Mag Star Research Articles

Observation of the First Gravitational Microlensing Event in a Sparse Stellar Field: The Tago Event

We report the observation of the first gravitational microlensing event in a sparse stellar field, involving the brightest (V = 11.4 mag) and closest (~1 kpc) source star to date. This event was discovered by an amateur astronomer, A. Tago, on 2006 October 31 as a transient brightening, by ~4.5 mag during a ~15 day period, of a normal A-type star (GSC 3656-1328) in the Cassiopeia constellation. Analysis of both spectroscopic observations and the light curve indicates that this event was caused by gravitational microlensing rather than an intrinsically variable star. Discovery of this single event over a 30 year period is roughly consistent with the expected microlensing rate for the whole sky down to V = 12 mag stars. However, the probability for finding events with such a high magnification (~50) is much smaller, by a factor of ~1/50, which implies that the true event rate may be higher than expected. This discovery indicates the potential of all sky variability surveys, employing frequent sampling by telescopes with small apertures and wide fields of view, for finding such rare transient events, and using the observations to explore Galactic disk structure and search for exoplanets.

Astrometric precision of observations at VLT/FORS2

In this paper we test the astrometric precision of VLT/FORS2 observations using a series of CCD frames taken in the Galactic bulge area. A special reduction method based on symmetrization of reference fields was used to reduce the atmospheric image motion. The positional precision of unsaturated mag star images at a 17 s exposure and 0.55″ seeing was found to be equal to 300 μas. The total error of observations was decomposed into components. It was shown that astrometric error depends mainly on the photon centroiding error of the target (250 μas for 16 mag stars), while the image motion was much less (110 μas). At galactic latitudes to about 20°, precision for a series of frames with a 10 min total exposure was estimated to be 30–50 μas for 14–16 mag stars, providing the images were not overexposed and the filter Rspecial was used. Error estimates for fields with smaller sky star density are given. We conclude that astrometric observations with large telescopes, under optimal reduction, are never atmospheric limited. The bias caused by differential chromatic refraction and the residual chromatism of LADC are considered and expressions that are valid for correcting color effects in the measured positions are given.

First Attempt at Spectroscopic Detection of Gravity Modes in the Long‐Period Pulsating Subdwarf B Star PG 1627+017

The first spectroscopic campaign on a PG 1716 variable (long-period pulsating subdwarf B star) has successfully detected low-level velocity variations due to g-mode pulsations in PG 1627+017, superimposed on the known orbital motion. The strongest velocity mode is barely detectable at 1.0-1.5 km s-1, although the target is one of the brightest (V = 12.9) and largest amplitude (~0.03 mag) stars in its class. Forty nights of observations on 2 m class telescopes in Arizona, South Africa, and Australia provided 84 hr of time-series spectroscopy over a time baseline of 53 days, with typical velocity errors of 5-6 km s-1. The derived radial velocity amplitude spectrum, after subtracting the orbital component, shows three potential pulsational modes 3-4 σ above the mean noise level of 0.365 km s-1, at 7201.0 s (138.87 μHz), 7014.6 s (142.56 μHz), and 7037.3 s (142.10 μHz). Only one feature is statistically likely to be real, but all three are tantalizingly close to, or a 1 day alias of, the three strongest periodicities found in the concurrent photometric campaign. The velocity spectrum also shows an unexpected component at twice the orbital frequency of PG 1627+017, possibly evidence of a slightly elliptical orbit, supporting Edelmann et al.'s recent results for other short-period sdB binaries. We further attempted to detect pulsational variations in the Balmer line amplitudes. The single detected periodicity of 7209 s, although weak, is consistent with theoretical expectations as a function of wavelength, and it rules out a degree index of l = 3 or 5 for that mode. Given the extreme weakness of g-mode pulsations in PG1716 stars, we conclude that future efforts will require larger telescopes, higher efficiency spectral monitoring over longer time baselines, improved longitude coverage, and increased radial velocity precision.

The Kyiv Meridian Axial Circle catalogue of stars in fields with extragalactic radio sources

A catalogue of astrometric (positions, proper motions) and photometric ( B , V , R , r' , J ) data of stars in fields with ICRF objects has been compiled at the Observatory of the National Academy of Sciences of Ukraine and the Kyiv University Observatory. All fields are located in the declination zone from 0° to +30°; the nominal field size is 46 ' (right ascension$)\times 24'$ (declination). The observational basis of this work is 1100 CCD scans down to $V=17$ mag which were obtained with the Kyiv meridian axial circle in 2001–2003. The catalogue is presented in two versions. The version KMAC1-T contains 159 fields (104 796 stars) and was obtained with reduction to the Tycho2 catalogue. For another 33 fields, due to a low sky density of Tycho2 stars, the reduction was found to be unreliable. Transformation to the ICRF system in the second version of the catalogue (KMAC1-CU) was performed using the UCAC2 and CMC13 catalogues as a reference; it contains 115 032 stars in 192 fields and is of slightly better accuracy. The external accuracy of one catalogue position is about 50–90 mas for $V<15$ mag stars. The average error of photometry is better than 0.1 mag for stars down to 16 mag.

Discovery of an X-ray binary in the outer SMC wing

The discovery of an X-ray binary RX J0209.6−7427 in the outer wing of the SMC is reported from two archival ROSAT PSPC observations. The data show variability in the X-ray light curve with a timescale of ∼40 days that probably is related to the binary orbital period of the system. A V = 14 mag star in the X-ray error circle of the source is the likely optical counterpart. The optical spectrum of the star shows Hα emission with an equivalent width of −10.8 ± 0.2 A. The spectral type of the star is constrained as B0-B1.5IV-Ve and consistent with that of a Be star. RX J0209.6−7427 is therefore a Be-type X-ray binary. The mean X-ray luminosity of the source is derived to be ∼1.0 × 10 38 erg s −1 (for a distance of 60 kpc), which is comparable to luminosities derived for Be-type X-ray binaries in the body of the SMC. This makes RX J0209.6−7427 the first candidate Be-type X-ray binary discovered in the outer SMC wing that directly extends into the Magellanic Bridge. This finding is consistent with the existence of a young (∼10-30 Myr) stellar population in the outer region of the SMC wing. Star formation due to an encounter of the SMC with the LMC and the Magellanic system with the Galaxy may explain this likely high-mass X-ray binary in an area far from the main body of the SMC.

The Structure of Pluto's Atmosphere from the 2002 August 21 Stellar Occultation

We have observed the 2002 August 21 occultation by Pluto of theR ¼ 15:7mag star P131.1, using 0.5 s cadence observations in integrated white light with the Williams College frame-transfer, rapid-readout CCD at the 2.24 m University of Hawaii telescope. We detected an occultation that lasted 5 minutes, 9:1 � 0:7 s between half-light points. The ‘‘kinks’’ in the ingress and egress parts of the curve that were apparent in 1988 had become much less pronounced by the time of the two 2002 occultations that were observed, indicating a major change in the structure ofPluto’satmosphere.AnalysisofourlightcurvesshowsthatthepressureinPluto’satmospherehasincreasedatall the altitudes that we probed. Essentially, the entire pressure scale has moved up in altitude, increasing by a factor of 2 since 1988. Spikes in our light curve reveal vertical structure in Pluto’s atmosphere at unprecedentedly high resolution.Wehaveconfirmationofourspikesatlowertimeresolutionaspartofobservationsoftheemersionmade at 1.4 s and 2.4 s cadence with the 3.67 m AEOS telescope on Maui.

Photometry of a Sodium Laser Guide Star at the Starfire Optical Range

ABSTRACTMeasurements of a laser‐pumped sodium guide star produced over the Starfire Optical Range in 2002 November show that the brightness of the spot produced by 11.5 W of linearly polarized power on the sky was equivalent to a V = 8.0 mag star. However, taking into account that the transmission through a V filter is only 55% at the wavelength of sodium, its corrected magnitude, V1, was 7.4, or 800 photons s−1 cm−2 at the top of the telescope. In 2003 March, tests with linearly and then circularly polarized beams out of the telescope showed that a circularly polarized beam from 12 W of power out of the telescope produced a spot with V1 = 7.1 (1015 photons s−1 cm−2 at the top of the telescope), 0.7 mag brighter than a linearly polarized beam from 11.1 W of power out of the telescope. Over the 4 nights of experiments over two seasons, the apparent 2σ width of the spot varied between 36 and 46, or 1.6 and 2.0 m at 92 km altitude, and its length through the sodium layer was 4.6–8.5 km, but no variation of spot size with power on the sky was seen.

Two Active Nuclei in 3C 294

The z = 1.786 radio galaxy 3C 294 lies < 10'' from a 12 mag star and has been the target of at least three previous investigations using adaptive optics (AO) imaging. A major problem in interpreting these results is the uncertainty in the precise alignment of the radio structure with the H- or K-band AO imaging. Here we report observations of the position of the AO guide star with the Hubble Space Telescope Fine Guidance Sensor, which, together with positions from the second United States Naval Observatory's CCD Astrograph Catalog (UCAC2), allow us to register the infrared and radio frames to an accuracy of better than 01. The result is that the nuclear compact radio source is not coincident with the brightest discrete object in the AO image, an essentially unresolved source on the eastern side of the light distribution, as Quirrenbach and coworkers had suggested. Instead, the radio source is centered about 09 to the west of this object, on one of the two apparently real peaks in a region of diffuse emission. Nevertheless, the conclusion of Quirrenbach and coworkers that 3C 294 involves an ongoing merger appears to be correct: analysis of a recent deep Chandra image of 3C 294 obtained from the archive shows that the nucleus comprises two X-ray sources, which are coincident with the radio nucleus and the eastern stellar object. The X-ray/optical flux ratio of the latter makes it extremely unlikely that it is a foreground Galactic star.

Photometry of the Globular Cluster NGC 3201 and Its Variable Stars

We present time series BVI photometry of the brighter stars in the halo globular cluster NGC 3201. We derive high-quality light curves for 58 RR Lyrae stars, all of which appear to be members of the cluster. We determine the foreground reddening of each RR Lyrae star from the star's color and confirm that the reddening varies significantly across the face of the cluster. We obtain a mean value of E(B-V) = 0.25 ± 0.02 mag for stars within 20 of the cluster center. Using the reddening values of the individual RR Lyrae stars, we made dereddened color-magnitude diagrams that reach from the red giant branch tip to ~2 mag below the main-sequence turnoff. Analysis of these diagrams indicates that NGC 3201 has an age of 13–14 Gyr, typical of other globular clusters of its metallicity. We detect low-level brightness variations in most of the cluster's brighter red giant stars and tentatively associate this variation with the small-amplitude red variables seen in the field. We also discovered several δ Scuti variable candidates, two of which appear to be cluster members. The distances derived from the δ Scuti and RR Lyrae stars are in good agreement, indicating d = 4.7 ± 0.2 kpc.

General relativistic satellite astrometry

The non-perturbative general relativistic approach to global astrometry introduced by de Felice et al. ([CITE]) is here extended to account for the star motions on the Schwarzschild celestial sphere. A new expression of the observables, i.e. angular distances among stars, is provided, which takes into account the effects of parallax and proper motions. This dynamical model is then tested on an end-to-end simulation of the global astrometry mission GAIA. The results confirm the findings of our earlier work, which applied to the case of a static (angular coordinates only) sphere. In particular, measurements of large arcs among stars (each measurement good to ~$100 \mu$arcsec, as expected for $V\sim17$ mag stars) repeated over an observing period comparable to the mission lifetime foreseen for GAIA, can be modeled to yield estimates of positions, parallaxes, and annual proper motions good to ~$15 \mu$arcsec. This second round of experiments confirms, within the limitations of the simulation and the assumptions of the current relativistic model, that the space-born global astrometry initiated with Hipparcos can be pushed down to the 10 -5 arcsec accuracy level proposed with the GAIA mission. Finally, the simplified case we have solved can be used as reference for testing the limiting behavior of more realistic models as they become available.

On the detection of the first extragalactic classical chemically peculiar stars

We report on the detection of the first extragalactic, classical chemically peculiar (CP) stars more than 100 years after the discovery of this group. Using the tool of CCD -photometry, eight definite positive detections in the globular-like stellar cluster NGC 1866 and its surrounding in the Large Magellanic Cloud were made. More than 1950 objects within a radius of 9.5′ of the cluster center down to 20.5 magnitude were observed in this three filter, narrow band photometric system. Averaging more than 70 individual frames for all three filters yielded a 3σ detection limit of 0.015 mag for stars with mag. Furthermore we found three objects with significantly negative -values indicating either an Ae/shell or λ Bootis nature. The overall percentage of chemically peculiar stars for NGC 1866 (1.5% ) seems lower than for the majority of open clusters in our Milky Way whereas the incidence within the LMC field is decisively less (0.3% ).

The recovery of the old nova RS Car (1895)

Low-resolution spectra, taken at La Silla (ESO), identify the old nova RS Car (1895) as a ∼18 mag star located 7 arcsec southwest from the previous published position. This suggests a much brighter absolute magnitude of the old nova, Mv∼5.4, than previously suspected. The spectrum reveals a continuum energy distribution typical of optically thick accretion discs and quite a high excitation state of the gas. The possible detection of the AlO λ4843 emission band is discussed.

Discovery and Photometric Observation of the Optical Counterpart of a Possible Galactic Halo X-Ray Transient, XTE J1118+480

Abstract We discovered an optical counterpart of about 13 mag of a soft X-ray transient, XTE J1118+480 on 2000 March 30. We performed astrometry and provided an accurate position of R.A. = 11h18m10s.85, Decl. = +48°02′12″.9. The outbursting object has been identified with a 18.8 mag star in the USNO catalog. Our pre-discovery data shows another outburst during 2000 January, again coinciding with an outburst detected in X-rays. Through CCD time-series photometry, we found the presence of a periodic variation with an amplitude of 0.055 mag and a period of 0.17078 ± 0.00004 d, which we consider to be a promising candidate of the orbital period. Because of the high galactic latitude and faint quiescence magnitude of 18.8, XTE J1118+480 is possibly the first firmly identified black hole candidate X-ray transient in the galactic halo.

Calibration of the MACHO Photometry Database

ABSTRACTThe MACHO Project is a microlensing survey that monitors the brightnesses of ∼60 million stars in the Large Magellanic Cloud (LMC), Small Magellanic Cloud, and Galactic bulge. Our database presently contains about 80 billion photometric measurements, a significant fraction of all astronomical photometry. We describe the calibration of MACHO two‐color photometry and transformation to the standard Kron‐Cousins V and R system. Calibrated MACHO photometry may be properly compared with all other observations on the Kron‐Cousins standard system, enhancing the astrophysical value of these data. For ∼9 million stars in the LMC bar, independent photometric measurements of ∼20,000 stars with V≲18 mag in field‐overlap regions demonstrate an internal precision σV = 0.021, σR = 0.019, σV - R = 0.028 mag. The accuracy of the zero point in this calibration is estimated to be ±0.035 mag for stars with colors in the range −0.1 mag<V−R<1.2 mag. A comparison of calibrated MACHO photometry with published photometric sequences and new Hubble Space Telescope observations shows agreement. The current calibration zero‐point uncertainty for the remainder of the MACHO photometry database is estimated to be ±0.10 mag in V or R and ±0.04 mag in V−R. We describe the first application of calibrated MACHO data: the construction of a color‐magnitude diagram used to calculate our experimental sensitivity for detecting microlensing in the LMC.

The ESO Precise RV Survey for Extra-solar Planets: Results from the First Five Years

AbstractResults are presented from the first five years of the high precision RV survey carried out with the 1.4m CAT+CES spectrograph at ESO La Silla. This RV survey of 37 solar-type stars was begun in Nov. 1992. Using an iodine gas absorption cell for self-calibration we currently achieve a long-term precision of 20 m/s in a 30-min exposure of a 5.5 mag star. This value is the typical ‘working’ precision in survey work, i.e. an average over all observing conditions.We present the RV data for our most variable star, ι Hor (G0V), with a possible periodicity of 600 d that could imply a companion of minimum mass m sin i = 2.0 MJup. However, these data are severely complicated by stellar activity to which a large part of the variability must be attributed. Our RV data for ϕ2 Pav (F8V) contain a possible period of 43 d which would imply a planet with m sin i = 0.69 MJup. It must be stressed that for both stars the significance of the periods is still insufficient to conclude that planets are indeed orbiting them. We also show our data for a Hipparcos astrometric brown dwarf candidate, GJ 570A, that do not confirm expectations for a correspondingly high RV signal. Longer monitoring of the star is required in order to confirm a possible low amplitude variation.

Arops : The ASIAGO Red Objective-Prism Survey

AROPS is an objective-prism survey conducted at high galactic latitudes with the 67/92 cm (f=205 cm) Schmidt telescope of the Asiago Astrophysical Observatory (Italy). The basic technical data are: – TP4415 high resolution plates, 20×20 cm, treated in forming gas (H2+N2) and covering 24 deg2 on sky– 4° UBK7 objective prism, for a dispersion of 1490 Åmm−1 at NaI D– blue side cut-off at 4980 Å using a 2mm GG14 filter, for a recorded λ range 5000–7000 Å– untrailed spectra to record V ∼ 16.5 mag stars with 40 min exposures

On Flamsteed's supernova Cas A

The Galactic supernova remnant Cas A is believed to be the result of a recent (∼ 300 yrs) and nearby (D ∼ 3 kpc) explosion of a massive star. The discovery of gamma-ray line emission due to radioactive 44Ti with COMPTEL supports the massive star origin of Cas A. However, the ejection of 44Ti should be accompanied by large amounts of 56Ni and 57Ni, which would have made this a very bright supernova. No 17th century sightings of this event exist, with the notable exception of the ∼ 6th mag star 3 Cas reported by Sir John Flamsteed. This mystery may be solved by assuming that the supernova was enshrouded by a dusty envelope, perhaps provided by a pre-supernova wind. X-ray observations with ROSAT provide evidence for this scenario through the simultaneous analysis of absorption along the line of sight and the extended X-ray scattering halo around Cas A. The supernova shock destroyed the dust, reducing the line-of-sight opacity to a value consistent with a 3 kpc path through an average ISM. ROSAT detects the evaporated dust as excess absorption.

Photometry of GX 349+2: Evidence for a 22-Hour Period

view Abstract Citations (25) References (28) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Photometry of GX 349+2: Evidence for a 22-Hour Period Wachter, Stefanie ; Margon, Bruce Abstract The intense galactic x-ray source GX 349+2 (Sco X-2) belongs to the class of persistently bright low-mass x-ray binaries called Z sources. GX 349+2, although observed in x rays for more than 30 years, has only recently been optically identified with a 19th mag star. Of the six known Z sources, only two (Sco X-i and Cyg X-2) have been studied in the optical. It has been suggested that Z sources as a group are characterized by evolved companions and correspondingly long orbital periods (Sco X-i, P=0.8 d; Cyg X-2, P =9.8 d). Recently Southwell et al. (1996) have presented spectroscopic observations of GX 349+2 suggesting a 14 d orbital period. We have obtained broadband photometry of the system on six consecutive nights, and find a statistically significant 21. .4 h (3σ) period of 0.14 mag half amplitude, superposed on erratic flickering typical of Sco X- 1 type objects. As with other Z sources, caution will be needed to insure that the variations are truly periodic, and not simply due to chaotic variability observed over a relatively short time span. Depending on the origin of the brightness variations, our proposed period could be either the orbital or half the orbital period. If our period is confirmed, then the nature of the 14 d spectroscopic variation found by Southwell et al. (1996) is unclear. There is evidence that the mass function of GX 349+2 is similar to that of Sco X-1. Publication: The Astronomical Journal Pub Date: December 1996 DOI: 10.1086/118212 arXiv: arXiv:astro-ph/9608120 Bibcode: 1996AJ....112.2684W Keywords: X-RAYS: STARS; BINARIES:; STARS: INDIVIDUAL: GX 349+2; Astrophysics E-Print: 17 pages including 3 tables and 5 figures full text sources arXiv | ADS | data products SIMBAD (14)

The TAUVEX space astronomy experiment

The TAUVEX space astronomy experiment will image wide sky areas in the 190–280 nm spectral region. The experiment is part of the SODART telescope complex on the Spectrum X-Gamma (SRG) spacecraft, and functions as a separate scientific instrument and service system for the spacecraft. TAUVEX consists of three bore-sighted telescopes with 20 cm diameter Ritchey-Chretien optics. Each telescope has a four-position filter wheel and can select one of six UV bands in the spectral region of operation. The photon-counting, imaging detectors cover a field of view of 0.9 degrees, with 80% of the energy from a point source within 10 arcsec, sampled at 3 arcsec intervals. The sensitivity allows the detection of 10–11 mag stars in 2 sec, and 20 mag stars in a typical SRG pointing of 5 hr. TAUVEX provides off-line aspect solutions for the SODART focal plane instruments and on-line fine pointing information to the SRG attitude and control system. The experiment is being constructed by El-Op Electro-Optical Industries Ltd, and is financially supported by the Government of Israel, through the Israel Space Agency, and the Ministry of Science and Arts. By mid-1994, four models of TAUVEX had been produced and supplied to the SRG integrators: size and mass models in 1992, a thermal model in early 1993, and an engineering model in spring 1994. A qualification model is being tested at El-Op and the flight model will be ready, after testing, burn-in and calibration, in the first quarter of 1997.

Globular Cluster Photometry With the Hubble Space Telescope. V. WFPC Study of M15's Central density Cusp

We describe images of the center of the dense globular cluster M15 (NGC 7078) obtained with the Hubble Space Telescope Wide Field and Planetary Camera 2 (WFPC2). Data taken in the F336W, F439W, and F555W filters (approximately U, B, and V) are used to study the surface density distribution of the ∼3×10<SUP>4</SUP> stars detected in a 5 arcmin<SUP>2</SUP> region within r&lt;2' (6.7 pc) of the cluster center. Realistic simulated images have been used to estimate photometric errors and incompleteness in the star counts, which are strong functions of stellar brightness and radius. We have used a combination of point-spread-function fitting and aperture photometry, a technique that yields more accurate photometry than either method alone on the undersampled WFPC2 images of crowded star fields. The error in photometry is 1σ∼0.05 mag for stars with V&lt;18; this increases to 1σ∼0.2 mag at V=20.5, which is 1.5 mag fainter than the main-sequence turnoff. The surface density of stars in M15 (after correction for the effects of incompleteness and photometric bias/scatter) is well represented by a power law in radius: N(r)∼r<SUP>-0.82±0.12,</SUP>, over the radial range 0:3 (0.017 pc) to 6". The observed power law is remarkably similar to what is expected if the center of the cluster harbors a massive black hole. Nonparametric estimates of the density profile show a monotonic rise with decreasing radius all the way in to r=0".3, the smallest radius at which the density can be reliably measured; there is no indication that the profile flattens at smaller radii. Any flat core of radius larger than 2" (0.11 pc) in the stellar distribution is ruled out at the ≳9S% significance level. The star count profile is consistent with that expected from core-collapse models or with the predicted distribution around a massive (few times 10<SUP>3</SUP> M<SUB>sun</SUB>) black hole. The close triplet of bright stars, AC 214, is within 0".5 (1.5σ) of the cluster centroid position. The projected density distribution of stars within the central 15" of M15 departs from circular symmetry at the 95% level, with an ellipticity e=0.05±0.04 (90% confidence limits) at a position angle of +60°±25°, consistent with the rotation measured by Gebhardt et al. (1995) in this region of the cluster.