! Hubble Space Telescope Cycle 6 (1996) Phase II Proposal Template ! $Id: 6580,v 6.1 1996/05/17 20:24:02 pepsa Exp $ ! ! Refer to the HST Phase II Proposal Instructions to fill this out ! ! Anything after a "!" is ignored, and may be deleted ! ! All keywords with multiple entries are comma delimited except the ! Visit_Requirements and Special_Requirements keywords which can be ! delimited with carriage returns or semi-colons, but not commas ! ! For help call your Program Coordinator: Karla Peterson ! Phone: 410-338-4774 , E-mail: peterson@stsci.edu ! ! This partially completed template was generated from a Phase I proposal. Proposal_Information ! Section 4 Title: Hot Stars and Young Super Star Clusters in the Wolf-Rayet Starburst Galaxy He 2- 10 Proposal_Category: GO Scientific_Category: GALAXIES & CLUSTERS Cycle: 6 Investigators PI_name: Peter S. Conti PI_Institution: JILA CoI_Name: William D. Vacca CoI_Institution: Institute for Astronomy Contact: ! Y or N (designate at most one contact) CoI_Name: Claus Leitherer CoI_Institution: Space Telescope Science Institute Contact: ! Y or N (designate at most one contact) Abstract: ! Free format text (please update) Wolf-Rayet (W-R) galaxies are a subset of starburst galaxies observed shortly after the onset of the star formation event, when a large number of the most massive stars have evolved to the W-R stage. HST UV images of 17 of these galaxies, obtained as part of Cycle 2, revealed that the O, B, and W-R stars in the starbursts are clustered within intense, compact, starburst knots, now generally referred to as ``super star clusters''. We propose a detailed investigation, employing both optical images and UV spectra, of the recently discovered starburst knots in the prototype W-R galaxy He 2-10. WFPC optical images of this blue compact dwarf galaxy will yield magnitudes and colors, which can then be used to better constrain the burst ages, masses, and stellar populations, for the individual starburst knots. GHRS UV spectra of the starburst regions will be analyzed with a sophisticated spectral synthesis code to determine the parameters of the massive star population directly from the observed spectral signatures. The proposed observations will allow us to investigate the processes occurring during the earliest stages of the starburst phenomenon. Questions ! Free format text (please update) Observing_Description: 4pt noindent WFPC2 imaging 4pt We selected four passbands to optimally constrain the stellar content of the starburst knots: F439W, F555W, F658N, and F814W. The choice of filters is a trade-off between exposure times (which become prohibitively long in the ultraviolet), and suitability to constrain the young stellar population (by separating age and extinction effects). The data obtained with these filters will be combined with the previously-obtained FOC UV (F220W) images. The primary filter to be used to detect hot, ionizing stars is F658N, for HAlpha estimates. Measurement of the HAlpha equivalent width will provide an excellent age discriminant for a young starburst. W(HAlpha) is a normalized quantity, and therefore does not depend on the star-formation rate, the total stellar mass, or the low-mass end of the IMF. (At the same time it is not very useful to constrain the IMF slope. This will be done with our UV spectroscopy). The continuum points will be taken from the V and I measurements, with the interpolation at HAlpha following from our population synthesis models. Ground-based measurements of distant H II regions yield equivalent widths which are generally lower than model predictions (Garc'\ia- Vargas & D'\iaz 1994), due to dilution of the continuum by the surrounding field population. The observed B (F439W), V (F555W), and I (F814W) magnitudes will be used to determine extinctions towards the knots and detect underlying older stellar populations in the surroundings. The B and V magnitudes are sensitive to age effects as well. Having constrained the age of the knots with HAlpha, we will use (B - V) to determine the interstellar extinction, which is substantial for He 2-10 (A_V = 1.6). With images taken through the F814W filter, we can then construct (B - V) versus (V - I) color-color diagrams. If red supergiants are present in the starburst regions, they will be detectable by their red colors (cf. Cervi\ no & Mas Hesse 1994). Although in general we do not expect to observe red supergiants in a singular burst when W-R stars are present, the starburst regions may have a very complex star formation history. We will determine absolute magnitudes M_B and M_V and determine a relationship between continuum magnitude and linear size of the knots. With the ages of the starburst events in the knots estimated from the H Alpha fluxes, we can then determine the masses of the knots by comparing the observed luminosities with those predicted from spectral synthesis models. We will construct luminosity and mass functions for the various knots and examine any variations among the starburst regions. We will compare the derived parameters to those of the recently formed globular clusters in the elliptical/merging galaxies NGC1275 (Holtzman et al. 1992) and NGC7252 (Whitmore et al. 1993). This will serve as a first step towards determining a physical relationship between starburst regions in W-R galaxies, recently formed clusters in merging galaxies, and globular clusters in general. Exposures times for the proposed images were calculated based on the fluxes measured on our pre-COSTAR FOC images at 2200 Angstrom\ and ground- based optical spectra. The total integration times per galaxy are 50 min (F439W), 30 min (F555W), 30 min (F814W), and 60 min (F656N). The exposures will be split into four sub- exposures to allow two dithered positions, and each CR-SPLIT will be used to identify the cosmic rays. Taking into account the overhead, we can accomplish the imaging in four orbits. 4pt noindent GHRS spectroscopy 4pt Even with a relatively flat IMF, stars with masses above 10 M_\odot contribute little to the optical continuum and their spectral features in this wavelength range are correspondingly weak. The situation improves dramatically in the UV at wavelengths below 2000 Angstrom\ where massive stars display a variety of strong stellar absorption lines. Our goal is to obtain HST GHRS UV spectra of two starburst regions in He 2-10. We expect that these will be similar to that obtained for NGC 4214 (see Fig. 2). We will use our spectral synthesis models to determine the parameters of the massive star population (e.g., burst age, numbers of hot stars, slope of the upper end of the IMF, upper cut-off mass) within the knots. We need S/N = 20 in the continuum to detect and model age- and IMF- sensitive stellar absorption lines, based on our experience with previous modeling of IUE and HST data (Robert et al. 1993; Vacca et al. 1995; Leitherer et al. 1995). Lower S/N makes it difficult to resolve spectral lines which are blends of stellar and interstellar contributions. We compared the capabilities of the FOS and G130H grating and of the GHRS and G140L grating. After taking into account instrumental overheads, sensitivity, scattered light in the far-UV, wavelength coverage, aperture sizes, and spectral resolution, we find that the GHRS is the instrument of choice. Above all, the most important strategic lines used to characterize the burst (He II 1640, C IV 1550, Si IV 1400, N V 1240) have wavelengths near or below 1600 Angstrom where the GHRS sensitivity is clearly superior. The GHRS has the required sensitivity, scattered light behavior, and spectral resolution (needed to separate the emission line from nearby geocoronal LyAlpha emission) to provide high-quality observations of N V. The fluxes of the starburst regions are around ~ 3* 10^-15 erg s^-1 cm^-2 AA^-1 at 1400 Angstrom, based on our FOC images at 2200 Angstrom. We need two grating positions to cover the essential wavelength region from 1200 Angstrom\ to 1700 Angstrom. At the shorter wavelengths, S/N ~ 20 is reached in 1.5 hours. 2.5 hours are required at the longer wavelength setting. We propose to observe 2 starburst regions in He 2-10. The regions are sufficiently close that the guide star and target acquisition has to be performed only once. The first orbit will be used for an FOS-assisted target acquisition. 5 orbits are needed to reach S/N = 20. The grand total for the spectroscopy is 5 orbits for 2 starburst regions. Real_Time_Justification: none OVRO and VLA measurements of the CO and HI in He 2-10 indicate the quantity and distribution of the molecular gas and the neutral hydrogen content (Kobulnicky et al. 1995). The spatial morphology strongly suggests that a merger/interaction has occurred. Calibration_Justification: ! Move appropriate text from Real_Time_Justification Additional_Comments: Fixed_Targets ! Section 5.1 Target_Number: 1 Target_Name: HE2-10 Alternate_Names: Description: GALAXY,DWARF COMPACT,STARBURST Position: RA=8H 36M 15.42S +/- 0.1S, DEC=-26D 24' 34.3" +/- 1" Equinox: 2000 RV_or_Z: RA_PM: ! Units are seconds of time per year Dec_PM: ! Units are seconds of arc per year Epoch: Annual_Parallax: Flux:F-CONT(2200) = 3.0 E-14 Comments: Target_Number: 2 Target_Name: HE2-10A Alternate_Names: Description: GALAXY,DWARF COMPACT,STARBURST Position: RA=8H 36M 15.14S +/- 0.1S, DEC=-26D 24' 34.0" +/- 1" Equinox: 2000 RV_or_Z: RA_PM: ! Units are seconds of time per year Dec_PM: ! Units are seconds of arc per year Epoch: Annual_Parallax: Flux:F-CONT(2200) = 6.0 E-15 Comments: Target_Number: 3 Target_Name: HE2-10B Alternate_Names: Description: GALAXY,DWARF COMPACT,STARBURST Position: RA=8H 36M 15.81S +/- 0.5S, DEC=-26D 24' 34.7" +/- 1" Equinox: 2000 RV_or_Z: RA_PM: ! Units are seconds of time per year Dec_PM: ! Units are seconds of arc per year Epoch: Annual_Parallax: Flux:F-CONT(2200) = 1.0 E-15 Comments: ! This is a template for a single visit containing a single exposure ! Repeat exposure and visit blocks as needed Visits ! Section 6 Visit_Number: 1 Visit_Requirements: ! Section 7.1 ! Uncomment or copy visit level special requirements needed ! Most of these requirements (including ORIENT) will limit scheduling ! PCS MODE [Fine | Gyro] ! GUIDing TOLerance ! ORIENTation TO ! ORIENTation TO FROM ! ORIENTation TO FROM NOMINAL ! SAME ORIENTation AS ! CVZ ! PARallel ! AFTER [BY [TO ]] ! AFTER ! BEFORE ! BETWEEN AND ! GROUP WITHIN