Tip Of Red Giant Branch Research Articles

Color Transformations and Bolometric Corrections for Galactic Halo Stars: α‐Enhanced versus Scaled‐Solar Results

We have performed the first extensive analysis of the impact of an [?/Fe] > 0 metal distribution on broadband colors in the parameter space (surface gravity, effective temperature, and metal content) covered by Galactic globular cluster stars. A comparison of updated and homogeneous ATLAS 9 UBVRIJHKL synthetic photometry, for both ?-enhanced and scaled-solar metal distributions, has shown that it is impossible to reproduce ?-enhanced (B - V) and (U - B) color transformations with simple rescalings of the scaled-solar ones. At [Fe/H] ~ -2.0, ?-enhanced transformations are well reproduced by scaled-solar ones with the same [Fe/H], but this good agreement breaks down at [Fe/H] larger than about -1.6. As a general rule, (B - V) and (U - B) ?-enhanced colors are bluer than scaled-solar ones at either the same [Fe/H] or the same [M/H], and the differences increase with increasing metallicity and decreasing Teff. A preliminary analysis of the contribution of the various ?-elements to the stellar colors shows that the magnesium abundance (and to a lesser extent oxygen and silicon) is mainly responsible for these differences. On the contrary, the bolometric correction to the V-band and other infrared colors predicted by ?-enhanced transformations are well reproduced by scaled-solar results because of their weak dependence on the metal content. Key parameters, such as the turnoff and zero-age horizontal branch V magnitudes, as well as the red giant branch tip I magnitude obtained from theoretical isochrones, are in general unaffected when using the appropriate ?-enhanced transformations in place of scaled-solar ones. We have also studied, for the first time, the effect of boundary conditions obtained from appropriate ?-enhanced model atmospheres on the stellar evolutionary tracks in the log L/L?-Teff plane. We find that, for both scaled-solar and ?-enhanced metal mixtures, the integration of a solar T(?) relationship provides?at least for masses larger than 0.5-0.6 M??tracks very similar to those computed using boundary conditions from the appropriate model atmospheres.

The distance to the Sagittarius dwarf spheroidal galaxy from the red giant branch tip

We derived the distance to the central region of the Sagittarius (Sgr) dwarf spheroidal galaxy from the red giant branch tip. The obtained distance modulus is (m - M) 0 = 17.10 ± 0.15, corresponding to a heliocentric distance D = 26.30 ± 1.8 Kpc. This estimate is in good agreement with the distance obtained from RR Lyrae (RR Ly) stars of the globular cluster M 54, located in the core of the Sgr galaxy, once the most accurate estimate of the cluster metallicity and the most recent calibration of the M V (RR Ly) versus [Fe/H] relation are adopted.

The calibration of the RGB Tip as a Standard Candle

New empirical calibrations of the Red Giant Branch Tip in the I, J, H and K bands based on two fundamental pillars, namely ω Centauri and 47 Tucanae, have been obtained by using a large optical and near infrared photometric database. Our best estimates give , , and at (ω Cen) and , , and at (47 Tuc). With these new empirical calibrations we also provide robust relations of the TRGB magnitude in the I, J, H, and K bands as a function of the global metallicity. It has also been shown that our calibrations self-consistently provide a distance modulus of the Large Magellanic Cloud in good agreement with the standard value ()).

The distance of M 33 and the stellar population in its outskirts

We present deep V, I photometry of two field in the outer regions of the M 33 galaxy. We obtain a robust detection of the luminosity of the Red Giant Branch Tip ( ± 0.08) from which we derived a new estimate of the distance modulus of M 33, ± 0.15, corresponding to a distance ± 60 kpc. By comparison of the color and magnitude of the observed Red Giant Branch stars with ridge lines of template globular clusters we obtained the photometric metallicity distribution of the considered fields in three different metallicity scales. The derived metallicity distributions are very similar over a range of distances from the galactic center , and are characterized by a well defined peak at (, in the Zinn & West scale) and a weak metal-poor tail reaching . Our observations demonstrate that Red Giant Branch and Asymptotic Giant Branch stars have a radial distribution that is much more extended than the young MS stars associated with the star-forming disc.

Iron abundances from high-resolution spectroscopy of the open clusters NGC 2506, NGC 6134, and IC 4651

This is the first of a series of papers devoted to deriving the metallicity of old open clusters to study the time evolution of the chemical abundance gradient in the Galactic disk. We present detailed iron abundances from high resolution () spectra of several red clump and bright giant stars in the open clusters IC 4651, NGC 2506 and NGC 6134. We observed 4 stars of NGC 2506, 3 stars of NGC 6134 and 5 stars of IC 4651 with the FEROS spectrograph with the ESO 1.5 m telescope; moreover, 3 other stars of NGC 6134 were observed with the UVES spectrograph on Kueyen (VLT UT2). After excluding the cool giants near the red giant branch tip (one in IC 4651 and one in NGC 2506), we found overall [Fe/H] values of , dex (2 stars) for NGC 2506, , dex (6 stars) for NGC 6134 and , dex (4 stars) for IC 4651. The metal abundances derived from a line analysis for each star were extensively checked using spectrum synthesis of about 30 to 40 Fe I lines and 6 Fe II lines. Our spectroscopic temperatures provide reddening values in good agreement with literature data for these clusters, strengthening the reliability of the adopted temperature and metallicity scale. Also, gravities from the Fe equilibrium of ionization agree quite well with expectations based on cluster distance moduli and evolutionary masses.

The Metal-Poor Inner Spheroid Globular Cluster NGC 6558

We discuss near-infrared photometric and spectroscopic observations of stars in and around the globular cluster NGC 6558, obtained with the FLAMINGOS-I imaging spectrograph on the Gemini South telescope. Efforts to estimate the metallicity of NGC 6558 from the slope of the red giant branch (RGB) are biased by contamination from bulge stars. The near-infrared spectroscopic properties of the two brightest stars projected against the main body of the cluster are consistent with them belonging to the bulge. After removing probable bulge members, we find that (1) the brightest metal-poor giant near the center of NGC 6558 has MK = -4.6, which is 1–1.5 mag fainter than the expected RGB tip brightness, and (2) [Fe/H] = -1.5 ± 0.5 based on the RGB slope measured from stars that are likely cluster members. This metallicity estimate is consistent with the nondetection of the first-overtone CO bands in the near-infrared spectra of giants midway down the RGB of NGC 6558. The K-band luminosity function (LF) of NGC 6558, corrected for contamination from bulge stars, parallels that of NGC 6121 on the lower RGB and subgiant branch (SGB), indicating that any depletion of giants in NGC 6558 is restricted to the upper portion of the RGB. When scaled according to the number of stars midway up the giant branch, the K LFs of NGC 6121 and NGC 6558 agree to within ±0.1 dex on the SGB, indicating that their ages do not differ by more than ±2 Gyr.

The distance to the Leo I dwarf spheroidal galaxy from the red giant branch tip

We present V and I photometry of a 9.4 × 9.4 arcmin 2 field centred on the dwarf spheroidal galaxy Leo I. The I magnitude of the tip of the red giant branch (RGB) is robustly estimated from two different data sets (I TRGB = 17.97 +0.05 −0.03 ). From this estimate, adopting [M/H] �− 1.2 from the comparison of RGB stars with Galactic templates, we obtain a distance modulus (m − M)0 = 22.02 ± 0.13, corresponding to a distance D = 254 +16 −19 kpc. Ke yw ords: stars: Population II ‐ galaxies: distances and redshifts ‐ Local Group.

Star-to-Star Na and O Abundance Variations along the Red Giant Branch in NGC 2808

We report for the first time Na and O abundances from high-resolution, high S/N echelle spectra of 20 red giants in NGC 2808, taken as part of the Science Verification program of the FLAMES multi-object spectrograph at the ESO VLT. In these stars, spanning about 3 mag from the red giant branch (RGB) tip, large variations are detected in the abundances of oxygen and sodium, anticorrelated with each other; this is a well known evidence of proton-capture reactions at high temperatures in the ON and NeNa cycles. One star appears super O-poor; if the extension of the Na-O anticorrelation is confirmed, NGC 2808 might reach O depletion levels as large as those of M 13. This result confirms our previous findings based on lower resolution spectra (Carretta et al. 2003) of a large star-to-star scatter in proton capture elements at all positions along the RGB in NGC 2808, with no significant evolutionary contribution. Finally, the average metallicity for NGC 2808 is [Fe/H]= -1.14 +/- 0.01 dex (rms=0.06) from 19 stars.

Variations in Star Formation History and the Red Giant Branch Tip

We examine the reliability of the tip of the red giant branch (TRGB) as a distance indicator for stellar populations with different star formation histories (SFHs) when photometric errors and completeness corrections at the TRGB are small. In general, the TRGB-distance method is insensitive to the shape of the SFH except when it produces a stellar population with a significant component undergoing the red giant branch phase transition. The I-band absolute magnitude of the TRGB for the middle and late stages of this transition (~1.3-1.7 Gyr) is several tenths of a magnitude fainter than the canonical value of M_I ~ -4.0. If more than 30% of all stars formed over the lifetime of the Universe are formed at these ages, then the distance could be overestimated by 10-25%. Similarly, the TRGB-distance method is insensitive to the metallicity distribution of stars formed except when the average metallicity is greater than = -0.3. If more than ~70% of all stars formed have [Fe/H] > -0.3, the distance could be overestimated by ~10-45%. We find that two observable quantities, the height of the discontinuity in the luminosity function at the TRGB and the median (V-I)_0 at M_I = -3.5 can be used to test if the aforementioned age and metallicity conditions are met.

Red giant branch in near-infrared colour-magnitude diagrams - II. The luminosity of the bump and the tip

We present new empirical calibrations of the red giant branch (RGB) bump and tip based on a homogeneous near-infrared data base of 24 Galactic globular clusters. The luminosities of the RGB bump and tip in the J, H and K bands and their dependence on the cluster metallicity have been studied, yielding empirical relationships. By using recent transformations between the observational and theoretical planes, we also derived similar calibrations in terms of bolometric luminosity. Direct comparisons between updated theoretical models and observations show an excellent agreement. The empirical calibration of the RGB tip luminosity in the near-infrared passbands presented here is a fundamental tool to derive distances to galaxies far beyond the Local Group, in view of using the new ground-based adaptive optics facilities and, in the near future, the James Webb Space Telescope.

The Stellar Populations in the Outer Regions of M33. I. Metallicity Distribution Function

We present deep CCD photometry in the VI passbands using the WIYN 3.5m telescope of a field located approximately 20' southeast of the center of M33; this field includes the region studied by Mould & Kristian in their 1986 paper. The color-magnitude diagram (CMD) extends to I~25 and shows a prominent red giant branch (RGB), along with significant numbers of asymptotic giant branch and young main sequence stars. The red clump of core helium burning stars is also discernable near the limit of our CMD. The I-band apparent magnitude of the red giant branch tip implies a distance modulus of (m-M)_I = 24.77 +/- 0.06, which combined with an adopted reddening of E(V-I)=0.06 +/- 0.02 yields an absolute modulus of (m-M)_0 = 24.69 +/- 0.07 (867 +/- 28 kpc) for M33. Over the range of deprojected radii covered by our field (~8.5 to ~12.5 kpc), we find a significant age gradient with an upper limit of ~1 Gyr (~0.25 Gyr/kpc). Comparison of the RGB photometry to empirical giant branch sequences for Galactic globulars allows us to use the dereddened color of these stars to construct a metallicity distribution function (MDF). The primary peak in the MDF is at a metallicity of [Fe/H] ~ -1.0 with a tail to lower abundances. The peak does show radial variation with a slope of d[Fe/H]/dR_{deproj} = -0.06 +/- 0.01 dex/kpc. This gradient is consistent with the variation seen in the inner disk regions of M33. As such, we conclude that the vast majority of stars in this field belong to the disk of M33, not the halo as previously thought.

The Stellar Content and Star Formation History of the Late-Type Spiral Galaxy NGC 300 fromHubble Space TelescopeObservations

We present the first Hubble Space Telescope (HST) WFPC2 V and I photometry for the Sculptor Group galaxy NGC 300 in four fields ranging from the center to the outer edge. We have made the first measurement of the star formation histories in two disk fields: the oldest stars were born at similar epochs and formation activity increased but at different mean rates. The main disk stellar population is predominantly old, consisting of red giant branch (RGB) and asymptotic giant branch (AGB) stars, based on a synthetic color-magnitude diagram analysis. The metallicity Z is found to have been less than 0.006 (or 0.33 Z⊙), with no evidence for significant change in mean Z over time in both disk fields. In the central region, we find a dearth of bright stars with respect to the two disk fields that cannot be explained by observational effects. Taken at face value, this finding would agree with the Davidge report of suppressed star formation there during the past 109 yr with respect to his disk fields at larger radii; but the possibility remains that significant central extinction affects our finding. We have also determined the first distance modulus estimate based on the tip of the red giant branch method. On the Cepheid distance scale of Ferrarese et al., we find (m-M)0 = 26.56 ± 0.07(±0.13) mag and a similar value from the Cepheid-independent empirical method of Lee, Freedman, & Madore, both in good agreement with the Cepheid distance determined by Freedman et al. A discrepancy between this value and the theoretical calibration of the red giant branch tip magnitude method remains. Finally, we report a newly detected young (up to about 10 Myr) stellar association of about average size (∼140 pc) in one of the disk fields.

The distance to the giant elliptical galaxy NGC 5128

The distance to NGC 5128, the central galaxy of the Centaurus group and the nearest giant elliptical to us, has been determined using two independent distance indicators: the Mira period-luminosity (PL) relation and the luminosity of the tip of the red giant branch (RGB). The data were taken at two different locations in the halo of NGC 5128 with the ISAAC near-IR array on ESO VLT. From more than 20 hours of observations with ISAAC a very deep K s -band luminosity function was constructed. The tip of the RGB is detected at K s = 21.24 ± 0.05 mag. Using an empirical calibration of the K-band RGB tip magnitude, and assuming a mean metallicity of [M/H] = -0.4 dex and reddening of E(B - V) = 0.11, a distance modulus of NGC 5128 of (m - M) 0 = 27.87 ± 0.16 was derived. The comparison of the H-band RGB tip magnitude in NGC 5128 and the Galactic Bulge implies a distance modulus of NGC 5128 of (m - M) 0 = 27.9 ± 0.2 in good agreement with the K-band RGB tip measurement. The inner halo field has larger photometric errors, brighter completeness limits and a larger number of blends. Thus the RGB tip feature is not as sharp as in the outer halo field. The population of stars above the tip of the RGB amounts to 2176 stars in in the outer halo field (Field 1) and 6072 stars in the inner halo field (Field 2). The large majority of these sources belong to the asymptotic giant branch (AGB) population in NGC 5128 with numerous long period variables. Mira variables were used to determine the distance of NGC 5128 from a period-luminosity relation calibrated using the Hipparcos parallaxes and LMC Mira period-luminosity relation in the K-band. This is the first Mira period-luminosity relation outside the Local Group. A distance modulus of 27.96 ± 0.1 was derived, adopting the LMC distance modulus of 18.50 ± 0.04. The mean of the two methods yields a distance modulus to NGC 5128 of 27.92 ± 0.19 corresponding to D = 3.84 ± 0.35 Mpc.

Stellar Associations in the Tail of NGC 4038

We have used the Hubble Space Telescope and Wide Field Planetary Camera 2 to image the putative tidal dwarf galaxy located at the tip of the Southern tidal tail of NGC 4038/9, the Antennae. We resolve individual stars, and identify two stellar populations. Hundreds of massive stars are present, concentrated into tight OB associations on scales of 200 pc, with ages ranging from 2-100 Myr. An older stellar population is distributed roughly following the outer contours of the neutral hydrogen in the tidal tail; we associate these stars with material ejected from the outer disks of the two spirals. The older stellar population has a red giant branch tip at I = 26.5 ± 0.2 from which we derive a distance modulus (m - M)0 = 30.7 ± 0.25. The implied distance of 13.8 ± 1.7 Mpc is nearly a factor of two closer than commonly quoted distances for NGC 4038/9. In contrast to the previously studied core of the merger, we find no super star clusters. One might conclude that SSCs require the higher pressures found in the central regions in order to form, while spontaneous star formation in the tail produces the kind of O-B star associations seen in dwarf irregular galaxies.

Mass motions and chromospheres of RGB stars in the globular cluster NGC 2808

We present the results of the first observations, taken with FLAMES during Science Verification, of red giant branch (RGB) stars in the globular cluster NGC 2808. A total of 137 stars was observed, of which 20 at high resolution ( 47 000) with UVES and the others at lower resolution ( 19 000–29 000) with GIRAFFE in MEDUSA mode, monitoring ~3 mag down from the RGB tip. Spectra were taken of the Hα, Na i D and Ca ii H and K lines. This is by far the largest and most complete collection of such data in globular cluster giants, both for the number of stars observed within one cluster, and for monitoring all the most important optical diagnostics of chromospheric activity/mass motions. Evidence of mass motions in the atmospheres was searched from asymmetry in the profiles and coreshifts of the Hα, Na i D and Ca ii K lines, as well as from Hα emission wings. We have set the detection thresholds for the onset of Hα emission, negative Na D2 coreshifts and negative K3 coreshifts at 2.5, 2.9 and 2.8, respectively. These limits are significantly fainter than the results found by nearly all previous studies. Also the fraction of stars where these features have been detected has been increased significantly with respect to the previous studies. Our observations confirm the widespread presence of chromospheres among globular cluster giants, as it was found among Population I red giants. Some of the above diagnostics suggest clearly the presence of outward mass motions in the atmosphere of several stars.

Resolving the Stellar Population of the Standard Elliptical Galaxy NGC 3379

Using the Near Infrared Camera and Multi-Object Spectrometer (NICMOS) on board the Hubble Space Telescope, we have obtained F110W (J-band) and F160W (H-band) images of three fields in NGC3379, a nearby normal giant elliptical galaxy. These images resolve individual red giant stars, yielding the first accurate color-magnitude diagrams for a normal luminous elliptical. The photometry reaches 1 magnitude below the red giant branch tip with errors of < 0.2 mags in F110W-F160W. A strong break in the luminosity function at F160W = 23.68 +/- 0.06 is identified as the tip of the red giant branch (RGB); comparison with theoretical isochrones implies a distance of 10.8 +/- 0.6 Mpc, in good agreement with a number of previous estimates using various techniques. The mean metallicity is close to solar, but there is an appreciable spread in abundance, from at least as metal poor as [Fe/H] = -1.5 to as high as +0.8. There is a significant population of stars brighter than the RGB tip by up to about 1 magnitude. The observations of each field were split over two epochs, separated by 2-3 months, allowing the identification of candidate long period variables; at least 40% of the stars brighter than the RGB tip are variable. Lacking period determinations, the exact nature of these variables remains uncertain, but the bright AGB stars and variables are similar to those found in metal rich globular clusters and are not luminous enough to imply an intermediate age population. All of the evidence points to a stellar population in NGC3379 which is very similar to the bulge of the Milky Way, or an assortment of Galactic globular clusters covering a large metallicity spread.

Thermal Stability of Rotating Low‐Mass Subgiants and Red Giants

A linear analysis of the thermal stability of rotating low-mass stars evolving from the zero-age main sequence (ZAMS) to the red giant branch (RGB) tip has been carried out. Two stellar models are considered: one for 1.2 M☉ with solar metallicity (Z = 0.0188), and the other for 0.8 M☉ with Z = 0.0005. An instability in a thermonuclear burning shell on the RGB in the first of these cases could potentially be related to the phenomenon of Li-rich red giants, while a hydrogen shell instability in the second model may help to explain the observed chemical abundance anomalies in globular cluster red giants. A range of surface rotational velocities in MS stars has been considered, and we have assumed that, beginning at the ZAMS, the stars (including their convective envelopes) rotate differentially with depth. This assumption, which leads to significantly higher centrifugal accelerations in the H-burning shell than with the case of solid-body rotation on the MS, is expected to favour the development of thermal instabilities. However, all of our models for rotating low-mass stars—even those that had much higher rotation rates on the MS than those observed for F-, G-, and K-type dwarfs—were found to be thermally stable throughout their evolution from the ZAMS to the RGB tip. Only when helium burning was ignited to end the first ascent of the giant branch did we find a thermal instability (the helium flash).

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 Stellar Content of the Southern Tail of NGC 4038/4039 and a Revised Distance

We have used the Hubble Space Telescope and Wide Field Planetary Camera 2 to image the putative tidal dwarf galaxy located at the tip of the Southern tidal tail of NGC 4038/4039, the Antennae. We resolve individual stars and identify two stellar populations. Hundreds of massive stars are present, concentrated into tight OB associations on scales of 200 pc, with ages ranging from 2 to 100 Myr. An older stellar population is distributed roughly following the outer contours of the neutral hydrogen in the tidal tail; we associate these stars with material ejected from the outer disks of the two spirals. The older stellar population has a red giant branch tip at I = 26.5 ± 0.2 from which we derive a distance modulus (m - M)0 = 30.7 ± 0.25. The implied distance of 13.8 ± 1.7 Mpc is significantly smaller than commonly quoted distances for NGC 4038/4039. In contrast to the previously studied core of the merger, we find no super–star clusters (SSCs). One might conclude that SSCs require the higher pressures found in the central regions in order to form, while spontaneous star formation in the tail produces the kind of OB star associations seen in dwarf irregular galaxies. The youngest population in the putative tidal dwarf has a total stellar mass of ≈2 × 105 M⊙, while the old population has a stellar mass of ≈7 × 107 M⊙. If our smaller distance modulus is correct, it has far-reaching consequences for this prototypical merger. Specifically, the luminous to dynamical mass limits for the tidal dwarf candidates are significantly less than 1, the central SSCs have sizes typical of Galactic globular clusters, rather than being 1.5 times as large, and the unusually luminous X-ray population becomes both less luminous and less populous.

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.