8237( 6) - 10/04/99 11:23 - [ 1] PROPOSAL FOR HUBBLE SPACE TELESCOPE OBSERVATIONS ST ScI Use Only ID: 8237 Version: 6 Check-in Date: 11-May-1999 09:48:58 1.Proposal Title: Detecting Circumstellar ``Hydrogen Wall'' Emission Around a Nearby, Sun-like Star ------------------------------------------------------------------------------------ 2. Proposal For 3. Cycle GO 8 ------------------------------------------------------------------------------------ 4. Investigators Contact? PI: Brian Wood Harvard-Smithsonian Center for Astrophysics CoI: Prof. Jeffrey Linsky JILA N CoI: Dr. Gary Zank Bartol Research Institute N ------------------------------------------------------------------------------------ 5. Abstract Using the long-slit spectroscopy capabilities of STIS, we propose to try to detect for the first time nebular Lyman- Alpha emission surrounding a Sun-like star produced by the interaction of its stellar wind with the ISM. Such ``hydrogen walls'' have likely been detected in absorption around the Sun and several other nearby stars using GHRS Lyman-Alpha spectra. However, most of these detections are tentative due to the difficulty in separating the H-wall absorption from the interstellar H I absorption. Furthermore, even if one accepts the reality of the detected hot H I absorption components, it is impossible to prove that circumstellar material is in fact responsible. We propose to circumvent these difficulties by detecting a hydrogen wall in emission around 40 Eri A, which is one of the stars for which a tentative H-wall detection already exists. A successful detection of the expected circumstellar emission would validate the previous Lyman-Alpha aborption line studies, a nd the combined spectroscopic and spatial information provided by long-slit spectroscopy would contribute valuable new information on the stellar wind of 40 Eri A and how it interacts with the ISM, especially when compared with models that we will construct of 40 Eri A's ``astrosphere.'' This new information includes a direct measurement of the distance to the stellar bow shock, information that we do not possess for any other nearby star, including the Sun. ------------------------------------------------------------------------------------ 8237( 6) - 10/04/99 11:23 - [ 2] Observations Description ------------------------ Our observations will use the 52X0.2F1 long slit with the G140M grating to observe the 1194--1249 Angstrom spectral region. Because the G140M grating does not have sufficient resolution to fully separate the H-wall and geocoronal lines, it will be necessary to minimize the geocoronal emission to prevent the wings of the geocoronal line from overwhelming the weak H-wall signal (see below). Thus, we must observe in Shadow Time. It is also highly desirable that the slit be oriented within 15^degrees of the upwind direction in the stellar rest frame (with a position angle of 32 degrees). This is where the visibility of the H-wall emission should be at a maximum, and the distance to the bow shock should be at a minimum. (See the ``Special Requirements'' section for information on the effects these constraints have on schedulability.) Our target is too bright to be acquired using the preferred F28X50LP CCD aperture, so we will acquire 40 Eri A with the F28X50OII aperture. Af t e r acquisition, we will peakup in the special aperture designed for use with the occulting bar associated with the 52X0.2F1 slit. Blocking the star with the occulting bar will minimize the stellar emission in the field of view which might obscure the weak H-wall emission we hope to detect around the star. We will observe 40 Eri A for 5 orbits, which is the maximum visit length for a single target due to the South Atlantic Anomaly. Because the Shadow Time requirement reduces the available observing time per orbit, the overheads associated with the acquisition, peakup, and first (and only) science observation are enough to use up practically all of the first orbit. We have run a crude Phase II version of this proposal through the RPS2 software (see the ``Special Requirements'' section) which predicts that for the remaining 4 orbits, about 30 minutes of exposure time will be available after guide star reacquisition. We assume any other overheads (such as those for ``wavecal s' ') will take place outside the narrow Shadow Time window. Thus, we expect our single science observation to be about 4* 30=120 minutes. In order to determine if this exposure time is sufficient to detect the H-wall of 40 Eri A, we had to perform the simulation described below. The first thing we need to do to assess the feasibility of our proposal is to estimate the size of the stellar H-wall projected onto the sky The column density of the H-wall component in Figure 1(b) is log N(H I)=13.75. Heliospheric models predict H I densities of about 0.2 cm^-3 within the solar H -wall (e.g. Zank et al. 1996). There are various competing effects that might make the H-wall density greater or lower for 40 Eri A, but we believe 40 Eri A's density will probably also be about 0.2 cm^-3 to within a factor of 2 or so. When combined with the column density, this yields a thickness of 19 AU for the H-wall along the line of sight, which is 58 degrees from the upwind direction. The distance fro m the star to the bow shock marking the outer edge of th e H-wall should be about twice this value (38 AU), based on heliospheric models which should also be crudely applicable here (e.g. Zank et al. 1996). Note that the upwind bow shock distance will be somewhat less than this. At the 4.8 pc distance of 40 Eri A the STIS field of view is about 120 X 120 AU, so the bow shock should fit into the field of view in the upwind hemisphere. Based on the shapes of solar bow shocks illustrated in the literature and the estimated 38 AU bow shock distance 58^degrees from the upwind direction, we estimate the area of the upwind hemisphere of the H-wall to be 2000 AU^2. The second step is to estimate the surface brightness of the stellar H-wall. We base our estimates on the properties of the H-wall component in Figure 1(b) (Wood & Linsky 1998), which suggests the stellar H-wall scatters about 1.0* 10^ -12 ergs cm^-2 s^-1 in the upwind hemisphere. At the distance of 40 Eri A, 1 arcsec = 4 .8 AU. Therefore, the average surface brightness of the H-wall should be about 1.2* 10^-14 ergs cm^-2 s^-1 arcsec^-2 . The third step is to convert this surface brightness to an emission line profile. For an optically thin plasma, the profile will simply be a Gaussian with a width appropriate for an H-wall temperature of 3.6* 10^5 K, the temperature suggested by the fit in Figure 1(b) (Wood & Linsky 1998). However, the 40 Eri A H-wall has an optical depth of 0.6 based on the fit in Figure 1(b), so we will make a crude correction for opacity effects. Away from the star we should also see the back side of the H-wall, so our final estimate for the total optical depth is 1.2. Using techniques described by Wood et al. (1996b), we derive an emission line profile for an optical depth of 1.2, a temperature of 3.6* 10^5 K, and a total surface brightness of 1.2* 10^-14 ergs cm^-2 s^-1 arcsec^-2. This profile is shown as a dotted histogram in Figure 2(a). The central wavelength of th e H- wall emission should vary with location, but it should generally be between the rest frames of the star and t he undisturbed LISM (1215.0 Angstrom and 1215.76 Angstrom, respectively). We assumed a centroid of 1215.6 Angstrom for the H- wall emission in Figure 2(a). The fourth step is to correct for interstellar absorption, which we do using the LISM parameters derived from the fit in Figure 1(b). We then convolve the emission line with the instrumental profile of the G140M grating combined with the 52X0.2F1 slit. The result is shown as a dotted line in Figure 2(a). The fifth step is to add in the geocoronal and interplanetary medium (IPM) H I emission lines. Since we are observing in Shadow Time, we use the minimum geocoronal flux listed in the STIS Instrument Handbook. The time of year of our observations is constrained by Shadow Time and slit placement requirements (see ``Special Requirements'' section), so the central wavelength of the geocoronal emission is known. T he IPM H I flux is taken from Clarke et al. (1995), and the emission is assumed to be at the projected velocit y of the LISM flow vector. The geocoronal and IPM emission c omponents, after convolution with the instrumental profile, are shown as dashed and dot-dashed lines in Figure 2(a). Finally, we conduct Monte Carlo simulations to see if the H-wall emission is apparent in the expected 120 minute exposure time. Figure 2(b) shows our best estimate of what we should see if we rebin our data by 10 pixels in the spatial direction, resulting in a spatial resolution along the slit of 1.2 AU. Some of the noise evident in the spectrum is produced by the dark count rate of the detector, although we have subtracted the ``continuum'' produced by the dark counts. The H-wall is indeed evident as a flux excess on the blue wing of the geocoronal H I line. Note that the spectra observed beyond the bow shock, where the H-wall emission will not be present, will provide us with an accurate ge ocoronal plus interplanetary H I line profile that we can subtract from the data where H-wall emission is present. If necessary, we can also improve S/N by further reb in ning of the data in both the spatial and spectral directions. Thus, we conclude that the H- wall emission, though weak, is detectable with STIS and can be at least partially separated from the geocoronal+IPM line. We can now explain why we chose 40 Eri A as our target star. This star is particularly attractive because of the extremely large 123 km s^-1 interstellar wind velocity seen by the star. This large velocity produces a very hot, compact astrosphere. The high temperature and compactness result in a surface brightness of the H-wall that is large enough to detect. The high temperature is necessary to produce an emission profile (dotted histogram in Fig. 2a) broad enough to escape complete obliteration by interstellar absorption, and broad enough to be partially separable from the geocoronal emission . References: Baranov, V. B., & Malama, Y. G. 1995, JGR, 100, 14755 Clarke, J. T., Lallement, R., & Bertaux, J. -L., & Qu'emerais, E. 1995, ApJ, 448, 893 Dring, A. R., et a l. 1997, ApJ, 488, 760 Holzer, T. E. 1972, JGR, 77, 5407 Holz er, T. E. 1989, ARA&A, 27, 199 Lallement, R., & Bertin, P. 1992, A&A, 266, 479 Linsky, J. L., & Wood, B. E. 1996, ApJ, 463, 254 Linsky, J. L., & Wood, B. E. 1997, in Primordial Nuclei and their Galactic Evolution, ed. N. Prantzos, M. Tosi, & R. von Steiger (Dordrecht: Kluwer), in press Pauls, H. L., Zank, G. P., & Williams, L. L. 1995, JGR, 100, 21595 Qu'emerais, E., Sandel, B. R., Lallement, R., & Bertaux, J. -L. 1995, A&A, 299, 249 Wallis, M. 1975, Nature, 254, 207 Wood, B. E., Alexander, W. R., & Linsky, J. L. 1996a, ApJ, 470, 1157 Wood, B. E., Harper, G. M., Linsky, J. L., & Dempsey, R. C. 1996b, ApJ, 458, 761 Wood, B. E., & Linsky, J. L. 1998, ApJ, 492, 788 Zank, G. P., Pauls, H. L., Williams, L. L., & Hall, D. T. 1996, JGR, 101, 21639 Real Time Justification ----------------------- Because the G140M grating does not have sufficient resolution to fully separate the H-wall and geocoronal lines, it will be necessary to minimize the geocoronal emission to prevent the wings of the geocoronal line from overwhelming the weak H- wall signal (see Fig. 2). Thus, we must observe in Shadow Time. It is also highly desirable that the long slit be placed within 15^degrees of the upwind direction in the stellar rest frame (with a position angle of 32degrees). This is where the visibility of the H-wall emission should be at a maximum, and the distance to the bow shock should be at a minimum. Both of the above requirements place serious restrictions on when the observations can be scheduled. In order to determine if the proposal is schedulable at all if constrained in this fashion, we have used the Phase II RPS2 software to make a preliminary assessment of our proposed observations. We find that there is indeed a month long scheduling window between December 19 and J anuary 20. Calibration Justification ------------------------- Additional Comments ------------------- ------------------------------------------------------------------------------------ 8237( 6) - 10/04/99 11:23 - [ 3] Data Distribution Paper Products: NO Media: ELECTRONIC Blocking Factor: 10 Ship To: PI_Address Ship Via: Email: ------------------------------------------------------------------------------------ 8237( 6) - 10/04/99 11:23 - [ 4] TARGET LIST Fixed Targets ------------------------------------------------------------------------------------------------------------------------------------ Tar| Target | Target | Target |Coord | Radial | Flux data No | Name | Description | Position |Eqnx | Vel. | ------------------------------------------------------------------------------------------------------------------------------------ 1 HD26965 STAR, K V-IV, ISM, WIND RA=4H 15M 16.32S +/- 0.05S, DEC= J2000 V = -42 V = 4.43+/-0.02 -7D 39' 9.9" +/- 0.5" B-V = 0.82+/-0.02 TYPE=K1V F-LINE(1216)=4.0+/-1.0e-12 W-LINE(1216)=0.5+/-0.1 F-CONT(1220)=5.0+/-3.0e-15 Comments: Coordinates are from the Hipparcos Input Catalog. Epoch of Position RA proper motion (seconds of time/yr) DEC Proper Motion (arcsec/yr) Annual Parallax (arcsec) J2000 -0.1497 -3.418 0.198 8237( 6) - 10/04/99 11:23 - [ 5] Visit: 01 Visit Priority: Visit Requirements: ORIENT 58D TO 98D ORIENT 238D TO 278D VISIBILITY INTERVAL 35 M On Hold Comments: Additional Comments: Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exposure| Target |Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Number | Name |Config| Mode |or FOV |Element |Waveln.| Parameters |Exp| | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 10 HD26965 STIS/C ACQ F28X50O MIRROR ACQTYPE=POINT 1 0.1 S ONBOARD ACQ FOR 30-40 CD II ------------------------------------------------------------------------------------------------------------------------------------ 30 HD26965 STIS/F ACCUM 52X0.2F G140M 1218 1 21 M SHADOW UV-MAM 1 A ------------------------------------------------------------------------------------------------------------------------------------ 40 HD26965 STIS/F ACCUM 52X0.2F G140M 1218 4 31.5 M SHADOW UV-MAM 1 A ------------------------------------------------------------------------------------------------------------------------------------ 8237( 6) - 10/04/99 11:23 - [ 6] Summary Form for Proposal 8237 Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Apertures F28X50OII 52X0.2F1 ------------------------------------------------------------------------------------------------------------------------------------ Configurations STIS/CCD STIS/FUV-MAMA ------------------------------------------------------------------------------------------------------------------------------------ Opmodes ACQ ACCUM ------------------------------------------------------------------------------------------------------------------------------------ Optional Parameters ACQTYPE=POINT ------------------------------------------------------------------------------------------------------------------------------------ Proposal Category GO ------------------------------------------------------------------------------------------------------------------------------------ Special Requirements ORIENT 58D TO 98D ORIENT 238D TO 278D VISIBILITY INTERVAL 35 M ONBOARD ACQ FOR 30-40 SHADOW ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements MIRROR G140M ------------------------------------------------------------------------------------------------------------------------------------ Target Names HD26965 ------------------------------------------------------------------------------------------------------------------------------------ Wavelengths 1218 ------------------------------------------------------------------------------------------------------------------------------------