GK Workshop - M. Romaniello et al.

Proceedings of the Workshop
"The Magellanic Clouds and Other Dwarf Galaxies"
of the Bonn/Bochum-Graduiertenkolleg

HST Study of the Stellar Populations

around SN 1987A^*

Martino Romaniello^1,2, Nino Panagia^1,3, Salvo Scuderi⁴,

and the SINS collaboration

¹Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218
²Scuola Normale Superiore, Pisa, Italy
³On assignment from Astrophysics Division, Space Science Department of ESA
⁴Osservatorio Astrofisico di Catania, Viale A. Doria 6, I-95125, Catania, Italy

Received 14th March 1998

Abstract. We present a short account of a study of the stellar population around SN 1987A based on an analysis of multi-band HST-WFPC2 images. The effective temperature, radius and reddening of each star were determined by fitting the measured broad band magnitudes to the ones calculated with model atmospheres. In addition, all stars with Halpha equivalent widths in excess of 8 Å were identified. An inspection to the HR diagram reveals the presence of several generations of stars, with ages between 1 and 150 Myr, superposed on a much older field population. The youngest stars in the field appear to be T Tau stars, characterized by strong Halpha excesses. We conclude that SN 1987A is associated with a region in which star formation has been active over a long time interval and is still very active at present.

^* Based on observations with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by AURA, Inc., under NASA contract NAS 5-26555

1. Introduction

SN 1987A is located at the SW edge of the Tarantula Nebula, some 20 arcminutes away from its center. The whole area contains a large number of early type stars interspersed with H II regions and SNR shells. The OB association closest to SN 1987A is LH 90, which is located about 5 arcminutes to the NE of the supernova (Lucke & Hodge 1970) and whose age is much younger than that of SN 1987A progenitor, i.e., about 4 Myr as compared with the 10-11 Myr as estimated for Sk -69 202 (e.g. Van Dyk et al. 1998). It is clear that the study of SN 1987A neighborhood offers a unique opportunity to place the supernova explosion in the proper context of stellar evolution and the evolution of stellar populations.

2. Observations and Data Reduction

SN 1987A has been observed with various instruments on board HST since 1990. In particular, as part of the SINS project (Supernova INtensive Study, PI Kirshner), we have now a series of multifilter images, that give an excellent coverage over an area of about 130" radius, i.e., about 30 pc, centered on SN 1987A. Here we present the results of the analysis of the SINS images taken with the WFPC2 camera, using the F255W, F336W, F439W, F502N, F555W, F656N, F675W, F814W filters (in the following we shall refer to the broad bands as UV, U, B, V, R, and I although they do not coincide with any of the canonical ground based color systems). We also use an archival F656N image taken in early 1994 under project #5203 (PI Trauger).

Figure 1 shows a composite of all available B, V, and I broad band plus the [O III] and Halpha narrow band images. As mentioned in the introduction, SN 1987A appears to belong to a group of early type stars, suggesting the presence of a physical group and/or a small cluster.

[Click here to see Fig. 1!]

After processing the observations through the PODPS (Post Observation Data Processing System) pipeline for bias removal, flat fielding and cosmic ray removal, we performed aperture photometry following the prescriptions of Gilmozzi (1990) and Gilmozzi et al. (1994) with the refinements as described by Romaniello et al (1998). The flux calibration was obtained using the internal calibration of the WFPC2, which is typically accurate to better than ±5%. In this way we obtained the photometry of a total of 21 955 stars. More than 15 000 of them have a photometric accuracy better than 0.1 mag in the V, R and I filters. This number drops to 6 825 in the B band and only 786 stars have a UV filter uncertainty smaller than 0.2 mag.

3. HR Diagram: Stellar Populations, Ages and IMF

The large number of bands available (6 broad band filters) which cover a wide baseline (more than a factor of 3 in wavelength, extending from 2550 Å to 8140 Å) provide us with a sort of wide-band spectroscopy which defines the continuous spectral emission distribution of each star quite well. Therefore, by comparison with model atmospheres (Kurucz 1993; Castelli, private communication), one can fit the 6 band observations of each star and solve for 3 unknowns simultaneously, namely the effective temperature, T_eff, the reddening, E(B-V), and the angular radius, R/D. In practice, this can be done only for stars with effective temperatures higher than about 10 000 K and lower than 6 300 K because for the other stars the solution in the plane T_eff - E(B-V) is not unique. Therefore, we first solved for the full set of parameters, T_eff, E(B-V), and R/D only for stars suitably selected on the basis of reddening-free colors. For each of the remaining stars, we adopt the average reddening of its first neighbors and solve for only two parameters, T_eff, and R/D. Finally the stellar luminosity is computed from the derived T_eff and R/D, adopting a distance to SN 1987A of 51.4 kpc (Panagia 1998; Panagia et al. 1998).

By comparing the R band magnitudes with the ones measured in the Halpha narrow band filter we identified the stars with strong Halpha excess (R-m(Halpha)>0.25, i.e. W_eq(Halpha)>8 Å). We identify the luminous and bright ones (5 stars), which are near the MS, as Be stars whereas we believe that the redder and fainter ones (154 stars) are T Tauri stars, i.e. pre-MS stars with circumstellar material, remnant of their proto-stellar cocoons. For the latter group, pre-MS isochrones (Siess et al. 1997) indicate a range of ages between 1-2 Myr up to 10-20 Myr.

The resulting HR diagram (log (L/L_sun) vs. log (T_eff) plot, Fig. 2) confirms the early findings of Walker & Suntzeff (1989) and Walborn et al. (1993) and reveals that:

The distribution of stars in the HR diagram is clearly bound toward high temperatures, identifying a ZAMS that corresponds to a metallicity Z≅Z_sun/3.
The positions of the most luminous blue stars fall on isochrones corresponding to ages around 10-12 Myr, which make them coeval to SN 1987A progenitor and star 2 (Scuderi et al. 1996).
There are a number of stars at intermediate luminosities and temperatures (say, log (L/L_sun)∼2-4 and log (T_eff)∼3.8-4.2) that indicate distinct stellar generations, with ages in the range 40-150 Myr.
The lower MS and the red giants are mostly old populations, consistent with a metallicity either identical to, or slightly lower than the one of the young components. No single age can explain the distribution of the old population, and stellar generations between 600 Myr and 6 Gyr are required to account for the observations.
Among the stars with substantial Halpha excess (W_eq (Halpha)>8 Å) we identify 154 strong T Tauri stars, whose positions in the HR diagram indicate ages from 1-2 Myr up to 10-20 Myr.
The spatial distribution of early type, massive stars and the one of PMS stars (cf. Fig. 3) does not correlate well with each other (they are almost anti-correlated!) indicating that star formation processes for different ranges of stellar masses are rather different and/or require different initial conditions.

[Click here to see Fig. 2 - 3!]

In summary, we find evidence for continued star formation in the young population, starting a little earlier than 100 Myr ago until at least 1-2 Myr and, possibly, still ongoing. The old population requires continued star formation over a broad interval of 0.6-6 Gyr. There is a clear gap between the young and the old populations which implies that star formation in the interval 150-600 Myr was virtually suppressed relative to earlier and later epochs. It is clear that, since even the young stars are a mixture of populations of different ages, a study of the IMF is very hard and requires a proper separation of the various stellar generations to avoid systematic biases and errors.

References

Brocato E., Castellani V., 1993, ApJ 410, 99
Cassisi S., Castellani V., Straniero O., 1994, A&A 282, 753
Gilmozzi R., 1990, "Core aperture photometry with the WFPC", STScI Instrument Report WFPC-90-96
Gilmozzi R., Kinney E.K., Ewald S.P. Panagia N., Romaniello M., 1994, ApJ 435, L45
Kurucz R.L., 1993, ATLAS9 Stellar Atmosphere Programs and 2 km s^-1 grid (Kurucz CD-ROM No. 13)
Lucke P.B., Hodge P.W., 1970, AJ 75, 171
Panagia N., 1998, Invited Talk at the Workshop 'Views on Distance Indicators', Caputo F. (ed.), Mem.S.A.It., in press
Panagia N., Romaniello M., Scuderi S., Kirshner R.P., 1998, ApJ, to be submitted
Romaniello M., Panagia N., Scuderi S., Kirshner R.P., 1998, in preparation
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Siess L., Forestini M., Dougados C. 1997, A&A 325, 556
Van Dyk S., Hamuy M., Mateo M., 1998, in 'SN 1987A: Ten Years Later', Phillips M.M., Suntzeff N.B. (eds.), ASP Conf. Ser., in press.
Walborn N.R., Phillips M.M., Walker A.R., Elias J.H., 1993, PASP 105, 1240
Walker A.R., Suntzeff N.B., 1991, PASP 103, 958

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First version:	07th	August,	1998
Last update:	04th	October,	1998

Jochen M. Braun & Tom Richtler
(E-Mail: jbraun|richtler@astro.uni-bonn.de)