Spatial and Temporal Variations in the Physical Conditions of Clouds Shocked by the Vela Supernova Remnant

0 Assigned ID: 2358 ----- Attempting Submission 1 (Fri Apr 09 22:23:21 GMT 2021) ----- HST Phase I Proposal 2358 successfully submitted. Receipt: # 2358-1 This is a resubmission. We changed the central wavelength setting for some of the exposures from 1562 A to 1598 A. This was to ensure that the C I multiplet at 1656 A is included in the observations. 1031759447 ----- Attempting Submission 1 (Thu Jul 29 05:09:10 GMT 2021) ----- HST Phase II Proposal 16751 successfully submitted. Receipt: # 16751-1 ----- Attempting Submission 2 (Fri Mar 04 18:13:40 GMT 2022) ----- HST Phase II Proposal 16751 successfully submitted. Receipt: # 16751-2 ----- Attempting Submission 3 (Mon May 02 22:21:28 GMT 2022) ----- Spatial and Temporal Variations in the Physical Conditions of Clouds Shocked by the Vela Supernova Remnant The Vela supernova remnant (SNR) is an ideal laboratory for studying the physical processes associated with SNR evolution in a highly inhomogeneous interstellar medium (ISM). Previous UV and optical absorption-line studies have revealed the remarkable nature of the highly disturbed interstellar gas interacting with the Vela SNR. The most noteworthy features are (1) the high positive and negative velocity absorption components seen in numerous directions, (2) the strong absorption from excited fine-structure levels of neutral carbon detected in low and high velocity gas, and (3) the many different absorption components that exhibit significant ongoing temporal variations. However, our understanding of the cloud-shock interactions taking place in the Vela region is far from complete. Here, we propose the first large-scale systematic investigation of shocked gas in the Vela SNR to employ the high resolution, high sensitivity, and extensive wavelength coverage provided by HST/STIS. We will observe eight stars clustered along a ridge of high pressure gas near the western edge of the northern X-ray bright region in the Vela SNR. The high resolution and broad wavelength coverage of the STIS spectra will allow us to examine the radial velocities, gas densities, thermal pressures, ionization fractions, and dust depletions in individual clouds along each line of sight. Five of our targets have archival STIS and/or GHRS observations, which will allow us to determine how the physical conditions have changed over time. The result will be the most comprehensive analysis to date of spatial and temporal variations in the physical conditions of clouds shocked by the Vela SNR. We plan to obtain high-resolution (~2.5-3.7 km/s), moderately high S/N ratio (~30-60) far-UV STIS spectra of eight stars probing a ridge of high-pressure gas associated with the Vela Supernova Remnant. Three of our targets have existing high-resolution STIS spectra in the MAST archive and these archival data will be incorporated into our analysis. The purpose of our research program is to evaluate the physical conditions (i.e., the densities, pressures, ionization states, and depletion levels) along each of the multiple sight lines to explore how the conditions change (1) as a function of velocity along the line of sight, (2) as a function of position on the sky, and (3) as a function of time. The targets selected for our investigation are known to exhibit significant absorption from excited fine-structure levels of neutral carbon. Many of our targets also exhibit high-velocity interstellar absorption components (from both low and high ionization atomic species) and several exhibit time-variable components. All of our targets have Gaia DR2 distances that place them behind the Vela SNR (which is at a distance of ~290 pc). Our observations will be obtained using the STIS FUV MAMA detector and E140H grating. Two central wavelength settings (at 1307 A and 1598 A) will provide wavelength coverage in the range 1206-1699 A, with a gap between 1408 A and 1497 A. The broad wavelength coverage is necessary so that our observations cover all of the relevant C I multiplets, which we will use to derive quantitative measurements of thermal pressure. By carefully analyzing the absorption features arising from all of the various C I multiplets, from the strongest multiplets (i.e., 1277, 1328, 1560, and 1656) to the weakest (e.g., 1270 and 1276), we will be able to obtain accurate determinations of thermal pressure over a wide range of C I column density. Moreover, since the pattern of transitions from the three fine-structure levels varies within each multiplet, it is necessary to observe many different multiplets in order to disentangle the complex velocity structure within each J level. The spectra we obtain will include many additional absorption features that will allow us to more fully characterize the physical conditions along the various lines of sight. Absorption from excited fine-structure levels of O I and Si II will provide information on the densities and temperatures in the neutral and partially ionized gas components that exhibit these features. Strong to moderately strong transitions of Al II, Fe II, and Ni II that fall within our wavelength coverage will yield information on the depletion characteristics of the different velocity components. Finally, absorption features from the highly-ionized species C IV, Si IV, and N V will provide crucial information on the state of the shocks permeating the Vela SNR. For the faintest stars in our program (HD 71336, HD 72088, HD 72089, and HD 72648), the 0.2x0.2 arcsec entrance slit will be used, which will provide a velocity resolution of ~3.7 km/s. The use of this relatively wide slit has two advantages. No peakup observation is required to place the target on the slit, meaning that more of the orbital visibility period can be used for science exposures. And, the higher throughput from the wider slit will yield higher S/N ratios in the resulting spectra for these fainter targets. For two of the brighter stars (HD 72350 and HD 72537), the 0.2x0.09 arcsec slit is required so that the total count rate per image is below the bright limit for the FUV MAMA. This narrower slit will yield a somewhat higher velocity resolution of ~2.8 km/s. For the binary pair (HD 72127AB), which consists of a bright (V=5.19) primary and a much fainter (V=6.98) secondary separated by 4.65 arcsec, a neutral density filter (0.2x0.05ND) will need to be employed for both targets (since the two stars are separated by less than 5 arcsec). The exposure time calculations are based on the need for peak S/N values in the range 30-60, which will yield 1-sigma equivalent width uncertainties of ~0.5-1.0 mA for weak lines with small intrinsic widths. This level of precision is required for our derivations of thermal pressures from C I and O I fine-structure excitations since the results can be sensitive to small changes in the relative populations. All of our targets (except HD 72127B) have previously been observed by the IUE satellite. Thus, we used the archival IUE spectrum (preferably obtained with the large aperture) as the input spectrum for the ETC calculations. For HD 72537, the only available IUE spectrum was obtained with the small aperture, which has been known to underestimate the true flux of the star. To correct the flux for this target, we matched the IUE spectrum to the overlapping portion of the available FUSE spectrum of this star. Since HD 72127B was not observed with IUE, we based the ETC calculations for this target on a synthetic spectrum derived from the spectral type (B2.5 V), V magnitude, and reddening (E(B-V)=0.04), assuming an average Galactic extinction curve. However, we checked that the flux in the synthetic spectrum matches available UV observations of this star taken with the GHRS (see Wallerstein et al. 1995, ApJ, 455, 590). With (absolute) declinations in the range 43-45 degrees, the visibility period for our targets is 54 minutes per orbit. In cases where we are using narrow slits (0.2x0.09 or 0.2x0.05ND), the targets will be acquired with an ACQ followed by an ACQ/PEAK. For all of our targets (except the binary pair), the visits have been structured so that each star has its own visit. Each target (except HD 72127B) will be observed with both central wavelength settings of the E140H grating. We can achieve the required S/N at both settings in one orbit each for HD 71336, HD 72089, HD 72350, HD 72537, and HD 72648, and in three orbits for HD 72088. Both components of the binary pair will be observed in one 4-orbit visit so that we can take advantage of a small angle maneuver to move from the primary to the secondary. A peakup will be performed after acquiring the primary and after moving to the secondary. While HD 72127A will be observed with both central wavelength settings, HD 72127B will be observed with the setting at 1307 A only. Due to the increased exposure time caused by the neutral density filter, it will not be feasible to obtain high quality data at both settings toward HD 72127B with the number of orbits allocated. This will not affect our science program, however. The purpose of observing the binary pair is to examine small scale structure primarily in the C I absorption profiles. There will still be many C I multiplets available with just the setting at 1307 A. Dr. Adam Ritchey has expertise in the reduction and analysis of HST spectra and is the lead author on several publications focused on the abundances and kinematics in interstellar gas associated with supernova remnants. He is the PI of three recent HST programs, which utilized high-resolution HST/STIS spectra to examine the abundances and physical conditions in the Galactic supernova remnant IC 443, and the physical conditions and dust depletions in the general Galactic interstellar medium. Dr. Edward Jenkins has considerable expertise in analyzing and interpreting ultraviolet absorption lines seen in the spectra of background targets located both in our Galaxy and in external galaxies. He has been the lead author on numerous publications that examined the abundances and physical conditions in shocked interstellar gas associated with the Vela supernova remnant and has led a number of efforts to derive thermal pressures from C I fine-structure excitations. Stellar Populations and the Interstellar Medium 6 false false true false false false false SUPPORTED STAR B3-B5 III-I B3-B5 V-IV J2000 J2000 F(1350) = 3.2E-11, F(1510) = 2.5E-11 STAR B3-B5 III-I B3-B5 V-IV J2000 J2000 F(1350) = 9.4E-12, F(1510) = 7.2E-12 STAR B3-B5 III-I J2000 J2000 F(1350) = 2.4E-11, F(1510) = 1.8E-11 STAR B0-B2 III-I J2000 J2000 F(1350) = 1.0E-9, F(1510) = 6.2E-10 STAR B0-B2 V-IV J2000 J2000 F(1290) = 1.5E-10, F(1540) = 7.1E-11 STAR B3-B5 V-IV J2000 J2000 F(1350) = 7.4E-11, F(1510) = 6.3E-11 STAR B3-B5 V-IV J2000 J2000 F(1350) = 4.8E-11, F(1510) = 3.6E-11 STAR B0-B2 III-I J2000 J2000 F(1350) = 2.1E-11, F(1510) = 1.6E-11 STIS/FUV-MAMA Spectroscopic No No E140H 1307 STIS/FUV-MAMA Spectroscopic No No E140H 1562 STIS/FUV-MAMA Spectroscopic No No E140H 1307 STIS/FUV-MAMA Spectroscopic No No E140H 1562 STIS/FUV-MAMA Spectroscopic No No E140H 1307 STIS/FUV-MAMA Spectroscopic No No E140H 1562 STIS/FUV-MAMA Spectroscopic No No E140H 1307 STIS/FUV-MAMA Spectroscopic No No E140H 1562 STIS/FUV-MAMA Spectroscopic No No E140H 1307 STIS/FUV-MAMA Spectroscopic No No E140H 1562 STIS/FUV-MAMA Spectroscopic No No E140H 1307 STIS/FUV-MAMA Spectroscopic No No E140H 1562 STIS/FUV-MAMA Spectroscopic No No E140H 1307 STIS/FUV-MAMA Spectroscopic No No E140H 1562 STIS/FUV-MAMA Spectroscopic No No E140H 1307 STIS/FUV-MAMA Spectroscopic No No E140H 1562 1627529360804 C:\Users\Adam\Documents\HST Proposal\Vela\OP-cache\vela-v01.xml The scheduling data was generated by CASM 30.1. 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