0 Assigned ID: 252 ----- Attempting Submission 1 (Thu Apr 07 18:07:14 GMT 2016) ----- HST Phase I Proposal 252 (tbrown_ocen.apt) successfully submitted. Receipt: # 252-1 0 ----- Attempting Submission 1 (Thu Jul 14 16:18:26 GMT 2016) ----- What Happens in the Atmospheres of Hot Horizontal Branch Stars Near 20,000K? In the color-magnitude diagrams (CMDs) of many globular clusters, the horizontal branch (HB) exhibits a long blue tail extending to high effective temperatures. In such clusters, two discontinuities appear within the HB locus. The first discontinuity occurs at ~12,000K, and was discovered by Grundahl et al. (1998). It is associated with the radiative levitation of metals and the gravitational settling of helium in the atmospheres of HB stars hotter than 12,000K. The hot subdwarf stars of the Galactic field population exhibit the same phenomenon. The second discontinuity occurs at ~20,000K, and was discovered by Momany et al. (2002). Its origin is unknown, but it appears at the same effective temperature in all globular clusters hosting HB stars near 20,000K, regardless of cluster properties (age, chemical composition, mass, etc.). We propose STIS long-slit spectroscopy of 6 HB stars that straddle this feature in the HB distribution of omega Cen, the nearest globular cluster where the feature is well populated. With this approach, we can efficiently obtain high-quality UV and blue spectra that span the full wavelength range of the photometric bands where this CMD feature is most prominent - a range this is only accessible by HST. The resulting spectra will unambiguously reveal the nature of this phenomenon - one that is universal in the atmospheres of hot evolved stars - and will yield new insight into the role of diffusion and radiative levitation in these stars. Our observations are straightforward, and similar to previous programs that also obtained UV spectroscopy of hot stars in the core of Galactic globular clusters with parallel broad- band imaging in the outskirts (e.g., GO-11665). We will obtain long-slit near-UV (NUV/G230L) and blue (CCD/G430L) STIS spectroscopy of omega Cen using the 52x0.5" slit, which will be truncated at a length of 25" in the NUV observations. Each slit position will center a pair of EHB stars along the slit axis, with dithering along the slit to remove detector artifacts. Each pair will include one star falling immediately to the blue of the M-jump and one star falling immediately to the red of the M-jump. Although each slit position has its own specific orientation, the Visit Planner shows ample schedulability at a distinct time of year for each position. The three 2-orbit visits are independent of each other, and 2-orbit visits present no scheduling challenges (unlike 4- or 5-orbit visits). We will offset to these three slit positions after an acquisition on an isolated bright star further from the cluster center. The offsets can be determined to high accuracy using extant HST imaging of the cluster. The extensive UV imaging also ensures the field is safe to observe, providing UV brightnesses and colors for all objects in the vicinity of our slit positions. The brightest star in the vicinity of each NUV/G230L observation is noted in the comments for each visit. The photometric catalogs of omega Cen enable accurate determination of the spectral quality we would obtain in these observations. For 5 of our targets, we will obtain a signal-to-noise ratio of ~30 per resolution element in the near-UV (where the metal absorption lines are most prominent), and ~100 per resolution element in the optical (for the Balmer lines and break). For the faintest target, these numbers will be 20 and 80, respectively. Although the field is crowded, the 52x0.5" slit is sufficiently narrow to exclude objects in the dispersion direction that would contaminate our spectra. These spectra would unambiguously characterize the spectral features responsible for the M-jump, given that the M-jump manifests itself as a ~10% change in a broad-band photometric color index. In parallel to the STIS spectroscopy, we will obtain WFC3 photometry in the same 3 filters of the UV Treasury survey of Galactic globular clusters (GO-13297; F275W, F336W, and F438W). The dithering of the prime STIS observations will only be in one axis, so the offsets will not be sufficient for resampling of the point spread function in these images, but they will enable removal of detector artifacts. In each visit, we perform a target acquisition on a relatively isolated star, and then offset to the site of the spectroscopic observations. The offset must not incur a change in guide stars, in order to place the targets accurately in the long STIS slit. If the offset employed in any of the three visits cannot be achieved on a single set of guide stars, due to the size of the offset, we can search for acquisition stars tailored to be nearby each of the three slit positions, but we would prefer to use the same acquisition star for all three visits. Once the offset executes in a given visit, we will obtain an image through the slit, prior to the spectroscopy, to characterize source placement within the slit and inform the data reduction. Stellar Populations 12 false true false false false false AVAILABLE STELLAR CLUSTER GLOBULAR CLUSTER The coordinates above fall midway between the pair of stars. Coordinates come from HLA image, but shifted to agree with position of acquisition star in HSC, 2MASS, and GSC, which all agree to within a few WFC3/UVIS pixels in the HLA image. In the native HLA image (WFC3/F275W or WFC3/F814W), the position would be 13 26 36.8237 -47 28 27.02. This can be used to see the slit placement in Aladin. The photometric information above is for the fainter of the two stars in the slit (specifically, the southwestern star in the pair), as determined in the F275W HLA image of the cluster. The other star in the pair has the following photometry: V=16.6 m275 = 15.4 vegamag J2000 J2000 m275=15.6 vegamag STELLAR CLUSTER GLOBULAR CLUSTER The coordinates above fall midway between the pair of stars. Coordinates come from HLA image, but shifted to agree with position of acquisition star in HSC, 2MASS, and GSC, which all agree to within a few WFC3/UVIS pixels in the HLA image. In the native HLA image (WFC3/F275W or WFC3/F814W), the position would be 13 26 48.6637 -47 30 35.68. This can be used to see the slit placement in Aladin. The photometric information above is for the fainter of the two stars in the slit (specifically, the northwestern star in the pair), as determined in the F275W HLA image of the cluster. The other star in the pair has the following photometry: V=16.6 m275 = 15.4 vegamag J2000 J2000 m275 = 16.3 vegamag STELLAR CLUSTER GLOBULAR CLUSTER The coordinates above fall midway between the pair of stars. Coordinates come from HLA image, but shifted to agree with position of acquisition star in HSC, 2MASS, and GSC, which all agree to within a few WFC3/UVIS pixels in the HLA image. In the native HLA image (WFC3/F275W or WFC3/F814W), the position would be 13 26 47.7087 -47 29 07.27. This can be used to see the slit placement in Aladin. The photometric information above is for the fainter of the two stars in the slit (specifically, the northeastern star in the pair), as determined in the F275W HLA image of the cluster. The other star in the pair has the following photometry: V=16.8 m275 = 15.6 vegamag J2000 J2000 m275=15.7 vegamag STAR HORIZONTAL BRANCH STAR NO V-magnitude comes from inspection of HLA WFPC2/F555W image. Aperture photometry on this image implies V=13.05 mag. HSC says m555=13.55 mag. Anywhere in this range will be fine for acquisition, regardless of spectral type (M to O), providing SNR>40 and well below saturation. Coordinates come from HLA image, but shifted to agree with position of acquisition star in HSC, 2MASS, and GSC, which all agree to within a few WFC3/UVIS pixels in the HLA image. In the native HLA image (WFC3/F275W or WFC3/F814W), the position would be 13 26 45.5506 -47 29 17.53. This can be used to see the ACQ placement in Aladin. J2000 STIS/NUV-MAMA Spectroscopic No No G230L 2376 STIS/CCD Spectroscopic No No G430L 4300 STIS/NUV-MAMA Spectroscopic No No G230L 2376 STIS/CCD Spectroscopic No No G430L 4300 STIS/NUV-MAMA Spectroscopic No No G230L 2376 STIS/CCD Spectroscopic No No G430L 4300 WFC3/UVIS Imaging No No F275W WFC3/UVIS Imaging No No F336W WFC3/UVIS Imaging No No F438W WFC3/UVIS Imaging No No F275W WFC3/UVIS Imaging No No F336W WFC3/UVIS Imaging No No F438W WFC3/UVIS Imaging No No F275W WFC3/UVIS Imaging No No F336W WFC3/UVIS Imaging No No F438W /Users/tbrown/hstprops/cycle24/ocen/phase2/OP-cache/14759-v01.xml The scheduling data was generated by CASM 24.2. CASM was run in Phase II mode. Brightest star in F275W within 40 arcsec radius of the slit center in the NUV images is at RA = 13:26:35.5395, Dec = -47:28:29.998. It has m275=13.9 vegamag. Although this star should not fall within the slit, if it did, it would be safe to observe in NUV/G230L. (Tic-Visit :proposal-id 14759 :visit-id "01" :proposal-version -1 :trans-version "20160601" :date "08-Jul-2016 11:12:31.0" :trans-errors '(:WARNINGS 0 :ERRORS 0 :HEALTH-AND-SAFETY 0 :FATAL-ERRORS 0 :CONTINUABLE-FATALS 0) :number-of-orbits 2 :saa-model "29" :si-bright-earth-limb-angle 20.0 :data-volume 2068512 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 14759 :visit-id "01" :obset-id "01" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 14759 :visit-id "01" :orbit-number 1 :pcs-time 333 :si-vis-utilization 2980 :al-overhead-at-end 13 :internal-dur-at-end 58 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3352 :pcs-gap 0 ) (Tic-Orbit :proposal-id 14759 :visit-id "01" :orbit-number 2 :pcs-time 222 :si-vis-utilization 3099 :al-overhead-at-end 104 :internal-dur-at-end 1062 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3352 :pcs-gap 0 ) true 2 0 Image through the slit to help characterize position of sources relative to slit midline. /Users/tbrown/hstprops/cycle24/ocen/phase2/OP-cache/14759-v02.xml The scheduling data was generated by CASM 24.2. CASM was run in Phase II mode. Brightest star in F275W within 40 arcsec radius of the slit center in the NUV images is at RA = 13:26:50.4875, Dec = -47:30:15.640. It has m275=14.1 vegamag. Although this star should not fall within the slit, if it did, it would be safe to observe in NUV/G230L. (Tic-Visit :proposal-id 14759 :visit-id "02" :proposal-version -1 :trans-version "20160601" :date "08-Jul-2016 11:12:33.0" :trans-errors '(:WARNINGS 0 :ERRORS 0 :HEALTH-AND-SAFETY 0 :FATAL-ERRORS 0 :CONTINUABLE-FATALS 0) :number-of-orbits 2 :saa-model "29" :si-bright-earth-limb-angle 20.0 :data-volume 2068512 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 14759 :visit-id "02" :obset-id "02" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 14759 :visit-id "02" :orbit-number 1 :pcs-time 333 :si-vis-utilization 2976 :al-overhead-at-end 13 :internal-dur-at-end 58 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3352 :pcs-gap 0 ) (Tic-Orbit :proposal-id 14759 :visit-id "02" :orbit-number 2 :pcs-time 222 :si-vis-utilization 3099 :al-overhead-at-end 104 :internal-dur-at-end 1062 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3352 :pcs-gap 0 ) true 2 0 Image through the slit to help characterize position of sources relative to slit midline. /Users/tbrown/hstprops/cycle24/ocen/phase2/OP-cache/14759-v03.xml The scheduling data was generated by CASM 24.2. CASM was run in Phase II mode. Brightest star in F275W within 40 arcsec radius of the slit center in the NUV images is at RA = 13:26:48.5958, Dec = -47:28:59.782. It has m275=14.8 vegamag. Although this star should not fall within the slit, if it did, it would be safe to observe in NUV/G230L. (Tic-Visit :proposal-id 14759 :visit-id "03" :proposal-version -1 :trans-version "20160601" :date "08-Jul-2016 11:12:36.0" :trans-errors '(:WARNINGS 0 :ERRORS 0 :HEALTH-AND-SAFETY 0 :FATAL-ERRORS 0 :CONTINUABLE-FATALS 0) :number-of-orbits 2 :saa-model "29" :si-bright-earth-limb-angle 20.0 :data-volume 2068512 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 14759 :visit-id "03" :obset-id "03" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 14759 :visit-id "03" :orbit-number 1 :pcs-time 333 :si-vis-utilization 2957 :al-overhead-at-end 13 :internal-dur-at-end 58 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3352 :pcs-gap 0 ) (Tic-Orbit :proposal-id 14759 :visit-id "03" :orbit-number 2 :pcs-time 222 :si-vis-utilization 3099 :al-overhead-at-end 104 :internal-dur-at-end 1062 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3352 :pcs-gap 0 ) true 2 0 Image through the slit to help characterize position of sources relative to slit midline.