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An Atlas of Spectrophotometric Landolt Standard Stars
We present CCD observations of 102 Landolt standard stars obtained withthe Ritchey-Chrétien spectrograph on the Cerro TololoInter-American Observatory 1.5 m telescope. Using stellar atmospheremodels, we have extended the flux points to our six spectrophotometricsecondary standards, in both the blue and the red, allowing us toproduce flux-calibrated spectra that span a wavelength range from 3050Å to 1.1 μm. Mean differences between UBVRI spectrophotometrycomputed using Bessell's standard passbands and Landolt's publishedphotometry were determined to be 1% or less. Observers in bothhemispheres will find these spectra useful for flux-calibrating spectra,and through the use of accurately constructed instrumental passbands,will be able to compute accurate corrections to bring instrumentalmagnitudes to any desired standard photometric system (S-corrections).In addition, by combining empirical and modeled spectra of the Sun,Sirius, and Vega, we calculate and compare synthetic photometry toobserved photometry taken from the literature for these three stars.
| Spectral Irradiance Calibration in the Infrared. XIV. The Absolute Calibration of 2MASS
Element by element, we have combined the optical components in the threecameras of the Two Micron All Sky Survey (2MASS), and incorporateddetector quantum efficiency curves and site-specific atmospherictransmissions, to create three relative spectral response curves (RSRs).We provide the absolute 2MASS attributes associated with ``zeromagnitude'' in the JHKs bands so that these RSRs may be usedfor synthetic photometry. The RSRs tie 2MASS to the``Cohen-Walker-Witteborn'' framework of absolute photometry and stellarspectra for the purpose of using 2MASS data to support the developmentof absolute calibrators for the Infrared Array Camera and pairwisecross-calibrators between all three SIRTF instruments. We examine therobustness of these RSRs to changes in water vapor within a night. Wecompare the observed 2MASS magnitudes of 33 stars (converted from theprecision optical calibrators of Landolt and Carter-Meadows intoabsolute infrared calibrators from 1.2 to 35 μm) with ourpredictions, thereby deriving 2MASS ``zero-point offsets'' from theensemble. These offsets are the final ingredients essential to merge2MASS JHKs data with our other absolutely calibrated bandsand stellar spectra, and to support the creation of faint calibrationstars for SIRTF.
| Spectral Irradiance Calibration in the Infrared. XIII. ``Supertemplates'' and On-Orbit Calibrators for the SIRTF Infrared Array Camera
We describe the technique that will be used to develop a set of on-orbitcalibrators for the Infrared Array Camera (IRAC) and demonstrate thevalidity of the method for stars with spectral types either K0-M0 III orA0-A5 V. For application to SIRTF, the approach is intended to operatewith all available optical, near-infrared (NIR), and mid-infrared (MIR)photometry and to yield complete absolute spectra from UV to MIR. Oneset of stars is picked from Landolt's extensive network of optical(UBVRI) calibrators, the other from the Carter-Meadows set of faint IRstandards. Traceability to the ``Cohen-Walker-Witteborn'' framework ofabsolute photometry and stellar spectra is assured. The method is basedon the use of either ``supertemplates,'' which represent the intrinsicshapes of the spectra of K0-M0 III stars from far-UV (1150 Å) toMIR (35 μm) wavelengths, or Kurucz synthetic spectra for A0-A5 Vstars. Each supertemplate or Kurucz model is reddened according to theindividual star's extinction and is normalized using availablecharacterized optical photometry. This paper tests our capability topredict NIR (JHK) magnitudes using supertemplates or models constrainedby Hipparcos/Tycho or precision ground-based optical data. We provideabsolutely calibrated 0.275-35.00 μm spectra of 33 Landolt andCarter-Meadows optical standard stars to demonstrate the viability ofthis technique, and to offer a set of IR calibrators 100-1000 timesfainter than those we have previously published. As an indication ofwhat we can expect for actual IRAC calibration stars, we have calculatedthe absolute uncertainties associated with predicting the IRACmagnitudes for the faintest cool giant and hot dwarf in this new set ofcalibration stars.
| Hipparcos red stars in the HpV_T2 and V I_C systems
For Hipparcos M, S, and C spectral type stars, we provide calibratedinstantaneous (epoch) Cousins V - I color indices using newly derivedHpV_T2 photometry. Three new sets of ground-based Cousins V I data havebeen obtained for more than 170 carbon and red M giants. These datasetsin combination with the published sources of V I photometry served toobtain the calibration curves linking Hipparcos/Tycho Hp-V_T2 with theCousins V - I index. In total, 321 carbon stars and 4464 M- and S-typestars have new V - I indices. The standard error of the mean V - I isabout 0.1 mag or better down to Hp~9 although it deteriorates rapidly atfainter magnitudes. These V - I indices can be used to verify thepublished Hipparcos V - I color indices. Thus, we have identified ahandful of new cases where, instead of the real target, a random fieldstar has been observed. A considerable fraction of the DMSA/C and DMSA/Vsolutions for red stars appear not to be warranted. Most likely suchspurious solutions may originate from usage of a heavily biased color inthe astrometric processing.Based on observations from the Hipparcos astrometric satellite operatedby the European Space Agency (ESA 1997).}\fnmsep\thanks{Table 7 is onlyavailable in electronic form at the CDS via anonymous ftp tocdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/397/997
| V803 Aquilae: A newborn W Ursae Majoris Siamese twin?
A complete photometric analysis of BVRI photometry of the physicallycompact, eclipsing binary V803 Aquilae is presented. Six mean epochs ofminimum light were determined from observations covering three primaryand three secondary eclipses. A period study covering 54 years ofobservation or nearly 77,000 orbital revolutions reveals three distincteras of constant period with two major period jumps of +0.1 s and -0.3s. The light curves shows that the primary and secondary eclipse depthsare identical in V, and are nearly identical in B, R, and I, indicatingthat the components have nearly the same temperatures. Standardmagnitudes were determined and a reddening estimate was made. Asimultaneous solution of the four light curves was computed using theWilson-Devinney synthetic light-curve code. The solution indicates thatthe system consists of twin approximately K4 stars in shallow contactwith a fill-out of approximately 8%. A mass ratio of 1.000 was computedwith a negligible temperature difference of only 6 K. Thus, based on ourpurely photometric solution, V803 Aql is made up of 'Siamese' (contact)twin components. Theory would indicate that the twins have just recentlycome into contact, and the lack of other equal-mass W Ursae Majorissystems would indicate that it is in a very transient or unusual state.
| Photometric investigation of a very short period W UMa-type binary - Does CE Leonis have a large superluminous area?
A complete photometric analysis of BVRI Johnson-Cousins photometry ofthe high northern latitude galactic variable, CE Leo is presented. Theseobservations were taken at Kitt Peak National Observatory on May 31,1989-June 7, 1989. Three new precise epochs of minimum light weredetermined and a linear and a quadratic ephemeris were computed fromthese and previous data covering 28 years of observation. The lightcurves reveal that the system undergoes a brief 20 min totality in theprimary eclipse, indicating that CE Leo is a W UMa W-type binary. Asystemic velocity of about -40 km/s was determined. Standard magnitudeswere found and a simultaneous solution of the B, V, R, I light curveswas computed using the new Wilson-Devinney synthetic light curve codewhich has the capability of automatically adjusting star spots. Thesolution indicates that the system consists of two early K-type dwarfsin marginal contact with a fill-out factor less than 3 percent. Evidencefor the presence of a large (45 deg radius) superluminous area on thecooler component is given.
| UBV(RI)c photometry of equatorial standard stars - A direct comparison between the northern and southern systems
UBV(RI)c photometry of 212 stars from Landolt's list of equatorialstandards is presented. The observations are tied to the system definedby Cousin's E-region standards. A comparison of the present results withLandolt's reveals reasonably good agreement for (V-R)c and (V-I)c, butmarked systematic differences for (B-V) and (U-B). The UBV systems ofCousins and Landolt are evidently not the same and both probably differfrom Johnson's original system.
| UBVRI photometric standard stars around the celestial equator
It is pointed out that accurate, internally consistent, and readilyaccessible standard star photometric sequences are necessary for thecalibration of the intensity and color data which astronomers obtain atthe telescope. The photometric results provided in connection with thepresent study represent the first part of an effort which is concernedwith the presentation of UBVRI photoelectric photometric standard starsin the magnitude range from 7 to 17 over as broad a range in color aspossible. All of the photometric observations were made with a 31034type photomultiplier used in a pulse counting mode. Some 15 to 25standard stars chosen from Cousins' lists (1973, 1976) in the E-regionswere observed with an 0.4-m telescope each night along with the programstars. UBVRI standard stars were observed periodically throughout thenight. Observations with a 0.9-m telescope were also conducted. TheUBVRI photoelectric observations take into account 223 stars.
| Spectral classifications for Landolt's celestial equatorial standard stars
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1979AJ.....84..783D&db_key=AST
| Equatorial UBVRI photoelectric sequences
From 1335 BVRI observations of 189 stars in selected areas 92-115,Landolt's (1973) network of faint UBV standards has been extended to RI. Of these stars, 173 have four or more observations. The (U-B) valuesof Landolt are adopted, and a well-observed equatorial faint-starnetwork is presented on the Johnson UBVRI photometric system.
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