arXiv subjects

C. Alard

Publications and source records attributed to C. Alard.

44 records · Page 3Linked to original sources

arXiv1999

Variable Stars in the Globular Cluster M5. Application of the Image Subtraction Method

We present $V$-band light curves of 61 variables from the core of the globular cluster M5 obtained using a newly developed image subtraction method (ISM). Four of these variables were previously unknown. Only 26 variables were found in the same field using photometry obtained with DoPHOT software. Fourier parameters of the ISM light curves have relative errors up to 20 times smaller than parameters measured from DoPHOT photometry. We conclude that the new method is very promising for searching for variable stars in the cores of the globular clusters and gives very accurate relative photometry with quality comparable to photometry obtained by HST. We also show that the variable V104 is not an eclipsing star as has been suggested, but is an RRc star showing non-radial pulsations.

astro-ph↗

arXiv1999

The Optical Gravitational Lensing Experiment. Monitoring of QSO 2237+0305

We present results from 2 years of monitoring of Huchra's lens (QSO 2237+0305) with the 1.3 m Warsaw telescope on Las Campanas, Chile. Photometry in the V band was done using a newly developed method for image subtraction. Reliable subtraction without Fourier division removes all complexities associated with the presence of a bright lensing galaxy. With positions of lensed images adopted from HST measurements it is relatively easy to fit the variable part of the flux in this system, as opposed to modeling of the underlying galaxy. For the first time we observed smooth light variation over a period of a few months, which can be naturally attributed to microlensing. We also describe automated software capable of real time analysis of the images of QSO 2237+0305. It is expected that starting from the next observing season in 1999 an alert system will be implemented for high amplification events (HAE) in this object. Time sampling and photometric accuracy achieved should be sufficient for early detection of caustic crossings.

astro-ph↗

arXiv1999

ISOGAL Survey of Baade's Windows in the Mid-infrared

The ISOGAL mid-infrared survey of areas close to the Galactic Plane aims to determine their stellar content and its possible bearing on the history of the Galaxy. The NGC6522 and Sgr I Baade's Windows of low obscuration towards the inner parts of the Bulge represent ideal places in which to calibrate and understand the ISOGAL colour-magnitude diagrams. The survey observations were made with the ISOCAM instrument of the ISO satellite. The filter bands chosen were LW2(~7 microns) and LW3(~15 microns). The results presented here show that most of the detected objects are late M-type giants on the AGB, with a cut-off for those earlier than M3-M4. The most luminous members of these two Bulge fields at 7 microns are the Mira variables. However, it is evident that they represent the end of a sequence of increasing 15 micron dust emission which commences with M giants of earlier sub-type. In observations of late-type giants the ISOCAM 15 micron band is mainly sensitive to the cool silicate or aluminate dust shells which overwhelm the photospheric emission. However, in ordinary M-giant stars, the 7 micron band is not strongly affected by dust emission and may be influenced instead by absorption. The nu2 band of water at 6.25 microns and the SiO fundamental at 7.9 microns are likely contribitors to this effect. A group of late M stars has been found which vary little or not at all but have infrared colours typical of well-developed dust shells. Their luminosities are similar to those of 200-300 day Miras but they have slightly redder [7]-[15] colours which form an extension of the ordinary M giant sequence. The Mira dust shells show a mid-infrared [7]-[15] colour-period relation. ca 700 days.

astro-ph↗

arXiv1999

Image subtraction with non-constant kernel solutions

It is demonstrated that non-constant kernel solution, that can fit the spatial variations of the kernel can be obtained with minimum computing time. The CPU cost required with this new extension of the image subtraction method is almost the same as for fitting constant kernel solution. The method is demonstrated with a serie of Monte-Carlo images. Differential PSF variations and differential rotation between the images are simulated. It is shown that this new method is able to achieve an optimal results even in these difficult cases. It is clear that the most frequents instrumental problems are automatically corrected by the method. It is also demonstrated that the method does not suffer problems with under-sampling of the images. To conclude the method is applied to real images of crowded field. It is proved that much larger sub-areas of the images can be used for the fit, while keeping the same accuracy in the subtracted image. This result is especially important in case of variables located in low density fields, like the Huchra lens. Many other useful applications of the method are possible for major astrophysical problems. For instance supernovas search should benefit the method, but other applications like Cepheids surveys in other galaxies should also find some advantage in using the method. Many other applications will certainly show-up, since variability search is a major issue in astronomy.

astro-ph↗

arXiv1998

Analysis of the OGLE microlensing candidates using the image subtraction method

The light curves of the OGLE microlensing candidates have been reconstructed using the image subtraction method. A large improvement of the photometric accuracy has been found in comparison with previous processing of the data with DoPHOT. On the mean, the residuals to the fit of a microlensing light curve are improved by a factor of 2 for baseline data points, and by a factor of 2.5 during magnification. The largest improvement was found for the OGLE #5 event, where we get an accuracy 7.5 times better than with DoPHOT. Despite some defects in the old CCD used during the OGLE I experiment we obtain most of the time errors that are only 30 % to 40 % in excess of the photon noise. Previous experiment showed that with modern CCD chips (OGLE II), residuals much closer to the photon noise were obtained. The better photometric quality enabled us to find a low amplitude, long term variability in the OGLE #12 and OGLE #11 baseline magnitude. We also found that the shape of the OGLE #14 candidate light curve is fairly inconsistent with microlensing of a point source by a point lens. A dramatic change in the light curve of the OGLE #9 candidate was also found, which indicates that very large biases can be present in data processed with DoPHOT. To conclude we made a detailed analysis of the blending issue. It is found that OGLE #5 and OGLE #6 are very likely highly blended microlensing events. These events result from the magnification of a faint star that would have been undetectable without microlensing.

astro-ph↗

arXiv1997

A method for optimal image subtraction

We present a new method designed for optimal subtraction of two images with different seeing. Using image subtraction appears to be essential for the full analysis of the microlensing survey images, however a perfect subtraction of two images is not easy as it requires the derivation of an extremely accurate convolution kernel. Some empirical attempts to find the kernel have used the Fourier transform of bright stars, but solving the statistical problem of finding the best kernel solution has never really been tackled. We demonstrate that it is possible to derive an optimal kernel solution from a simple least square analysis using all the pixels of both images, and also show that it is possible to fit the differential background variation at the same time. We also show that PSF variations can also be easily handled by the method. To demonstrate the practical efficiency of the method, we analyzed some images from a Galactic Bulge field monitored by the OGLE II project. We find that the residuals in the subtracted images are very close to the photon noise expectations. We also present some light curves of variable stars, and show that, despite high crowding levels, we get an error distribution close to that expected from photon noise alone. We thus demonstrate that nearly optimal differential photometry can be achieved even in very crowded fields. We suggest that this algorithm might be particularly important for microlensing surveys, where the photometric accuracy and completeness levels could be very significantly improved by using this method.

astro-ph↗

arXiv1996

Lensing of unresolved stars towards the Galactic Bulge

Previous calculations of the rates and optical depths due to microlensing only considered resolved stars. However, if a faint unresolved star lens is close enough to a resolved star, the event will be seen by the microlensing experiments and attributed to the bighter star. The blending biases the duration, making the contribution of the unresolved stars very significant for short events. This contribution is confused with lensing by brown dwarfs. The exact rates of these blended events are extremly sensitive to the limiting magnitude achieved in the microlensing search. Appropriate calculations of the optical depth and rates are provided here, and illustrated in the case of the DUO and OGLE experiments. The additional contribution of unresolved stars is very significant and probably explains the high optical depth and rates observed towards the Galactic Bulge. The blended unresolved event can be identified using either the color shift or the light curve shape. However, neither of these two methods is apropriate to identify a large number of blended events towards the Bulge. In some cases of good photometry and small impact parameter, an identification is possible, as for the OGLE 5 event, which clearly appears as a case of lensing of an unresolved star. The recent results obtained by the PLANET collaboration indicate that a high resolution and dense sampling of the light curve is possible, and will probably provide a very interesting possibility to correct the blending bias, as demonstrated for OGLE 5. This possibility, is certainly better than a statistical estimation of the lensing rates, which are always prone to some uncertainty. But, at this time, the contribution of unresolved stars must be included in the analyses of microlensing experiments.

astro-ph↗

arXiv1995

Object DUO 2: A New Binary Lens Candidate

We present the light curve of an unusual variable object, DUO 2, detected during the search for microlensing events by the DUO project. The star remained stable for more than 150 days before it brightened by more than two magnitudes in 6 days in the B and R bands. The light curves are achromatic during the variability. We consider possible explanations of the photometric behavior, with particular emphasis on the binary lens interpretation of the event. The masses of the lenses are quite small, with the companion possibly in the range of a brown dwarf or even a few times of Jupiter. We report evidence of blending of the source by a companion through the first detection of shift in the light centroid among all the microlensing experiments. This shift sets a lower limit of $0.3^{\prime\prime}$ on the separation between the stars. The best lens model obtained requires moderate blending, which was what motivated us to check the centroid shift that was subsequently found. The best lens model predicts a separation of $1^{\prime\prime}$ between the two blended stars. This prediction was recently tested using two CCD images taken under good seeing conditions. Both images show two components. Their separation and position angle are in good agreement with our model.

astro-ph↗