Page 1 PROPOSAL FOR HUBBLE SPACE TELESCOPE OBSERVATIONS ST ScI Use Only ID 5713 Report Date: 18-Jul-95:17:53 Version: ********** Check-in Date: ********** 1.Proposal Title: THE DISTANCE OF HIGH-VELOCITY CLOUDS ------------------------------------------------------------------------------------ 2. Scientific Category 3. Proposal For 4. Proposal Type 5. Continuation ID INTERSTELLAR MEDIUM GO/CAR 4536 Sub Category ABSORPTION LINES ------------------------------------------------------------------------------------ 6. Principal Investigator Institution Country Telephone Dr. B. P. Wakker UNIVERSITY OF ILLINOIS USA 217-244-4207 POSTDOC ------------------------------------------------------------------------------------ 7. Abstract We request time on the HST to obtain high-resolution (20 km/s) spectra around the 2800A MgII doublet of two stars and one quasar which lie projected toward the high- velocity HI cloud complex A. The prime purpose of this project is to determine the distance to complex A. Using these spectra we will be able to derive for the first time a firm upper or lower distance limit of 2.5 kpc for a HVC, which allows us to exclude several of the many proposed explanations. If we find that the object is beyond 2.5 kpc this will have important implications for the understanding of the structure of the Galaxy, especially for the energy balance and dynamics of interstellar matter and the exchange of material between the halo and the disk. A byproduct of the observations will be the first measurement of the MgII abundance in a HVC. Metal abundances are important for the problem of the origin of HVCs. We discuss the principle and problems of the method used to estimate distances and describe the current status of knowledge. Ground-based observations have been limited by sensitivity and ST allows us to reach stars faint enough so that a clear answer can be found. ------------------------------------------------------------------------------------ ------------------------------------------------------------------------------------ 9. Est obs time (hours) pri: 7.00 par: 0 10. Num targs pri: 3 par: 0 ------------------------------------------------------------------------------------ 11. Instruments requested: HRS ------------------------------------------------------------------------------------ ------------------------------------------------------------------------------------ Page 2 I. GENERAL FORM Proposal 5713 PI: Dr. B. P. Wakker Proposal Title: THE DISTANCE OF HIGH-VELOCITY CLOUDS ------------------------------------------------------------------------------------ 1. Proposers: Proposers Institution Country ESA ------------------------------------------------------------------------------------ Pi B. P. Wakker UNIVERSITY OF ILLINOIS USA Co-I L. Danly SPACE TELESCOPE SCIENCE USA INSTITUTE Co-I U. J. Schwarz KAPTEYN LABORATORIUM, UNIVERSITY NETHERLANDS X OF GRONINGEN Co-I H. Van Woerden KAPTEYN LABORATORIUM, UNIVERSITY NETHERLANDS X OF GRONINGEN ------------------------------------------------------------------------------------ 3. Description of proposed observations. Below we list the selected objects. For each probe we want to get one around the 2800 A MgII doublet, a strong resonance transition from a species with high abundance in the ISM. The current state of the GHRS makes this the optimal line. Listed are estimated fluxes at 2800 A, in erg/cm^2/s/A. They were derived by assuming Teff=25000 K, a typical temperature for the cooler in the list of Moehler et al. (1990), who derived Teff from a detailed analysis. We used a Kurucz model with this temperature to find the ratio of fluxes between B magnitude and 2800 A. We can justify this method because IUE spectra we obtained earlier for a few other, brighter, sdB stars from the PG list fit the Kurucz distribution well. The spectral resolution we need is determined by the width of the emission at 21 cm. In complex A the widths vary with angular resolution and can be as narrow as 5 km/s FWHM (Wakker Schwarz 1991). However, this width is for the cooler gas in this HVC. Most of the emission comes from gas with widths of about 10-15 km/s. Thus, our instrument of choice is the medium-resolution spectrograph in the GHRS, combined with the large aperture, giving a resolution of 20 km/s at 2800 A. Page 3 ------------------------------------------------------------------------------------ 4. Justification of need for HST observations. The determination of the distance to high-velocity clouds has been an unsolved problem for almost 30 years. Recent progress shows that it is likely that HVCs are a few kpc distant (see Wakker, 1991a). So far, all attempts to determine distances directly via absorption-line studies have failed to give conclusive results. Songaila et al. (1988) and Lilienthal et al. (1990) showed complex A has D>300 pc. Schwarz et al. (1992) failed to detect absorption in PG0832+676 (B=14.3) at the level of 24 mA, with 5h integration. The distance is about 1.5 kpc [or 31 kpc if the analysis by Brown et al. is correct]. Since no absorptions have been found in nearby stars, it has become necessary to observe more distant stars. Unfortunately, only a few stellar types are usable as probes, and so we have to observe faint stars. Assuming a Ca^+ abundance of 3E-8 for HVCs, integration times for 5 sigma detections on 4-m class telescopes turn out to be about 100 hours. With HST we have the following advantanges: 1) the probes have larger fluxes in the UV than in the blue, 2) we can use the more sensitive lines of MgII, 3) HST is more sensitive in the UV than ground based telescopes in the violet, 4) no light is lost due to seeing. Our project would produce spectra of three probe stars at distances larger than 1 kpc with 20 km/s velocity resolution and S/N of 15 or better. If we find a detection, an upper limit to the distance to the HVC complex A will be known. If we do not find a detection in a stellar probe the spectrum of the quasar MK 106 will give us the Mg/H ratio. The HI column densities toward our targets are in the range E19-E20. For solar abundances (Spitzer 1978), this implies an Mg^+ column density of 3E14 /cm^2 (Mg^+ is the dominant ionization stage for Mg in neutral gas). Observations of IVCs indicate that most of the refractory elements are in the gas phase. However, even if the gas is depleted as heavily as is found in dense interstellar clouds, the predicted Mg^+ column density is still 1E13 /cm^2. Exposure times were calculated to achieve a S/N of 15 in the continuum at 2800 A. This permits a 5 sigma detection of a 35 mA line, corresponding to a MgII column density of 7E11 /cm^2, much lower than the expected column density. Only if the abundances are significantly less than in dense interstellar clouds (i.e. by at least a factor of 10), may the detection of HVCs at MgII become difficult. The proposed S/N ratio is therefore more than sufficient to detect Mg^+ absorption associated with the HVC, if the probe stars are more distant. Given the required S/N ratio, we calculated the exposure time using the fluxes found in Sect. 4a and the updated sensitivity curves for the G270M grating listed in "Updated Technical Information", 23 May, 1991. The value of counts/sec/diode was compared to the instrumental background (0.012 cnts/sec/diode for the red detector) to ensure that the resultant spectrum would not be dominated by system noise. ------------------------------------------------------------------------------------ 5. Description of special scheduling requirements. Wave calibrations are requested for all spectra. ------------------------------------------------------------------------------------ 6. Description of special calibration exposures. Wave calibrations are required for all spectra. This was not included in the Phase I proposal, but is now recognized as essential. Absolute velocity calibration to about 2-3 km/s is required. Each wave calibration must be performed with no gap immediately preceeding each spectral exposure. ------------------------------------------------------------------------------------ 8. Additional comments or special requests. Time has already been allocated at Westerbork to map the several probe fields in the 21 cm line. The probes will also be included in the ongoing program of spectroscopic distance determinations on which our collaborators in Germany (K.S. de Boer and others) are working, using a number of telescopes including those at Calar Alto in Spain and at La Silla in Chile. Page 4 ------------------------------------------------------------------------------------ 9. Description of previous HST work. unrelated: 2644 - The environment of starburst galaxies: absorption line studies of galactic outflows; PI C.Norman, Co-Is C.Blades, L.Danly, T. Heckman 3606 - The nature of gaseous loops in the Milky Way halo; PI L. Danly, Co-Is R. Benjamin, K. Kuntz, C.E. Albert, P. Shapiro ------------------------------------------------------------------------------------ 10. Resources to be supplied by investigator's institution(s). We expect that most of the HSR datareduction will be done at STScI, by Danly in collaboration with Wakker. For the reduction of the radio and further analysis a workstation for Wakker is requested. ------------------------------------------------------------------------------------ 11. Address Information Name: B. P. WAKKER Category: PI Institution: UNIVERSITY OF ILLINOIS Address: ASTRONOMY DEPARTMENT 1002 W. GREEN ST. City: URBANA State: IL Zip Code: 61801 Country: USA Telephone: 217-244-4207 Telex (or e-mail): NA, FAX 217-244-7638 ------------------------------------------------------------------------------------ TARGET LIST a) Fixed Targets ID = 5713 [ 5] ------------------------------------------------------------------------------------------------------------------------------------ 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 ------------------------------------------------------------------------------------------------------------------------------------ Tar| Target | Target | Target |Coord | Radial |Acqui|FLX| Flux data No | Name | Description | Position |Eqnx | Vel. |Prblm|REF| | | | | | | | | ------------------------------------------------------------------------------------------------------------------------------------ 1 MK106 E,313,G,509 RA = 9H 19M 55.236S +/- 2000 1 V = 16.15 0.5S, 3 B-V = 0.39 DEC= +55D 21' 36.66" +/- 4 U-B = -0.94 0.5", 5 F-CONT(2800) = 4.4 E-14 6 F-LINE(2795) = 4.4 E-14 7 W-LINE(2795) = 0.14 Comments: FLUXNUM: 1=V; 2=BPG; 3=B-V; 4=U-B 5=CONT; 6=F-LINE; 7-W-LINE DETECTION LIMIT FOR LINE FROM F-LINE=F-CONT*0.25; W- LINE=0.14 ------------------------------------------------------------------------------------------------------------------------------------ EXPOSURE LOGSHEET ID = 5713 [ 6] ------------------------------------------------------------------------------------------------------------------------------------ 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 |10 | 11 | 12 |13 |14| 15 ------------------------------------------------------------------------------------------------------------------------------------ Line | Seq | Target |Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | S/N |Flx|Pr| Special Number | Name | Name |Config| Mode |or FOV |Element |Waveln.| Parameters |Exp| |Rel. Time|Ref| | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 1.1 MK106 HRS ACQ 2.0 MIRROR-N2 SEARCH-SIZE=3 1 10S 1 CYCLE 4 ONBOARD ACQ FOR 1.3 SEQ 1.1-1.3 NO GAP ------------------------------------------------------------------------------------------------------------------------------------ 1.2 WAVE HRS ACCUM SC2 G270M 2800 STEP-PATT=5 1 DEF 1 CYCLE 4 CALIB FOR 1.3 ------------------------------------------------------------------------------------------------------------------------------------ 1.3 MK106 HRS ACCUM 2.0 G270M 2800 FP-SPLIT=STD 17 500S 20 5 10 CYCLE 4 STEP-PATT=5 6 CENSOR=YES 7 DOPPLER=ON ------------------------------------------------------------------------------------------------------------------------------------ Summary Form for Proposal 5713 [ 7] Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Configurations HRS ------------------------------------------------------------------------------------------------------------------------------------ Opmodes ACQ ACCUM ------------------------------------------------------------------------------------------------------------------------------------ Optional Parameters SEARCH-SIZE=3 STEP-PATT=5 FP-SPLIT=STD CENSOR=YES DOPPLER=ON ------------------------------------------------------------------------------------------------------------------------------------ Proposal for GO/CAR ------------------------------------------------------------------------------------------------------------------------------------ S/C Hours 7.00 ------------------------------------------------------------------------------------------------------------------------------------ Scientific Category INTERSTELLAR MEDIUM ------------------------------------------------------------------------------------------------------------------------------------ Scientific Sub-category ABSORPTION LINES ------------------------------------------------------------------------------------------------------------------------------------ Special Requirements CYCLE 4; ONBOARD ACQ FOR 1.3; SEQ 1.1-1.3 NO GAP CALIB FOR 1.3; ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements MIRROR-N2 G270M ------------------------------------------------------------------------------------------------------------------------------------ Target Names MK106 ------------------------------------------------------------------------------------------------------------------------------------