RR LYRAE VARIABLES IN M32 AND THE DISK OF M31 (original) (raw)
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HST/Acs Observations of RR Lyrae Stars in Six Ultra-Deep Fields of M31
The Astronomical Journal, 2011
We present HST/ACS observations of RR Lyrae variable stars in six ultra deep fields of the Andromeda galaxy (M31), including parts of the halo, disk, and giant stellar stream. Past work on the RR Lyrae stars in M31 has focused on various aspects of the stellar populations that make up the galaxy's halo, including their distances and metallicities. This study builds upon this previous work by increasing the spatial coverage (something that has been lacking in previous studies) and by searching for these variable stars in constituents of the galaxy not yet explored. Besides the 55 RR Lyrae stars we found in our initial field located 11kpc from the galactic nucleus, we find additional RR Lyrae stars in four of the remaining five ultra deep fields as follows: 21 in the disk, 24 in the giant stellar stream, 3 in the halo field 21kpc from the galactic nucleus, and 5 in one of the halo fields at 35kpc. No RR Lyrae were found in the second halo field at 35kpc. The RR Lyrae populations of these fields appear to mostly be of Oosterhoff I type, although the 11kpc field appears to be intermediate or mixed. We will discuss the properties of these stars including period and reddening distributions. We calculate metallicities and distances for the stars in each of these fields using different methods and compare the results, to an extent that has not yet been done. We compare these methods not just on RR Lyrae stars in our M31 fields, but also on a data set of Milky Way field RR Lyrae stars.
The ancient stellar population of M 32: RR Lyrae variable stars confirmed
Astronomy & Astrophysics, 2012
Using archival multi-epoch ACS/WFC images in the F606W and F814W filters of a resolved stellar field in Local Group dwarf elliptical galaxy M 32 we have made an accurate Colour−Magnitude Diagram and a careful search for RR Lyr variable stars. We identified 416 bona fide RR Lyr stars over our field of view, and their spatial distribution shows a rising number density towards the centre of M 32. These new observations clearly confirm the tentative result of Fiorentino et al. (2010), on a much smaller field of view, associating an ancient population of RR Lyr variables to M 32. We associate at least 83 RR Lyr stars in our field to M 32. In addition the detection of 4 Anomalous Cepheids with masses in the range 1.2−1.9 M ⊙ indicates the presence of relatively young, 1−4 Gyr old, stars in this field. They are most likely associated to the presence of the blue plume in the Colour−Magnitude Diagram. However these young stars are unlikely to be associated with M 32 because the radial distribution of the blue plume does not follow the M 32 density profile, and thus they are more likely to belong to the underlying M 31 stellar population. Finally the detection of 3 Population II Cepheids in this field gives an independent measurement of the distance modulus in good agreement with that obtained from the RR Lyr, µ 0 =24.33 ± 0.21 mag.
The ancient stellar population of M32: RR Lyr Variable stars confirmed
Using archival multi--epoch ACS/WFC images in the F606W and F814W filters of a resolved stellar field in Local Group dwarf elliptical galaxy M32 we have made an accurate Colour-Magnitude Diagram and a careful search for RR Lyr variable stars. We identified 416 bona fide RR Lyr stars over our field of view, and their spatial distribution shows a rising number density towards the centre of M32. These new observations clearly confirm the tentative result of Fiorentino et al. (2010), on a much smaller field of view, associating an ancient population of RR Lyr variables to M32. We associate at least 83 RR Lyr stars in our field to M32. In addition the detection of 4 Anomalous Cepheids with masses in the range 1.2-1.9 Mo indicates the presence of relatively young, 1-4 Gyr old, stars in this field. They are most likely associated to the presence of the blue plume in the Colour-Magnitude Diagram. However these young stars are unlikely to be associated with M32 because the radial distributio...
A Multicolor and Fourier Study of RR Lyrae Variables in the Globular Cluster NGC 5272 (M3)
The Astronomical Journal, 2005
We have performed a detailed study of the pulsational and evolutionary characteristics of 133 RR Lyrae stars in the globular cluster NGC5272 (M3) using highly accurate BVI data taken on 5 separate epochs. M3 seems to contain no less than ∼32% of Blazhko stars, and the occurrence and characteristics of the Blazhko effect have been analyzed in detail. We have identified a good number (∼ 14%) of overluminous RR Lyrae stars that are likely in a more advanced evolutionary stage off the Zero Age Horizontal Branch (ZAHB). Physical parameters (i.e. temperature, luminosity, mass) have been derived from (B-V) colors and accurate color-temperature calibration, and compared with Horizontal Branch evolutionary models and with the requirements of stellar pulsation theory. Additional analysis by means of Fourier decomposition of the V light curves confirms, as expected, that no metallicity spread is present in M3. Evolution off the ZAHB does not affect [Fe/H] determinations, whereas Blazhko stars at low amplitude phase do affect [Fe/H] distributions as they appear more metal-rich. Absolute magnitudes derived from Fourier coefficients might provide useful average estimates for groups of stars, if applicable, but do not give reliable individual values. Intrinsic colors derived from Fourier 167, 168, 170, 188 and 209. Finally, Car98 provided also I photometry with the warning that it may be affected by a zero-point error in the absolute calibration. We treat this problem in some detail in Sect. 4.1, but we anticipate here that indeed Car98 I data are most likely too faint by ∼ 0.083 mag. The same type of comparison with Kal98 data, using intensity integrated < V > magnitudes for both data sets (note however that the published Kal98 < V > are magnitude integrated), shows that the CC01 V magnitudes are brighter than Kal98 V photometry in the SOUTH and NORTH fields by 0.028 and 0.018 mags, respectively. Car98 and Kal98 data have been corrected by the above offsets, when they have been used along with CC01 data (e.g. for Blazhko stars). On the other hand, a comparison of CC01 photometry with 15 randomly selected secondary standard stars from Sandage (1970) shows that CC01 V magnitudes are fainter by 0.009 ± 0.024 mag, and the B magnitudes are brighter by 0.004 ± 0.014 mag, as already noted by CC01. Only the stars with well defined light curves in both B and V bands have been taken into account for the present study. This led us to consider a total of 133 stars out of the 201 RR Lyraes observed by CC01, in particular 23 RRc out of 43, 67 RRab out of 111, and 43 Blazhko stars out of 47. The stars we have not considered in the present study all have very noisy light curves, which may be due to photometric errors (contamination from companions) or to intrinsic phenomena such as double-mode pulsation or unidentified Blazhko modulation. They may be very interesting objects in themselves, and surely deserve further and more careful investigation (cf. Clementini et al. 2004). However, for the purpose of the present analysis, we shall use only those stars that show the "cleanest" light curves so as to keep the noise at the minimum level, taking advantage of the fact that M3 is probably the only cluster where one can afford to be very selective, due to the richness of its variable star population. 2.2. The Blazhko variables The Blazhko effect, first noticed and studied by Blazhko (1907), is a modulation of the basic pulsation variability that produces variations of the light curve shape showing as larger photometric scatter and significant changes in the light curve amplitude. The timescale of this modulation is typically tens of days but can be as large as a few hundred days. Several mechanisms have been proposed to explain the origin of this phenomenon, which however is still an open question. We refer the reader to Smith (1995) for a recent and comprehensive discussion on this topic. According to CC01 data no less than ∼32% of the total RR Lyrae variable star population in M3 is affected by Blazhko variability. This fraction might easily be larger if some of the stars with noisy light curves, that we have not considered in the present analysis, turn out to be Blazhko variables in future studies. The frequency of this phenomenon we find in M3 is consistent with previous results in other stellar systems (cf. Smith 1995) and has been recently confirmed in another cluster, NGC3201, where Piersimoni et al. (2002) have identified about 30% such stars. However,
RR Lyrae Variables in the Globular Clusters of M31: A First Detection of Likely Candidates
The Astrophysical Journal, 2001
The purpose of this paper is to show that RR Lyrae variables exist and can be detected in M31 globular clusters. We report on the first tentative identification of RR Lyrae candidates in four globular clusters of the Andromeda galaxy, i.e. G11, G33, G64 and G322. Based on HST-WFPC2 archive observations in the F555W and F814W filters spanning a total interval of about 5 consecutive hours we find evidence for 2, 4, 11 and 8 RR Lyrae variables of both ab and c Bailey types in G11, G33, G64 and G322, respectively. Several more candidates can be found by relaxing slightly the selection criteria. These numbers are quite consistent with the horizontal branch morphology exhibited by the four clusters, starting from the very blue HB in G11, and progressively moving to redder HBs in G64, G33 and G322.
RR LYRAE VARIABLES IN TWO FIELDS IN THE SPHEROID OF M31
The Astronomical Journal, 2009
We present Hubble Space Telescope observations taken with the Advanced Camera for Surveys Wide Field Channel of two fields near M32 -between four and six kpc from the center of M31. The data cover a time baseline sufficient -2for the identification and characterization of 681 RR Lyrae variables of which 555 are ab-type and 126 are c-type. The mean magnitude of these stars is V = 25.29 ± 0.05 where the uncertainty combines both the random and systematic errors. The location of the stars in the Bailey Diagram and the ratio of ctype RR Lyraes to all types are both closer to RR Lyraes in Oosterhoff type I globular clusters in the Milky Way as compared with Oosterhoff II clusters. The mean periods of the ab-type and c-type RR Lyraes are P ab = 0.557 ± 0.003 and P c = 0.327 ± 0.003, respectively, where the uncertainties in each case represent the standard error of the mean. When the periods and amplitudes of the abtype RR Lyraes in our sample are interpreted in terms of metallicity, we find the metallicity distribution function to be indistinguishable from a Gaussian with a peak at [Fe/H] =−1.50 ± 0.02, where the quoted uncertainty is the standard error of the mean. Using a relation between RR Lyrae luminosity and metallicity along with a reddening of E(B − V ) = 0.08 ± 0.03, we find a distance modulus of (m − M) 0 = 24.46 ± 0.11 for M31. We examine the radial metallicity gradient in the environs of M31 using published values for the bulge and halo of M31 as well as the abundances of its dwarf spheroidal companions and globular clusters. In this context, we conclude that the RR Lyraes in our two fields are more likely to be halo objects rather than associated with the bulge or disk of M31, in spite of the fact that they are located at 4-6 kpc in projected distance from the center.
The RR Lyrae Variables in M54 and the Sgr Dwarf Galaxy
Symposium - International Astronomical Union, 2002
We report on new B, V and I CCD photometry of the globular cluster M54 that was aimed at the study of its variable stars. With respect to the previous most recent work on M54 we have nearly doubled the number of detected variable stars: M54 can now be classified as intermediate in the Oosterhoff groups. The metallicity can be estimated for the cluster and field red giant stellar population, and for the variables.
RR Lyrae stars in the MACHO database
1996
The MACHO Project has catalogued ∼ 8000 RR Lyrae stars in the Large Magellanic Cloud, ∼ 1800 in the Galactic bulge, and ∼ 50 in the Sgr dwarf galaxy. These variables are excellent distance indicators, and are used as tools to study the structure of the Large Magellanic Cloud and the bulge. The large datasets also probe uncommon pulsation modes. A number of double-mode RR Lyrae stars (RRd) are found in the Large Magellanic Cloud sample. These stars provide important clues for understanding the formation and evolution of the inner Galaxy, the Large Magellanic Cloud and the Sgr dwarf galaxy. A large number of second overtone pulsators (RRe) are found in the LMC and bulge. Finally, the RR Lyrae belonging to the Sgr dwarf yield an accurate distance to this galaxy. Their presence also alerts us of the very interesting possibility of distant sources for bulge microlensing events.
The RR Lyrae Population of the Galactic Bulge from the MACHO Database: Mean Colors and Magnitudes
The Astrophysical Journal, 1998
Mean colors and magnitudes of RR Lyrae stars in 24 fields towards the Galactic bulge from the MACHO database are presented. Accurate mean reddenings are computed for these fields on the basis of the mean colors. The distribution along the line of sight of the RR Lyrae population is examined on the basis of the mean magnitudes, and it is shown that the bulk of the RR Lyrae population is not barred. Only the RR Lyrae in the inner fields -2closer to the Galactic center (l < 4 • , b > −4 • ) show evidence for a bar. The red giant clump stars in the MACHO fields, however, clearly show a barred distribution, confirming the results of previous studies. Given the different spatial distribution, the RR Lyrae and the clump giants trace two different populations. The RR Lyrae would represent the inner extension of the Galactic halo in these fields.
Monthly Notices of the Royal Astronomical Society, 2012
We present time series photometry of two fields near M32 using archival observations from the Advanced Camera for Surveys Wide Field Channel on-board the Hubble Space Telescope. One field is centred about 2 arcmin from M32, while the other is located 15 arcmin to the southeast of M31. The imaging covers a time baseline sufficient for the identification and characterization of a total number of 1139 RR Lyrae variables of which 821 are ab-type and 318 are c-type. In the field near M32, we find a radial gradient in the density of RR Lyraes relative to the centre of M32. This gradient is consistent with the surface brightness profile of M32, suggesting that a significant number of the RR Lyraes in this region belong to M32. This provides further confirmation that M32 contains an ancient stellar population formed around the same time as the oldest population in M31 and the Milky Way. The RR Lyrae stars in M32 exhibit a mean metal abundance of [Fe/H] ≈ −1.42 ± 0.02, which is ≈15 times lower than the metal abundance of the overall M32 stellar population. Moreover, the abundance of RR Lyrae stars normalized to the luminosity of M32 in the field analysed further indicates that the ancient metal-poor population in M32 represents only a very minor component of this galaxy, consistent with the 1-4.5 per cent in mass inferred from the colour-magnitude diagram analysis of Monachesi et al. We also find that the measured reddening of the RR Lyrae stars is consistent with M32 containing little or no dust. In the other field, we find unprecedented evidence for two populations of RR Lyraes in M31 as shown by two distinct sequences among the ab-type variables in the Bailey diagram. When interpreted in terms of metal abundance, one population exhibits a peak at [Fe/H] ≈ −1.3 and the other is at [Fe/H] ≈ −1.9. One possible interpretation of this result is that the more metal-rich population represents the dominant M31 halo, while the metal-poorer group could be a disrupted dwarf satellite galaxy orbiting M31. If true, this represents a further indication that the formation of the M31 spheroid has been significantly influenced by the merger and accretion of dwarf galaxy satellites.