaxy center occurs at K = 15.6, which is significantly fainter than measured in previous ground-based studies that sampled similar projected distances from the center of M31 but were affected by crowding. Within 26 of the center of M31, the number density of bright AGB stars scales with r-band surface brightness and the K brightness of the AGB tip does not vary measurably with radius. It is concluded that the infrared bright AGB stars (1) belong to the bulge and not the disk and (2) are well mixed throughout the inner bulge, suggesting that they formed at a time when the overall structural properties of the M31 bulge were imprinted. The bolometric luminosity functions (LFs) of the M31 bulge and Baade's window are in excellent agreement, while the brightest AGB stars in the M31 bulge, the Galactic bulge, and M32 have similar MK. Barring a fortuitous tuning of age and metallicity to produce AGB tips with similar brightnesses, it is suggested that the brightest stars in M32 and the bulges of M31 and the Milky Way belong to an old, metal-rich population; these stars are bright not because they have a young or intermediate age, but because they have a high metallicity.

The Detection of Bright Asymptotic Giant Branch Stars in the Nearby Elliptical Galaxy Maffei 1

We have used the adaptive optics system on the Canada-France-Hawaii Telescope to study a field 6' from the center of the heavily obscured elliptical galaxy Maffei 1. Our near diffraction-limited H and K' images reveal an excess population of objects with respect to a background field, and we conclude that the asymptotic giant branch (AGB) tip in Maffei 1 occurs at K = 20 ± 0.25. Assuming that stars at the AGB tip in Maffei 1 have the same intrinsic luminosity as those in the bulge of M31, then the actual distance modulus of Maffei 1 is 28.2 ± 0.3 if AV = 5.1, which corresponds to a distance of 4.4 Mpc. This is in excellent agreement with the distance estimated from surface brightness fluctuations in the infrared and confirms that Maffei 1 is too distant to affect the dynamics of the Local Group significantly.

The Peak Brightness and Spatial Distribution of Asymptotic Giant Branch Stars Near the Nucleus of M32

The bright stellar content near the center of the Local Group elliptical galaxy M32 is investigated with 012 FWHM H and K images obtained with the Gemini Mauna Kea telescope. Stars with K = 15.5, which are likely evolving near the tip of the asymptotic giant branch (AGB), are resolved to within 2'' of the nucleus, and it is concluded that the peak stellar brightness near the center of M32 is similar to that in the outer regions of the galaxy. Moreover, the projected density of bright AGB stars follows the visible light profile to within 2'' of the nucleus, indicating that the brightest stars are well mixed throughout the galaxy. Thus, there is no evidence for an age gradient, and the radial variations in spectroscopic indices and ultraviolet colors that have been detected previously must be due to metallicity and/or some other parameter. We suggest that either the bright AGB stars formed as part of a highly uniform and coherent galaxy-wide episode of star formation or they originated in a separate system that merged with M32.