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Science with the Hubble Space Telescope -- II
Book Editors: P. Benvenuti, F. D. Macchetto, and E. J. Schreier
Electronic Editor: H. Payne

Post--COSTAR UV Imaging of the Bulge of M31

L.M. Buson, A. Bressan
Astronomical Observatory, Padova, Italy

F. Bertola, M. Cappellari, C. Chiosi
University of Padova, Italy

D. Burstein
Arizona State University, Tempe, AZ 85287-1504 USA

and S. di Serego Alighieri
Arcetri Astrophysical Observatory, Italy

 

Abstract:

New post-COSTAR WFPC2 UV images of the inner bulge of M31 allow us to investigate in more detail the composite nature of the UV flux produced by its evolved stellar population. In particular, both luminosity function and UV colors of the detected UV-bright objects indicate that we are likely dealing with a minority of classical hot Post-AGB stars, whereas the relatively faint and cool stars providing the bulk of the UV emission observed in HUT2 spectra cannot be seen by HST as individual sources.

Introduction

It is now well established that evolved stars are the responsible of the typical UV-upturn shortward of 2000Å observed in early-type galaxies. This conclusion comes mainly from recent space-borne UV observations of bulge-dominated galaxies including both HST imaging (e.g., King et al. 1992, Bertola et al. 1995) and HUT spectroscopy down to the Lyman limit (Ferguson & Davidsen 1993, Brown et al. 1995). The most likely candidates as the sources of the UV flux are Hot-HB, Post-EAGB and AGB-manqué stars as well as classical P-AGB stars, whose relative contribution strictly depends on the metallicity (and/or age) mix of the stellar population, as discussed by Bressan, Chiosi & Fagotto (1994).

A large amount of UV observations focussed on the inner bulge of M31, the nearest high-metallicity `early-type' subsystem characterized by a remarkable flux rise at the shortest UV wavelengths. FOC HST images---which show individual UV-bright stars superimposed to a fainter diffuse UV component---as well as HUT spectra consistently indicate that the UV flux produced by this evolved population is intrinsically composite. In fact, while a large fraction of UV radiation can be ascribed to the presence of the variety of hot evolutionary stages (such as Extreme-HB stars and their descendants) which dominate the most metal-rich systems (Brown et al. 1995), in M31 a significant contribution comes also from the shorter-lived, UV-brighter P-AGB stars.

More precisely, our previous FOC/48 observations of M31 allowed us to detect 80 individual UV-bright stars within a field of 2323 (Bertola et al. 1995). Even if a single UV filter combination (F150W+F130LP) was available, the existence of comparable (FOC/48 F175W) observations by King et al. (1992) gave us the opportunity---by combining the two sets of data---to derive a first UV color for 48 stars in common. The subsequent comparison with proper HST c-m diagrams showed that the detected individual sources were consistent with being bright P-AGB stars. The main limitation of the above analysis---besides the obvious reduced imaging capabilities of the aberrated optics of the telescope---was the lack of a properly-spaced multi-color UV photometry. In addition, the interpretation of the FOC data was hampered by the redleak of the F150W+F130LP filter combination, as well as by its possible degraded UV sensitivity, as discussed in Bertola et al. (1995).

 
Figure: Luminosity Function of the 74 stars detected in the PC1 field. The zeropoint of the magnitude scale is such that a HST magnitude m= 19.53 corresponds to a count rate of 1 count s.

Obviously, the fully restored imaging capabilities of HST at the beginning of 1994 made this space-borne facility an even more attractive tool to achieve our current scientific goals. They still include the detection of the individual hot stars responsible for the UV upturn as well as the measurement of their UV luminosity function and color(s). In order to disentangle among the possible contributors to the UV flux, these pieces of information---coupled with proper post-HB evolution models---are absolutely essential.

With a view to maximizing the efficiency of the optical setup for our (post-refurbishment) carry-over observations, we moved from the FOC (whose maximum field of view is now only 1414) to the WFPC2 which, besides the much larger high-resolution field of PC1 (3535), provides an improved UV performance in comparison to the progenitor WFPC-1 and a proper filter set. Unfortunately, among the planned observations in three consecutive filters, namely F160BW, F218W and F336W, only the last one turned out to be of adequate S/N ratio for a meaningful analysis. However, the combination of the WFPC2/F336W new data with the previous King's et al. FOC/F175W data or our own FOC/F150W data, provides now a much larger wavelength basis to derive suitable UV colors.

 
Figure: C-m diagram for the 37 stars detected both in our FOC/48 and subsequent WFPC2 observations. HST magnitudes are defined in the text. A set of isochrones for P-AGB stars of fixed (solar) metallicity (z=0.02) and different ages (from 6.3 to 15.8Gyr, from top to bottom) are superimposed. The nominal efficiency curve of the FOC filter combination F150W+F130LP has been adopted for the comparison.

Observations and Reduction

The observations discussed here include two consecutive UV (WFPC2 F336W) images (2230 s exposure time in total) obtained in the context of the HST Cycle 4. The inner nucleus of M31 was centered on the PC1 detector which represents the highest resolution portion of the L-shaped field of the WFPC2. Both preliminary reduction and stellar photometry were carried out by means of standard IRAF packages. After successfully applying the gcombine task to get a cosmic ray-free co-added image, the daofind and phot tasks were used to identify individual point-like objects and to obtain aperture photometry, respectively. An aperture of 3 pixels was adopted, while the ``sky'' was measured within an annulus from 10 to 15 pixels. The zeropoint magnitude (19.53) for the filter F336W was taken from Whitmore (1995). This implies that for our derived HST magnitude a value m= 19.53 corresponds to a count rate of 1 count s. As already stressed, we took advantage of our own FOC/48+F150W+F130LP pre-existing photometry of an overlapping field to derive for as many stars as possible a UV color m-m, where m is the HST magnitude as defined in Bertola et al. (1995). This was made possible by the identical pattern shown by the brightest stars in both FOC and WFPC2 images; from the coordinates of such stars one can derive the proper transformation to identify corresponding objects (if any) from one camera (i.e., wavelength range) to another. This technique allowed us to obtain reliable UV colors for 37 stars.

 
Figure: The same as in Fig. 2, adopting for the isochrones a metallicity much lower than solar (z=0.008).

Results and Discussion

The present investigation is confined to the portion of galaxy covered by the PC1 detector (i.e., a field of 3535 arcsec); 74 individual stars have been reliably (3 ) identified. Even if no strict completeness tests have been applied, one should notice that there is no evidence of any dichotomy in the observed LF. As a consequence, even if one can expect---on the basis of both theoretical and observational arguments (Greggio & Renzini 1990, Bressan et al. 1994, Brown et al. 1995)---that more than one kind of hot component does contribute to the UV upturn, the UV stars detected by HST likely represent a single (the brightest) contributor to the observed flux.

The 37 stars for which we derived a FOC/WFPC2 UV color are shown in Fig. 2 and Fig. 3 on a proper c-m diagram (m vs. m-m) together with a set of isochrones computed for P-AGB stars of different ages (from 6.3Gyr to 15.8Gyr) and different metallicities (z=0.02 and z=0.008). For the theoretical computations the nominal efficiency curve of the FOC filter combination F150W+F130LP has been adopted.

The observed good match between observed stars and such theoretical predictions in spite of the degraded UV transmission noticed by Bertola et al. (1995) on the basis of pre-existing IUE data, must be considered to some extent preliminary and forces us to look further into the issue of FOC images calibration. However, the location of the observed objects even on these not-final diagrams, essentially assures that our HST UV images do show genuine hot P-AGB stars. As discussed above the derived LF itself is consistent with this conclusion. The observed spread of the points could reflect an intrinsic spread either in metallicity or age, or both.

In summary, the present framework provided by UV space-borne investigations (Astro-1, Astro-2 and HST) indicates that, whereas the bulk of the UV emission produced by (metal-rich) early-type galaxies comes from stars relatively cool (T20,000--23,000 K), a significant contribution can also come---as in the case of the inner bulge of M31---from classical hotter P-AGB stars. Since the latter stars are by far the brightest in the UV, they are the only objects detected by HST as individual sources.

Acknowledgments:

D. Burstein's research is supported by NASA HST Grant GO 5430.93A.

References:

Bertola, F., Bressan, A., Burstein, D., Buson, L.M., Chiosi, C., & di Serego Alighieri, S. 1995, ApJ, 438, 680

Bressan, A., Chiosi, C., & Fagotto, F. 1994, , 94, 63

Brown, T.M., Ferguson, H.C., & Davidsen, A.C. 1995, ApJ, 454, L15

Ferguson H.C. & Davidsen, A. F. 1993, ApJ, 408, 92

Greggio, L. & Renzini, A. 1990, ApJ, 364, 55

King, I.R. et al. 1992, ApJ, 397, L35

Whitmore, B. 1995, in Calibrating Hubble Space Telescope Post Servicing Mission, ed. A. Koratkar & C. Leitherer, STScI, 269



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