ASTRAL is a Hubble Space Telescope (HST) Large Treasury Project, whose aim was to collect high-quality ultraviolet spectra of representative bright stars utilizing the high-performance Space Telescope Imaging Spectrograph. In Cycle 18 (2010–2011), ASTRAL focused on eight iconic late-type stars, devoting 146 HST orbits to the purpose. In Cycle 21 (2013–2015), the program shifted gears to the warm side of the H-R diagram, to capture 21 diverse early-type objects with an allocation of 230 orbits. Main objective was to record the targets — including well-known bright stars like Procyon, Betelgeuse, Sirius, and Vega — with broad uninterrupted UV coverage (1150–3100 Å) at the highest signal-to-noise and highest echelle spectral resolution achievable within the alloted orbits and observing constraints. These UV "atlases" have enormous interpretive value in their own right, and complement efforts from ground-based observatories, which now routinely achieve comparably high resolution and S/N in optical and near-infrared spectra of bright stars.
Broad ultraviolet coverage was achieved by splicing together echellegrams taken in multiple FUV (1150–1700 Å) and NUV (1600–3100 Å) grating settings of STIS. The observing strategy was designed to maximize S/N by spreading the total desired exposure depth in each setting over 2–5 separate "visits" of up to four orbits each, in a series of independent relatively short integrations (1–3 kiloseconds duration). Because of a slight randomness in the STIS grating positioning mechanism, and because of the changing projected velocity of the target (owing to telluric and spacecraft motions), each independent exposure will shift slightly on the detector and thus experience a different "fixed pattern noise." Combining the independent spectra after the fact mitigates these systematics, improving S/N. In addition, the observing sequences were designed to have at least one exposure of each distinct setting immediately after a target-centering "peak-up" so that the velocity zero point of the echellegram, which can be affected by thermal drifts, would be as accurate as possible. Other exposures of that type can be registered to the reference observation by cross-correlation. Furthermore — if practical, depending on target brightness — a few exposures in each sequence were taken through the photometric aperture (0.2"×0.2"), and again close to a peak-up, to ensure that the radiometric scale of the final spliced spectrum would be as close to the true absolute level as practical.
The post-processing of the ASTRAL spectra follows protocols developed for the earlier StarCAT project, an extensive catalog of STIS echelle spectra of objects classified as "stars." A full description can be found in Ayres (2010: ApJS 187, 149). The author strongly encourages consulting the latter as a general introduction before attempting to use the ASTRAL spectra for analysis purposes. At the same time, the ASTRAL experience has uncovered a number of observation-related issues, which have required modifications and additions to the original protocols.
Post-processing begins at the level of the calstis pipeline "x1d" file, a tabulation of extracted wavelengths, flux densities, photometric errors, and data quality flags for the up to several dozen orders of the particular grating setting (e.g., E140M-1425, where the first part is the mode and the second is central wavelength in Å). The x1d file contains at least one — sometimes several — sub-exposures, which were treated as separate observations. The initial processing includes a post facto correction for subtle wavelength distortions identified in a previous study of the STIS dispersion relations ("Deep Lamp Project"); a Bayesian reformulation of the photometric error; and more aggressive edge trimming of E230 settings to avoid unflagged "dropouts" that sometimes occur at the beginning of the low orders. Also, spurious tilts of the spectra, due to an unfortunate interaction between narrow slits and the telescope "breathing," are corrected, if necessary. The x1d orders then are merged, averaging the overlap regions weighted by the individual sensitivity functions s λ, but accounting for bad pixels and wavelength gaps. During this process, an "active blaze correction" determines an optimum blaze shift to balance the fluxes in the overlaps between adjacent orders.
Next, a series of different layers of coaddition and splicing are applied to the sets of order-merged 1D spectra of each object:
STAGE ZERO — sub-exposures, if any, of an observation are combined. The individual spectra are aligned in velocity by cross-correlating against the observation with the highest apparent throughput, determined by comparing the total net count rates integrated over the central zone of the echelle format. With the narrow slits used extensively in ASTRAL Hot-Stars, the effective throughput can vary significantly (up to ~30%) during a single orbit owing to telescope breathing. Next, the sub-exposures are interpolated onto the wavelength scale of the reference spectrum; scaled to the reference exposure in flux density according to the total net count rates; then coadded, weighting by the total net counts, but taking into account bad pixels and gaps. Resulting files are called "o-type" and have the same rootname as the original observation, e.g., obkk52040.
STAGE ONE/TWO — Same-setting exposures of an object taken in different orbits of a visit, or in different visits, are combined. This is a hybrid of the Stages ONE and TWO described in StarCAT, to take advantage of the specially designed observing strategy of ASTRAL. As in Stage ZERO, the cross-correlation alignments are relative to the exposure exhibiting the highest apparent throughput. Also, the individual observations are scaled to the reference exposure according to flux ratios determined by a global average over high S/N intervals. Again, the weighting was by the total net counts, which allows for possibly different integration times in an exposure set (or use of different apertures in different visits). Resulting files are called "E-type," and have an appended aperture code and MJD date range to reflect the diversity of the constituent exposures, e.g., "E140M-1425_020X020_55543-55554."
STAGE THREE — Different wavelength segments of an object are spliced together to produce a seamless spectrum covering the full FUV + NUV range. As in the other steps, wavelengths of adjacent segments are aligned by cross-correlation. A "bootstrapping" calibration procedure takes advantage of the intentional broadly overlapping spectral coverage to refine the velocity zero point and the absolute flux scale (by pair-wise comparisons of the overlap regions in velocity and flux). Resulting file is called "U-type," e.g., "UVSUM_1X_55543-55554." The "UVSUM" part signals that a multi-wavelength splice was involved; the middle numeral indicates a particular grouping of spectra that were spliced; the adjacent letter tells whether the spectra all were medium resolution ("M"), all were high resolution ("H"), or mixed ("X"; latter is the case for all but one of the ASTRAL Cool Stars targets, but several of the Hot Stars are purely M and a few are purely H); and the trailing date range summarizes the minimum and maximum starting MJDs of the spliced group.
In the splicing procedure, the general philosophy was to minimize overlap regions between observations of different resolutions, to extent possible given desire to include enough overlap to determine accurate flux ratios, as well as at least one suitable cross-correlation feature. Exception was made for frequent combination E140H-1291 + E140M-1425, to boost S/N in key FUV interval below 1350 Å. In this instance, entire overlap zone (1150–1350 Å) was co-added. In all cases of mixed resolution, higher resolution spectrum was filtered with lower resolution line-spread-function, and vice versa. Thus, spectral resolution in mixed resolution overlap zone is convolution of the two lsf's, and is lower than lower of the two. Photometric errors were adjusted for the filtering, and co-addition weighting was according to smoothed version of respective inverse squared errors (in flux density units). Dual-filtering procedure avoids awkward lsf resulting from simply adding two mixed-resolution overlaps (as was done in StarCAT). New parameter "RESOL" (λ/Δλ) was added to U-type FITS files to keep track of places where mixed-resolution spectra were spliced. Finally, for Cool Stars targets, an E230M-2707 taken for flux calibration purposes was spliced only over small interval at longwavelength end of merged NUV H-res spectrum.
NOV 2018: datasets reprocessed (mid-2017) with new CALSTIS reference files (created by author). Improvements made to post-processing protocols (2018). New data product for sub-exposures.
NOV 2018: added two STIS Calibration White Dwarfs (G191-B2B and BD+28°4211).
JUN 2017: added four metal-poor turn-off stars from GO-14161 (Peterson, PI); and sunlike α Cen A & B from various historical and current STIS programs.
As of 2015 January, the STIS observations for the Hot Stars episode of ASTRAL had been successfully completed.
If highest resolution is desired, say for ISM studies, avoid top-level spliced (U-type) spectrum below 1350 Å, and instead consult E140H-1291 co-add one level down (if available).
There remain spurious defects in STIS spectra, especially noticeable in Hot Stars targets with long stretches of smooth continuum. Most appear as weak, narrow "emission lines," mainly in NUV. They apparently result from flat-field issues. Occasionally defects occur in FUV where local areas of sensitivity degradation on detector were not flagged: these appear as broad (few Å) shallow absorptions. Also, new flag (EPSILON=450) marks places between adjacent echelle settings where difference between co-added fluxes was larger than 3 times average photometric noise. Flag introduced to help identify possible processing glitches.
Because of necessity to de-tilt and rescale many sub-exposures, owing to "breathing" effects, ASTRAL Hot Stars material not well suited for temporal studies of continuum variations (0.2″×0.2″ Cool Stars o-type exposures should be better in time-domain regard). Also, fact that exposures of each setting were scaled to observation with maximum throughput could introduce bias, if source had been variable over span of STIS visits. Thus, absolute fluxes of ASTRAL spectra should be viewed cautiously; although relative flux distributions should be reliable.
Top-level object tables, below, summarize brief stellar characteristics, mostly taken from SIMBAD, and link to several layers of processed data. RE-LOAD PAGE FOR MOST UP-TO-DATE DATA (Note CALSTIS processing date).
First layer down (linked through object HD number [Column 1]) contains the final spectrum for each target, a U-type covering the full 1150–3100 Å range.
This "final spectrum" layer then links down to the constituent spectra in the splice; and each of these in turn points to the grouping of exposures and sub-exposures that constituted it. Thus, the lowest layers of a tree are sub-exposures; next layer up are co-added o-type exposures; the next layer up has the setting-specific E-type co-added spectra; and finally the top layer holds the broad-coverage U-type(s).
There are several kinds of processed data. First, each page displays a PNG preview of the coadded (o-type or E-type) or spliced (U-type) spectrum. Second, at the top level, a graphical timeline of all the observations is provided, color-coded by mode. Third, FITS files of the fundamental data are linked for downloading. Fourth, for the top-level datasets, an "ETC-ready" ASCII file is available. It is a highly streamlined version of the final spectrum, specifically intended to be used with an HST Exposure Time Calculator (or any that adheres to the HST ETC format standard), but NOT SUITABLE FOR ANALYSIS (see: ETC Summary). A description of the streamlining procedure and numerous warnings concerning the use of these ETC files can be found in ETC Summary. Finally, links to flat ASCII versions of all the coadded data files can be found here.
Fits File Data Formats. The "o-type" files (individual observations) have basic header information in 0-th extension describing target, exposure properties, and splice points from order-merging; and one or more trailing data extensions. If observation had only single sub-exposure, there is one extension (EXTEN=1), containing WAVE – wavelength (Å); FLUX – flux density (erg/cm²/s/Å); ERROR – photomeric error (same units as flux density); and DQ – data quality (0 for no issues; higher values to flag various conditions: bad pixels, camera blemishes, gaps, and so forth). If two or more sub-exposures, EXTEN=1 holds Stage ZERO co-added spectrum; subsequent extensions have parameters of individual sub-exposures: sub#n in EXTEN=n+1. EXTEN=0 header now contains additional information, concerning cross-correlation template and derived velocity shifts for Stage ZERO co-addition.
"E-type" co-added and "U-type" spliced spectra have similar FITS structure to single-exposure o-type, consisting of just two extensions. 0-th extension again lists basic information concerning target and exposure properties; and summaries of cross-correlation templates, velocity shifts, splice points (for U-types), and flux scale factors. Extension 1 contains spectral parameters for co-added and/or spliced spectrum. In all cases, including o-types, most refined dataset always is EXTEN=1.
ASTRAL Cool Stars. A few words concerning the eight late-type targets. They cover a wide range of spectral type, F-M, mainly giants and supergiants. All have been observed by IUE, and all previously by HST, usually GHRS but two by STIS. However, especially for GHRS, the spectral coverage was incomplete, the resolution was not as high as the STIS H modes, or the S/N did not meet our goals. To round out the original ASTRAL Cool Stars group, the nearby, bright cool dwarfs Alpha Centauri A (G2 V) and B (K1 V) were added (from an ongoing joint Chandra/HST program to track the coronal activity cycles). Key characteristics of the targets:Beta Cassiopeia (Caph: F2 IV)— Most extreme "X-ray deficient" case in a group of already anomalous fast-spinning Hertzsprung gap giants. These stars display powerful FUV emissions, but surprisingly underluminous X-ray coronae. Beta Cas, like Procyon (see next), falls at the edge of convection: an essential ingredient (together with rotation, which Beta Cas has in abundance, but Procyon does not) for the dynamo generation of magnetic fields, with their consequent effects on high-energy processes in the stellar outer atmosphere. Important link in magnetic evolution of Hertzsprung gap giants: soft 1 MK corona versus hard 10 MK for the later G0 IIIs. Alpha Canis Minoris A (Procyon: F5 IV-V)— Nearby, bright, warmer analog of the low-activity Sun. Important cool-corona object (2 MK). Chandra transmission grating spectrum mid-way between solar-like Alpha Cen A and its more active K-type companion Alpha Cen B. Alpha Centauri (Rigel Kentaurus) A (G2 V) and B (K1 V)— Nearest sunlike stars, only 1.3 pc away. Binary system with 80 year period and 17.5" semi-major axis, although only ~4" apparent separation in current epoch (ca. 2015). Resolved X-ray sources with Chandra, even at closest orbital approach (2016). Alpha Cen A is a near twin of Sun in its fundamental properties, including age and coronal activity; B is smaller, cooler, less luminous, but coronally more active. Important comparisons to more evolved stars of original Cool Stars sample. Alpha Aquarii (Sadalmelik: G2 Ib)— "Hybrid chromosphere" supergiant in the class originally discovered by L. Hartmann and colleagues in early 1980s: harboring cool massive wind, imprinting blueshifted circumstellar absorptions on Mg II; but also displaying hot FUV lines like C IV, a combination usually avoided in the "noncoronal" giants like Arcturus (Alpha Boo: K2 III) and Aldebaran (Alpha Tau: K5 III). Weak coronal X-ray source detected by Chandra. Important comparison to its sibling, Beta Aqr (G0 Ib), another certified hybrid star, previously observed by STIS (and Chandra). Beta Draconis (Rastaban: G2 Iab)— Yellow supergiants Beta Dra and Alpha Aqr, although superficially similar in spectral type and luminosity, are strikingly different at high energies: former is a strong X-ray source with bright FUV emissions; latter is cool-wind dominated star with suppressed FUV emissions and barely detected corona. A pivotal pair for understanding the dichotomy between coronally active and quiet supergiants. Beta Geminorum (Pollux: K0 IIIb)— Early-K giant with solar-like coronal properties; key comparison to the noncoronal red giants mentioned above. Important contrast as well to the equally puzzling class of super-active helium-core-burning "clump giants" like Iota Cap (G8 III) and Beta Cet (K0 III), previously observed by STIS. One of few giant stars with suspected planetary companion as well as detected weak magnetic field. Gamma Draconis (Etamin: K5 III)— Another hybrid chromosphere star, showing weaker fluoresced molecular lines (CO and H2) than archetype red giant Arcturus, latter extensively observed by STIS. Faint Chandra source, but stronger than the 3 events detected from Arcturus in a 19 kilosecond HRC-I pointing. Important link to more active hybrid stars like Alpha Aqr (above). Gamma Crucis (Gacrux: M3.5 III)— Classic M giant crucial for understanding complex atmosphere, wind, and spectrum of more exotic red supergiant Betelgeuse (next): similar in surface temperature, equally extreme nocoronal object, but with a simpler, cleaner UV spectrum (e.g., narrower, less blended chromospheric and wind emission lines), and yet significant mass outflow. Bridge to the warmer noncoronal K giants like Gamma Dra. Alpha Orionis (Betelgeuse: M2 Iab)— Iconic windy cool supergiant, with very clumpy surface convection and mysterious distant cold circumstellar shell, prominent in FUV absorptions of CO (as seen by GHRS at low resolution). An extreme object in terms of low surface temperature, high visual luminosity, and lack of coronal signatures. Again, an important player in the story of the hot-corona/cool-wind transition.
ASTRAL Hot Stars. The twenty-one targets of Hot Stars are too numerous to describe individually. Some brief characteristics are noted in the Hot Stars Table. The specific objects were chosen by the ASTRAL collaborators, after significant debate, to include: the full range of spectral types from early-O to early-A; Main sequence and evolved stars; normal plus chemically peculiar subtypes; fast and slow rotators; magnetic exotica; as well as nearby objects of relevance to ISM studies. A practical consideration was that early-type stars are UV-bright, and in many cases can trigger overlight conditions on STIS's highly sensitive MAMA cameras. While there is a pair of neutral density filters in the STIS slit wheel for bright targets, the initial ND step is 2 (100x attenuation relative to the normal clear slits), which means that many targets that are just over the bright limits will be very inefficient to observe with the supported ND2 aperture (or companion ND3). To work around this limitation, we carried out a calibration program in Cycle 19 ("Bridging STIS's Neutral Density Desert") to validate a set of three 31"×0.05" intermediate-ND slits (ND=0.6, 1.0, and 1.3) that were "available but not supported." This involved measurements of a hot WD to pin down the wavelength-dependent attenuation of each slit; and a spectral comparison between short ND2 (0.2"×0.05") and tall ND1.3 (31"×0.05") using the bright, relatively sharp-lined A0 star Vega (Alpha Lyrae) to determine whether the long slit degraded resolution or enhanced scattered light (neither turned out to be an issue). The addition of the intermediate-ND slits to the STIS toolkit greatly increased the number of potential objects that could be observed efficiently (albeit still requiring 6-12 orbits per target) to satisfy the objective of high S/N. This made it easier for the ASTRAL collaborators to select a list of candidates to meet the purely scientifically-motivated objectives. The case of Vega was deemed of special importance (zero point of photometric scale; ultra-fast rotator seen pole-on; possible Chandra X-ray source), even though it fell partly into the inefficient category. Vega accordingly was allocated additional orbits (20 total) to boost S/N for some of the underperforming echelle settings.
Metal Deficient Turn-Off Stars. Four metal-poor turn-off stars from GO-14161 (R. Peterson, PI) were added to ASTRAL, as examples of low-metallicity objects in contrast to more normal compositions of main sample. These (relatively faint) F–G dwarfs were recorded exclusively in the 5 NUV prime high-res settings with CENWAVE>2000 Å. About a dozen orbits, spread over several visits, were devoted to each target. Unfortunately, these metal-poor stars were too faint for FUV echelles. Nevertheless, NUV co-added, spliced spectra are of high quality.
Calibration White Dwarfs. Several hot White Dwarfs are HST standard stars. Most important for STIS are G191-B2B (photometric calibration, order-dependent blaze correction curves) and BD+28°4211 (long-term sensitivity changes, slit throughputs). G191-B2B was observed in all 44 STIS echelle settings near beginning of STIS operations, then again shortly after Servicing Mission 4 (2009, when STIS was restored to operating condition after long hibernation following electrical failure in 2004). BD+28°4211 has been recorded on regular basis (every few months) during STIS operations since beginning, in three primary medium-res echelle settings (M-1425, M-1978, M-2707), and two prime high-res settings (H-1416, H-2263). Co-adding and splicing these extensive data collections yield spectra of unusually high S/N and quality. However, because ASTRAL-modified CALSTIS was based mainly on post-SM4 calibration material, in some cases pre-SM4 echellegrams did not process well. Consequently, G191-B2B and BD+28°4211 datasets with processing defects were culled out. As result, not all of original spectra were included in co-added, spliced WD spectra here, but nevertheless final tracings are very high quality, exceeding most other ASTRAL stars.
Final word. The author would like to express appreciation to all the ASTRAL Co-Investigators who have contributed to the project, especially to STIS colleagues at STScI who have helped materially in planning and executing the complex observing program. In a number of cases, the ASTRAL Co-Is are obtaining supporting ground-based observations, which ultimately will be linked here when available.
Acknowledgments. Based on observations made with NASA/ESA Hubble Space Telescope, obtained from the Mikulsky Archive at Space Telescope Science Institute, operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555. Support for ASTRAL is provided by grants HST-GO-12278.01-A and HST-GO-13346.01-A from STScI. The project has made use of public databases hosted by SIMBAD, maintained by CDS, Strasbourg, France.