Comparison Stars Research Articles

Algorithm of the artificial comparison star for the CCD photometry

Algorithm of the artificial comparison star for the CCD photometry

The multicolor panoramic photometer-polarimeter with high time resolution based on the PSD

Multicolor panoramic photometer-polarimeter (MPPP) with a time resolution of 1 μs has been built based on a position-sensitive detector (PSD) and used at the 6-m telescope in SAO (Russia). The device allows registration of the photon fluxes in four photometric bands simultaneously and finding values of 3 Stokes parameters. MPPP consists of PSD, acquisition MANIA-system, polarization unit and a set of dichroic filters. MPPP gives a possibility of detecting photons in 2 pupils with a size of 10–15 arcsec centered on the object and comparison star positions simultaneously. The first half of the object photon flux passes through the phase rotating plate and polarizer, and the second one through the polarizer alone. MPPP registers in each of the 4 filters four images of the object with different orientations of polarization plane and one image of a comparison star. It allows measuring instantaneous Stokes parameters. The main astrophysical problems to be solved with MPPP are as follows: investigation of optical pulsars; study of GRB phenomenon in the optical range; searching for single black holes; study of fast variability of X-ray binaries. As an illustration of MPPP use, the results of observations at the 6-m telescope of Crab pulsar and soft gamma repeater are presented.

UBV photometry of the asteroid 44 Nysa

The color curves of Nysa connected to the rotation and phase angle were obtained from observations for the first time in 1982. The color curves showed the U − B change with rotation. The value of this change was 0.2 mag. Color variation indicates that the extended color spot is on the surface of 44 Nysa. The U − B color index decreases with the increase of the phase angle (the phase coefficient is 0.011 mag/deg). This decrease is stronger for the surface with color spots. Phase functions of the brightness for opposite sides of Nysa are different. The value of the opposition effect of 44 Nysa decreases as reflectance decreases. reflector of the Gissar Observatory of Tajikistan at UBV stan- dard spectral bands. Conditions of observation of the asteroid were difficult: the airmass of the asteroid was close to 2, the observations were made at full Moon and there were nights affected by the presence of dust in the Earth's atmosphere. Therefore, comparison stars were selected close to the aster- oid position and they were often compared with photometric standards and among themselves. Observations were carried out with the help of a photometer, working in the regime of the count of photons. Counting time of the signal was 30 s. 2 to 4 measurements of the asteroid's brightness were made between two measurements of a star's brightness. The stability of operation of the photometer was controlled by the sample, the brightness of which was measured for 40 min on average. The accuracy of single measurement of the asteroid's bright- ness was of 0. m 015 in the V band, 0.

A Reevaluation of the Super–LITHIUM‐rich Star in NGC 6633

We present a new abundance analysis of the super-Li-rich star J37 and a comparison star in NGC 6633. We confirm the result of Deliyannis et al. that J37 has a Li abundance well above the meteoritic value, and we also confirm that Al, S, Si, Ca, Fe, and Ni are supersolar and that C is subsolar. We additionally find that Na, Sc, Ti are supersolar, while O is subsolar. The abundance pattern of metals in J37 is generally consistent with the bulk composition of the Earth. We propose that accretion of circumstellar matter is the best explanation for these abundance anomalies, although we cannot rule out a secondary contribution from diffusion.

Asteroseismological studies of three Cephei stars: IL Vel, V433 Car and KZ Mus

We have acquired between 127 and 150 h of time-resolved multicolour photometry for each of the three β Cephei stars IL Vel, V433 Car and KZ Mus over a time-span of 4 months from two observatories. All three objects are multiperiodic with at least three modes of pulsation. Mode identification from the relative colour amplitudes is performed. We obtain unambiguous results for the two highest-amplitude modes of IL Vel (both are l = 1) and the three strongest modes of KZ Mus (l = 2, 0 and 1), but none for V433 Car. Spectroscopy shows the latter star to be a fast rotator (υ sin i = 240 km s - 1 ), whereas the other two have moderate υ sin i (65 and 47 km s - 1 , respectively). We performed model calculations using the Warsaw-New Jersey stellar evolution and pulsation code. We find that IL Vel is an object of about 12 M O . in the second half of its main-sequence evolutionary track. Its two dipole modes are most probably rotationally split components of the mode originating as p 1 on the zero-age main sequence; one of these modes is m = 0. V433 Car is suggested to be an unevolved 13-M O . star just entering the β Cephei instability strip. KZ Mus seems less massive (12.7 M O .) and somewhat more evolved, and its radial mode is probably the fundamental one. In this case its quadrupole mode would be the one originating as g 1 , and its dipole mode would be p 1 . Two of our photometric comparison stars also turned out to be variable. HD 90434 is probably a new slowly pulsating B-type star, the dominant mode of which is a dipole, whereas the variability of HD 89768 seems to be a result of binarity. It is suggested that mode identification of slowly rotating β Cephei stars based on photometric colour amplitudes is reliable; we estimate that a relative accuracy of 3 per cent in the amplitudes is sufficient for unambiguous identifications. Owing to the good agreement of our theoretical and observational results we conclude that the prospects for asteroseismology of multiperiodic slowly rotating β Cephei stars are good.

BVphotometry of UX Ari in the period 1987–2002

We present 14.3 years of previously unpublished photometric observations of UX Ari. The observations were obtained between 1987.9 and 2002.2 using the T3 0.4-metre Automatic Photoelectric Telescope at Fairborn Observatory and consist of 1228 B band and 1213 V band measurements. The comparison star was 62 Ari. We have analyzed the new data together with previously published photometric observations. The V magnitude shows variations with dominant periods of about 12 and 25 years, where the longest period seems to correspond to an activity cycle. The previously reported anticorrelation between the colour and V magnitude variations is confirmed.

Optical variability of the BL Lacertae object GC 0109+224

We present the most continuous data base of optical observations ever published on the BL Lacertae object GC 0109+224, collected mainly by the robotic telescope of the Perugia University Observatory in the period November 1994-February 2002. These observations have been complemented by data from the Torino Observatory, collected in the period July 1995-January 1999, and Mt. Maidanak Observatory (December 2000). GC 0109+224 showed rapid optical variations and six major outbursts were observed at the beginning and end of 1996, in fall 1998, at the beginning and at the end of 2000, and at the beginning of 2002. Fast and large-amplitude drops characterized its flux behaviour. The Rc magnitude ranged from 13.3 (16.16 mJy) to 16.46 (0.8 mJy), with a mean value of 14.9 (3.38 mJy). In the periods where we collected multi-filter observations, we analyzed colour and spectral indexes, and the variability patterns during some flares. The long-term behaviour seems approximatively achromatic, but during some isolated outbursts we found evidence of the typical loop-like hysteresis behaviour, suggesting that rapid optical variability is dominated by non-thermal cooling of a single emitting particle population. We performed also a statistical analysis of the data, through the discrete correlation function (DCF), the structure function (SF), and the Lomb-Scargle periodogram, to identify characteristic times scales, from days to months, in the light curves, and to quantify the mode of variability. We also include the reconstruction of the historical light curve and a photometric calibration of comparison stars, to favour further extensive optical monitoring of this interesting blazar.

The present-day chemical composition of the SMC from UVES spectra of the sharp-lined, B-type dwarf AV 304

High-resolution spectroscopic VLT/UVES observations are presented for the B-type main-sequence star, AV 304, in the Small Magellanic Cloud (SMC). These spectra have been analysed using LTE model-atmosphere techniques, to derive stellar atmospheric parameters and chemical compositions. As AV 304 is located within the hydrogen burning main-sequence band, its chemical composition should reflect that of the SMC interstellar medium (ISM). A detailed line-by-line differential analysis has been undertaken relative to a Galactic comparison star. A general metal deficiency for the a-process elements O, Si & S of -0.43 +/- 0.05 dex is found for AV 304, with iron having a similar underabundance. Oxygen may be relatively over-abundant by similar to0.1 dex and carbon and aluminium underabundant by similar to0.2 dex. A large nitrogen underabundance (of -1.2 dex relative to hydrogen and -0.7 dex relative to iron) is found. This is interpreted in terms of the CNO bi-cycle having been suppressed in the SMC. Furthermore, the large nitrogen deficiency is in excellent agreement with that found for SMC H II regions. Indeed, this represents a first for stellar astrophysics - confirming the low base-line nitrogen composition of the SMC ISM (viz. 12+log(N/H) similar to 6.66 +/- 0.10 dex).

Atmospheric Extinction Corrections for WET Observations

AbstractAccount of the extinction in the Earth’s atmosphere is an important part of the WET light curve reduction procedures. Usually, WET observations are not corrected for the second order extinction effects: the dependence of the extinction coefficient on spectral type and on air mass (the Forbes effect). The ignorance of these effects does not change seriously the derived pulsation amplitudes at the frequencies higher than 200 μHz but the increase of the noise at lower frequencies takes place. For obtaining the true extra-atmospheric magnitudes of stars, a modification of the extinction correction procedure is proposed. For photometry with the R647 phototube we recommend to use a filter cutting out the region below 310 nm, in order to decrease and stabilize the extinction coefficient. A method for estimation of spectral type of the comparison star from WET observations is proposed.

The present-day chemical composition of the LMC

High-resolution observations of five OB-type main-sequence stars in the Large Magellanic Cloud (LMC) have been obtained with the UCLspectrograph on the 3.9-m Anglo-Australian Telescope. These spectra have been analysed using LTE model-atmosphere techniques, to derive stellar atmospheric parameters and chemical compositions. As these stars are located within the hydrogen burning main-sequence band, their surface abundances should reflect those of the present-day interstellar medium. Detailed line-by-line differential analyses have been undertaken relative to Galactic comparison stars. We conclude that there exists a general metal deficiency of −0.31 ± 0.04 dex within the LMC, and find no significant abundance variations between cluster and field stars. There is also tentative evidence to suggest a lower oxygen to iron abundance ratio, and an over-deficiency of magnesium relative to the other α-elements. These are discussed in terms of previous abundance analyses and models of discontinuous (or bursting) star formation within the LMC. Finally, there is some evidence to suggest a greater chemical enrichment of material within the H  region LH104.

Chemical composition of stars in the galactic halo

The chemical compositions of the atmospheres of six metal-poor stars are analyzed. Spectra with signal-to-noise ratios of no less than 100 and a resolution of R≈17 000 were obtained using the 6-m telescope of the Special Astrophysical Observatory of the Russian Academy of Sciences. The abundances of Li, O, α-process elements (Mg, Si, Ca, Ti), Na, K, Sc, iron-peak elements (Cr, Mn, Fe, Ni, Cu, Zn), and s-process elements (Y, Ba) are derived. The star G251-54 ([Fe/H]=−1.55, T eff=5541 K, logg=3.58) is deficient in some elements compared to both stars with similar metallicities and the Sun. The atmosphere of G251-54 has the following elemental abundances relative to iron: [O/Fe]=+0.47, [α/Fe]≈−0.3, [Na/Fe]=−0.60, [Sc/Fe]=−0.57, [Cr, Ni, Fe]≈0, [Zn/Fe]=+0.16, [Cu/Fe]=−0.66, [Y/Fe]=−0.70, and [Ba/Fe]=−1.35. The remaining five stars have metallicities in the range −1.6<[Fe/H]<−1.3 and normal abundances for this metallicity interval and are used as reference stars for comparison with the chemical composition of G251-54. Possible explanations for deviations of the abundances of some elements from the mean relations established for halo objects are discussed.

Luminosity-metallicity relation for stars on the lower main sequence

We present a comparison of the predictions of stellar models with the luminosity of the lower main sequence ($5.5 < M_V < 7.3$) using K dwarfs in the Hipparcos catalog. The parallaxes of our comparison stars are known to better than 15% and metallicities have been recently determined from photometry. A major advantage of our comparison is that distances in our sample are known with good accuracy, while tests that involve open and globular clusters are constrained by potentially inaccurate distances. We show that the luminosity of the lower main sequence relative to a fiducial (solar metallicity) isochrone is a simple function of metallicity: $\Delta M_V = 0.84375 \times {\rm [ Fe/H]} - 0.04577$. We compare the data with a range of isochrones from the literature. None of the models fit all the data, although some models do clearly better than others. In particular, metal rich isochrones seem to be difficult to construct. The relationship between luminosity, colour and metallicity for K dwarfs is found to be very tight. We are thus able to derive metallicities for K dwarfs based on their position in the Hipparcos colour-magnitude diagram with accuracies better than 0.1 dex. The metallicity-luminosity relation for K dwarfs leads to a new distance indicator with a wide range of possible applications.

Five New δ Scuti Stars

ABSTRACTWe present high‐resolution spectroscopy and precision photometry of five new, relatively bright δ Scuti stars. They were originally chosen as photometric comparison stars in our program of automated, high‐precision photometry of solar‐type stars and subsequently recognized as new variable stars. We conducted follow‐up spectroscopic and photometric observations to determine the properties of the stars and their types of variability. All five of the stars presented here belong to the most common subgroup of low‐amplitude, Population I δ Scuti variables. One of the stars, HD 10502, is the third example of a δ Scuti variable with composite broad and narrow spectroscopic line profiles suggesting the presence of a circumstellar shell or disk.

IUE-newsips spectra of σ Geminorum

The IUE spectra of Gem have been taken from the NASA IUE archive using IDL (Interactive Data Language). The spectra have been observed in 1979-1986 period. All the spectra analysed in the present study have NEWSIPS reductions and consist of 25 LWP, 8 LWR and 5 SWP images in high resolution, and 2 LWP and 14 SWP images in low dispersion. The emission lines, identified as activity indicators and originating in the chromosphere and transition region, are seen in the spectra. The emission line fluxes and equivalent widths are computed based on Gaussian profile fitting procedures to examine the existence of any line flux variation that depends on time or orbital phase. It was found that there is a flux variation with time and orbital phase that is in good agreement with the photometric light curve variation. By comparing the flux variation with simultaneous light curve variation, it can be shown that there is a relation between the ultraviolet flux variation and the spot activity of the system, as shown by Ayres et al. (1984) and Engvold et al. (1988) based on their IUE (with IUESIPS reduction) spectral analysis. Moreover, it was inferred that there is no ultraviolet excess in Gem by comparing the spectra of Gem taken as a comparison star. The Mg II h and k radial velocity curves of Gem were in a good agreement with data obtained by Eker (1986) and Duemmler et al. (1997). The sinusoidal Mg II radial velocity curve solutions of the system give e = 0, Porb: = 19:607 0:008 days, K = 34:86 2:33 km s 1 , = 49:42 1:87 km s 1 and To = 2445972:53 0:28. Since the ultraviolet flux data are not conveniently distributed and are insucient to determine the activity cycle, the evaluation of the ultraviolet flux activity cycle was not successful.

The ZZ Ceti red edge

With a time-series CCD photometric survey, we have demonstrated clearly that the observed red edge for the ZZ Ceti stars instability strip at 11 000 K is real, with the pulsation amplitude decreasing at least by a factor of 50. Previous surveys for variability among hydrogen atmosphere white dwarfs around 11 000 K have been carried out using time-series photoelectric photometry, not dierential photometry, insensitive for small amplitude periodicities of 15 min and longer. In our survey we constantly monitor the sky brightness as well as one or more comparison stars through the same color lter, reducing the adverse eects of dierential extinction and sky fluctuations, obtaining true dierential photometry.

Measuring magnetic fields of early-type stars with $\vec{FORS{\it 1}}$ at the $\vec{VLT}$

This paper describes an experiment aimed at evaluating the capability of the FORS1 instrument at the VLT for measuring weak (0.1%) Zeeman circular polarization signatures in stellar H Balmer lines. We have obtained low-resolution polarized spectra at 3500–5800 Å of two bright A-type stars, HD 94660, and HD 96441. The former is a well known magnetic star, the latter an apparently non-magnetic, unpolarized star that was observed for comparison purposes. In order to test the possibility of performing multi-object spectropolarimetric measurements (e.g., for cluster studies), observations were taken both on-axis (i.e., at the CCD field center) and off-axis (i.e., at the edges of the CCD). In HD 94660 (the magnetic star), we detected a clear signal of circular polarization in all observed hydrogen Balmer lines (i.e., from Hβ to the Balmer limit), with a typical peak-to-peak amplitude of about 0.8%. From the analysis of Stokes V under the weak-field approximation, we have estimated a mean longitudinal field of G, a value fully consistent with previous studies of this star. Notably, we found that at wavelengths shorter than about 4100 Å, the polarization signal detected off-axis is about 20–30% smaller than that detected on-axis. No polarization due to the Zeeman effect was detected in the comparison star HD 96441, and we estimate that the contribution due to instrumental circular polarization is limited to the order of . This experiment demonstrates the effectiveness of FORS1 at the VLT as a tool for high-sensitivity diagnosis of the magnetic field in upper main sequence stars, providing the potential for measuring fields in fainter and more rapidly rotating stars than has previously been possible.

GSC 4778-00324, a New Small-Amplitude Multiperiodic δ Scuti Star

A careful analysis of the light curves of comparison stars to the intermediate amplitude δ Scuti star V1162 Ori observed in 1999/2000 within an international campaign (Arentoft et al., 2001) revealed two new variable stars: GSC 4778-00324, a small-amplitude δ Scuti star, and GSC 4778-00001, an eclipsing variable (see http://www.lunarpages.com/bho for a map of the field). Here, we will concentrate on the first of these.Since the colour indices (Handler, 2000) put GSC 4778-00324 right in the middle of the δ Scuti instability strip, we decided to monitor its variability (Van Cauteren &amp; Lampens, 2000) taking as much advantage as possible from the international campaign on V1162 Ori and adopting the same comparison stars. The new observations were performed between October 1999 and March 2001, mainly from the Greek and Belgian sites, making use of a standard V filter. We collected and used 2866 data points for GSC 4778-00324 (V = 10.26; b − y = 0.20).

HD 331319: A post-AGB F supergiant with He I lines

Based on CCD spectra taken with an echelle spectrometer attached to the 6-m telescope, we have determined for the first time the fundamental parameters and detailed chemical composition of HD 331319, an optical counterpart of the infrared source IRAS 19475+3119, by the model-atmosphere method. Helium lines were detected in the spectrum of this luminous (Mυ<−8m object with the effective temperature Teff=7200 K. This detection can be interpreted as a significant helium overabundance in the observed atmospheric layers and may be considered as a manifestation of helium synthesis during the preceding evolution. Nitrogen and oxygen were found to be overabundant, [N/Fe]⊙=+1.30 dex and [O/Fe]⊙=+0.64 dex, with the carbon overabundance being modest. The metallicity of the stellar atmosphere, [Fe/H]⊙=+0.25, differs only slightly from its solar value. The s-process metals are not overabundant but most likely underabundant relative to iron: [X/Fe]⊙=−0.68 for Y and Zr. Barium is also underabundant relative to iron: [Ba/Fe]⊙=−0.47. The heavier elements La, Ce, Nd, and Eu are slightly enhanced relative to iron: the mean [X/Fe]⊙=−0.16 for them. In general, the elemental abundances confirm that IRAS 19475+3119 is a post-AGB object. The metallicity in combination with the radial velocity Vr=−3.4 km s−1 and Galactic latitude \(\left| b \right| = 2_.^ \circ 7\) of the object suggest that it belongs to the Galactic disk population. The envelope expansion velocity, Vexp≈21 km s−1, was determined from the positions of the absorption bands that originate in the circumstellar envelope. A comparison of our results for the comparison star HD 161796=IRAS 17436+5003, a typical post-AGB object, with previously published data revealed an evolutionary increase in the effective temperature of HD 161796 at a mean rate of ≥50° per year.

Hydroxyl 1.563 Micron Absorption from Starspots on Active Stars

We present results from a study of starspots on active stars using a pair of vibrational-rotational absorption lines of the OH molecule near 1.563 μm. We detect excess OH absorption due to dark, cool starspots on several active stars of the RS CVn and BY Dra classes. Our results for the single-lined spectroscopic binaries II Pegasi, V1762 Cygni, and λ Andromedae augment those from a previous study that used a less sensitive detector. In this study, we were able for the first time to use molecular absorption features to measure starspot properties on double-lined spectroscopic binaries. Measuring the equivalent widths of these OH lines in inactive giant and dwarf stars of spectral types G, K, and M, we find that the total equivalent width of the line pair increases approximately linearly as effective temperature decreases from 5000 to 3000 K. We measure starspot filling factors by fitting the spectra of active stars with linear combinations of comparison star spectra representing the spot and nonspot regions of the star.

Study of FK Comae Berenices

We present 8 years of previously unpublished photometric observations of FK Com together with the determination of the stability of the primary comparison star HD 117567. The observations have been carried out between 1993 and 2001 at four different observatories and they consist of 5157 data points in total: U(903), B(994), V(1643), R(166), Ic(573), b(461) and y(417). We also analyse this new data together with the previously published photometric observations. The V magnitude shows variations with dominant periods of about 3, 6, 12, 14 and 31 years. The short-term light curve variations appear to be caused by rearrangement of approximately constant amount of cool spots. From the values for different colours obtained during the brightest season observed, corresponding to the supposedly unspotted surface, the spectral type of FK Com is determined to be G7 III.