HST STIS Echelle Spectral Catalog of Stars

Thomas Ayres, University of Colorado (CASA)


StarCAT is a catalog of high resolution ultraviolet spectra of objects classified as "stars," recorded by Space Telescope Imaging Spectrograph (STIS) during its initial seven years of operations (1997-2004). StarCAT is based on 3184 echelle mode observations of 545 distinct targets, with a total exposure duration of 5.2 Ms. For many of the objects, broad ultraviolet coverage has been achieved by splicing together echellegrams taken in two or more FUV (1150-1700 Å) and/or NUV (1600-3200 Å) settings. In cases of multiple pointings on conspicuously variable sources, spectra were separated into independent epochs. In cases of nonvariable, or lightly variable, objects, different epochs were combined to enhance signal-to-noise (S/N).

Here is the StarCAT Portal. Below is a brief summary of the project. A full description can be found in StarCAT Summary. The author strongly encourages consulting the latter before attempting to use the StarCAT spectra for analysis purposes.

Brief Description

The StarCAT sample was assembled by two crosscutting searches through the HSTonline catalog hosted at MAST. The initial search focussed on objects identified by the Guest Observer as "STAR-∗" for the first broad category keyword. The second search focussed on the second choices of GOs when the first choice was "STAR-∗." (Sometimes, a GO would use these less preferred secondary descriptors as the leading broad category keyword.)

A naming convention was adopted to unify the somewhat diverse target IDs assigned by the GOs, taking in order of priority: HD number, variable star name (including supernovae: SN), WD number (from McCook et al. 2006), and assigning "STARHHMM±DDMM" to all others (equinox/epoch 2000). In one case, "NOIDHHMM±DDMM" was tagged to an observation lacking a SIMBAD object within 20″.

Post-processing began at the level of the calstis pipeline "x1d" file; somewhat a misnomer because it actually is a 2D tabulation of 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 contained at least one — and sometimes several — subexposures, which were treated as separate observations. The initial processing included a post facto correction to compensate for subtle wavelength distortions identified in a recent study of the STIS dispersion relations (the "Deep Lamp Project"); a recalculation of the photometric error (replacing N½ with [N+1]½); and more aggressive edge trimming of E230 modes to avoid unflagged "dropouts" that sometimes occurred at the beginning of the low orders. The x1d orders then were merged, averaging the overlap regions weighted by the individual sensitivity functions s λ, but accounting for bad pixels and wavelength gaps.

Next, a series of different layers of coaddition and splicing were applied to the sets of order-merged 1D spectra of each object.

STAGE ZERO — the subexposures, if any, of an observation were combined. The individual spectra were aligned in velocity by cross-correlation against the first observation. The initial exposure should have the most reliable wavelength scale because it normally would have been taken closest to an acquisition and peak-up (the zero point is dependent on the accuracy of the target centering, which is affected over time by jitter and/or drifts). The subexposures then were interpolated onto the wavelength scale of the initial spectrum and coadded, weighting by exposure time, taking into account bad pixels and gaps. The resulting files are called "o-type" and have the same rootname as the original STIS observation, e.g., o61s01010.

STAGE ONE — the independent exposures of like mode and aperture, all taken in a well defined group within a single "visit," were combined. As in Stage Zero, the spectra were aligned by cross-correlation against the first of the sequence, interpolated onto that scale, then coadded, again weighting by exposure time. The resulting files are called "E-type," e.g., "E140M-1425_020X020_51613." The initial part is the particular mode/tilt (here, "E140M-1425" for the medium-res FUV echelle mode with central wavelength 1425 Å), followed by an aperture code (here, "020X020" for the 0.20″×0.20″ photometric slit), and ending with the start time of the first exposure expressed in integer days as a Modified Julian Date (MJD: J.D. - 2,400,000).

STAGE TWO — like-mode exposures of an object taken in different visits and/or using different apertures were combined. Now, the cross-correlation alignments were done relative to the highest S/N exposure of the group (perhaps a Stage 0 or 1 coadd), and the average shift was subtracted. Also, the coaddition weighted the constituent spectra by a factor closely related to the total net counts at each wavelength, to compensate for the different transmission factors of, say, a narrow versus broad slit. The multi-epoch coadditions were undertaken only if the object displayed minimal variability during its timeline. The resulting files, like Stage 1, are E-type, but with extended aperture codes and/or MJDs to reflect the diversity of the constituent exposures, e.g., "E230M-2707_020X020_020X006_51163-51202" for one extreme example.

STAGE THREE — the available wavelength segments of an object were spliced together, perhaps grouped by epoch if the object displayed noticeable variability. Again, wavelengths of adjacent segments were aligned by cross-correlation. Also, relative flux ratios were measured in the overlap zones to ensure coherent spectral energy distributions for objects with broadly overlapping coverage. The philosophy was to retain at each wavelength the highest resolution fluxes available, and to minimize coadding (in overlap zones) spectra of mixed resolution. The resulting file is called "U-type," e.g., "UVSUM_1M_52755." The "UVSUM" part signals that a multi-wavelength splice was involved; the middle numeral points to 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"); and the trailing date (or dates if a range) indicates the extreme starting MJDs of the spliced group.

Contents of the Catalog

The top-level object list (StarCAT Portal), lists brief stellar characteristics abstracted from SIMBAD, and links to several layers of data tables. It looks like this:


Notes. Stellar data from SIMBAD. Coordinates (α, δ) are equinox/epoch 2000, in degrees, of the HST pointing; V and B-V are in magnitudes; and parallax (π) is in arcseconds. The uncertainty in π typically is ±0.001″. Tildes indicate missing, or uncertain, values.

The first layer down (linked through object name [Column 1]) represents the final spectrum (or spectra) for each object, which might be only a single o-type, say from a GO "SNAPSHOT" program; or a single E-type if only one mode/setting had been utilized, but, say, with several independent o-type exposures; or a full-blown U-type covering all or part of the 1150-3200 Å range, perhaps as a function of epoch for variable targets.

This "final spectra" layer then links down to the constituent spectra in the splice, if any; and each of these in turn links down to whatever grouping, if any, of exposures constituted it. Thus, the lowest layers of a tree always are the o-type exposures; the next one or two layers up are the E-types; and finally the top layer holds the U-types (at best; an o-type at worst; and E-types in intermediate cases).

There are several kinds of processed data. First, the page will display a JPEG preview of the spectrum (or spectra, if more than one at the top level). Also, at the top level, a graphical timeline of all the observations is provided, color-coded by mode. Second, FITS files of the fundamental data are linked and can be downloaded. Finally, for the top-level datasets, an "ETC-ready" ASCII file is linked. It is a highly processed, compacted version of the final spectrum, specifically intended to be used with an HST Exposure Time Calculator (any that adheres to the HST ETC format standard). A description of the compactification procedure and numerous warnings concerning the use of these ETC files can be found in ETC Summary.

Data Formats. A word concerning the FITS file formats. The o-type files have basic header information in the zeroth extension (EXTEN=0) concerning the target, exposure properties, and splice points from the merging process; and one or more trailing data extensions. If the observation consisted of a single exposure, there would be only one data extension — EXTEN=1 — and the spectral parameters would be found there. The quantities stored are WAVE — wavelength (Å); FLUX — flux density (ergs/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 such as bad pixels, camera blemishes, gaps, and so forth). On the other hand, if there were two or more subexposures, EXTEN=1 would hold the Stage Zero coadded spectrum, while the trailing extensions would contain the parameters of the individual subexposures: subexp#1 in EXTEN=2, subexp#2 in EXTEN=3, and so forth. The EXTEN=0 header now also would list the cross-correlation template (a specific spectral feature, say an ISM absorption line, at wavelength λ, and the correlation window Δλ) and the derived velocity shifts.

The E-type (coadded) and U-type (spliced) data files are analogous, and consist of two extensions. EXTEN=0 again summarizes basic information concerning the target and exposure properties, including digests of cross-correlation templates, splice points, and flux scale factors (the latter two applying to the U-types only). EXTEN=1 contains the spectral parameters for the coadded or spliced spectrum. In all cases, including o-types, the most refined dataset always is in EXTEN=1.

Final word. The author would like to express his appreciation to all of the STIS investigators who have indirectly, and unwittingly, contributed to StarCAT. It was fascinating to view the wide diversity of UV spectra available in the MAST archive for the stars, and the often dramatic differences between objects. The fact that such differences exist, and are widespread, is testament to the enormous value of UV spectroscopy to stellar astronomy.

At the same time, the author cautions users of StarCAT that — as with any large scale project like this — there are inevitable errors and omissions that have escaped the albeit numerous checks that the author has conducted. The author would appreciate any feedback along these lines, or any other comments for that matter, so that corrections can be made and/or enhancements undertaken to improve the catalog for future users.

Acknowledgments. Based on observations made with the NASA/ESA Hubble Space Telescope, obtained from the Data Archive at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555. Support for StarCAT was provided by grant HST-AR-10638.01-A from STScI, and grant NAG5-13058 from NASA. The project has made use of public databases hosted by SIMBAD and VizieR, both maintained by CDS, Strasbourg, France.