STScI has developed a Bright Object Tool (BOT) to facilitate field checking prior to COS program implementation. The BOT is implemented within APT (the
Astronomer’s Proposal Tool), using the Aladin interface, and reads target and exposure information from the proposal. Help files and training movies are available within
APT. The BOT is based on displays of the Digital Sky Survey (DSS) and on automated analysis of the field using data from two catalogs: the second Guide Star Catalog (GSC2) and the
GALEX catalog.
Automated GALEX screening is now available within the APT/BOT. The AIS (all-sky) sources are screened as unreddened O5 stars and reported as either safe, unsafe, or unknown. Because it is based directly on UV fluxes,
GALEX screening can reveal, for example, previously unknown hot companions to late-type stars. If the field passes the BOT check, it is safe; unsafe and unknown objects require further investigation.
Caveats: (1) The
GALEX catalog does not cover the whole sky, so PIs must check that their COS field is fully covered by
GALEX. (2)
GALEX fluxes represent upper limits in crowded regions because of the instrument’s relatively low spatial resolution. (3) The
GALEX detectors suffer local non-linear effects at high count rates. The fluxes and magnitudes in the current version of the
GALEX catalog are not corrected for these effects and may be underestimated for the brightest stars. A preliminary correction is presented in Morrissey et al. (2007, ApJS, 173, 682). The BOT now applies this correction to the
GALEX catalog. As a result, it may report
GALEX magnitudes that are brighter than those given in the
GALEX catalog itself.
In general, a COS pointing with unconstrained telescope orientation requires the clearance of a field 43 arcsec in diameter. If any displacements from the default pointing (e.g., acquisition scans, POS TARGs, patterns, or mosaics) are specified, then the field to be cleared increases commensurately. Because both the PSA and BOA are exposed to the sky at all times, no unsafe or unknown star may fall within 7 arcsec of either aperture. The BOT automatically accounts for the reduced throughput of the BOA.
An existing UV spectrogram of the target or class may be imported directly into the ETC. When using
IUE data as input spectra in the
ETC, consider only low-resolution spectra taken through the large aperture. Note that the
ETC does not convolve imported spectra to the COS resolution. To be conservative, one must assume that the entire flux of an emission line falls within a single COS resolution element.
If model spectra are used in the ETC, the original Kurucz (not Castelli & Kurucz) models should be used for early-type stars. None of the provided models is adequate for late-type stars since the models lack chromospheric emission lines. Actual UV data must be used for late-type stars when possible. If a given star has only a V magnitude, it must be treated as an unreddened O5 star. If one color is available it may be processed as a reddened O5 (which will always have a greater UV flux than an unreddened star of the same color).
If two colors are available, then the spectral type and reddening can be estimated separately. In some cases, the 2MASS JHK may be the only photometry available for an otherwise unknown star. The ETC supports direct entry of observed J and H magnitudes with E(B−V). It is also possible to estimate V and E(B−V) from those data on the assumption of a reddened O5 star, and thus determine its count rates in the
ETC. Martins & Plez (2006, A&A, 457, 637) derive (J−H)
0 = −0.11 for all O stars, and (V−J)
0 = −0.67 and (V−H)
0 = −0.79 for early O types. (The K band should be avoided for BOP purposes because of various instrumental and astrophysical complications.) Bessell & Brett (1988, PASP, 100, 1134), Appendix B, give relationships between the NIR reddenings and E(B−V).
