Search

The astrometric monitoring of the natural planetary satellites is an important step to assess the formation and the evolution of these systems. However, in order to quantify relevant gravitational effects such as tidal forces, it is necessary to have very accurate observations over a long time interval. Unfortunately, the accuracy is decreasing as one considers older observations. To solve this issue, digitizing of old photographic plates is an attractive method, but a high accuracy in the measurement and the reduction of those plates is absolutely necessary. We have developed methods and algorithms adapted to specific plates provided by USNO, using the DAMIAN digitizer of ROB. From a set of 35 plates taken in 1974, we have been able to produce measurements with an accuracy better than 0.08 micrometers and after reduction using the UCAC2 catalogue, rms residuals of 35 mas (1.7 micrometers) for intersatellite positions (when the original reduction provided 100 mas) and of 65 mas for equatorial RA and Dec. positions (which were not possible to get with the original reduction). First results on the dynamics of the satellites and of the planet Jupiter are provided.

We report new nearby L and late-M dwarfs (d_phot = 3.0, later than M8.0) at low Galactic latitude (|b| =M8.0), L, and T dwarfs with accurate trigonometric parallaxes to calibrate the M_J versus I-J colour-luminosity relation. The resulting photometric distances have standard errors of ~15%, which we used to select candidates d_phot

We measured the proper motions of 1250 pre-main sequence (PMS) stars and of 104 PMS candidates spread over all-sky major star-forming regions. This work is the continuation of a previous effort where we obtained proper motions for 213 PMS stars located in the major southern star-forming regions. These stars are now included in this present work with refined astrometry. The major upgrade presented here is the extension of proper motion measurements to other northern and southern star-forming regions including the well-studied Orion and Taurus-Auriga regions for objects as faint as V ≤ 16.5. We improve the precision of the proper motions which benefited from the inclusion of new observational material. In the PMS proper motion catalogue presented here, we provide for each star the mean position and proper motion as well as important photometric information when available. We provide also the most common identifier. The rms of proper motions vary from 2 to 5 mas/yr depending on the available sources of ancient positions and depending also on the embedding and binarity of the source. With this work, we present the first all-sky catalogue of proper motions of PMS stars.

We present the results of a study of the FBS blue stellar objects with the goal of discovering new interesting objects. Forty-seven FBS objects have been observed with the Byurakan Observatory 2.6m telescope in 1987-1991 and scanned with the MAMA scanning machine at Observatoire de Paris in 1997-1999. 5 QSOs/Sy1s, 4 emission-line stars, five white dwarfs, and a number of continuous spectra objects (possible BL Lacs and DC white dwarfs) have been revealed.

we present the discovery of a previously unknown member of the immediate solar neighbourhood, DENIS-P J104814.7 395606.1 (hereafter DENIS 1048 39), identified while mining the DENIS database for new nearby stars. A HIRES echelle spectrum obtained with the 10-m Keck telescope shows that it is an M9 dwarf. DENIS 1048 39 has a very bright apparent magnitude (I = 12.67) for its spectral type and colour (I J = 3.07), and is therefore very nearby. If it is single its distance is only 4.1±0.6 pc, ranking it as between our 12 th and 40 th closest neighbour. It is also the closest star or brown dwarf with a spectral type later than M7V. Its proper motion was determined through comparison of Sky atlas Schmidt plates, scanned by the MAMA microdensitometer, with the DENIS images. At 1.52" yr 1 it primarily attests the closeness of DENIS 1048 39 and hence its dwarf status. These characteristics make it an obvious target for further detailed studies.

Astrometry (positions and proper motions) and photographic BVR photometry were determined from a set of 29 photographic plates and CCD data in a region of 2.3 degrees x 2.3 degrees in the area of open clusters NGC 1750 and NGC 1758 in Taurus. The iterative central overlap algorithm was used for the proper motion calculations. Plate-to-plate transformations and astrometric magnitude effects are discussed in detail. CCD data interact with photographic material ill three different ways: for the elimination of astrometric magnitude effects,, for the photometric standard calibration of photographic plates and for building a CCD pseudo-plate used for astrometric purposes as any other photographic plate. A preliminary analysis of the resulting photometric and astrometric catalogue con firmed the reality of two different clusters in the zone (NGC 1750 and NGC 1758). There were no indications for the existence of NGC 1746.

We present a catalogue of positions and pho- tographic R magnitudes at observed maximum and mini- mum brightness for 1061 variable stars newly detected in the direction of the Bright Cloud B in Sagittarius. The limiting R magnitude of the survey is about 18. Most of these stars are long period variables. Taking into account the previously published lists (Terzan et al. 1982, 1988, 1991), the total number of new variable objects discov- ered in the surveyed 100 square degree eld amounts to 4430.

We present identifications and accurate positions for about 280 planetary nebulae (PNe) in the Large Magellanic Cloud (LMC). These positions are needed for follow-up ground or space observations. Entire Schmidt plates have been scanned with the MAMA micro-densitometer, and PPM stars used for the astrometric reduction. The precision achieved is always better than , and better than for most of the objects not located close to the border of the plates. Photometry in the B band is provided with an accuracy of about 0.2 magnitude, except in too crowded zones where the background cannot be determined. Cross-identifications with IRAS sources have also been tabulated and discussed.

The large field of Schmidt plates allows monitoring of hundreds of thousands or even millions of stars, either for magnitude variations or for proper motions. CCD cameras benefit from the sensitivity, linearity, and easy interface to computers of detectors of increasing dimensions. However, especially for proper motion studies, there is no substitute to plates of first or intermediate epochs. In addition, the photographic plates, with their large sky coverage free of gaps, are irreplaceable for the accurate astrometry of optical counterparts of sources detected at other wavelengths.We first introduce the subject by mentioning a few problems concerning the replacement of Schmidt plates by CCD detectors. After a brief survey of astronomical applications requiring accurate astrometry or massive processing in large fields, we develop in more detail the results obtained in proper motion studies by several groups using the MAMA system (Berger et al. 1991). The last part of this paper is devoted to the studies of stellar variability and to the EROS and DUO microlensing programmes. Our conclusion (not a surprise, we think), is that Schmidt telescopes with photographic facilities should remain available to the general astronomical community for at least a decade and probably more.

A 20cm 2 CCD mosaic camera has been especially built to search for dark galactic halo objects by the gravitational microlensing effect. The sensitive area is made of 16 edge-buttable CCDs developped by Thomson-CTS, with 23×23 µm 2 pixels. The 35 kg camera housing and mechanical equipment is presented. The associated electronics and data acquisition system are described in a separate paper. The camera resides at the focal plane of a 40 cm, f/10, Ferson reflector. The instrument has been in operation since December 1991 at the La Silla Observatory (ESO).

To summarize, the readout and the control system of the CCD mosaic camera are running since December 1991 at the La Silla Observatory (ESO). The overall performance of the camera has been good. About 12000 pictures (data and flat-fields) have been successfully registered up to now. We will report in the near future preliminary scientific results of the EROS experiment.

We report the discovery of a new M9.0 dwarf at only 8.2 pc, which we identified in our search for nearby ultracool dwarf (I-J >= 3.0, later than M8.0) in the DENIS database. We measure a very high proper motion of 2.5 arc-sec/yr. The PC3 index measured from its low-resolution spectrum gives a spectrophotometric distance of 8.2 pc. This makes it the third closest M9.0 dwarf., revised version, accepted by MNRAS Letters

We present spectra of 36 nearby star candidates and 3 red giant candidates, identified in the DENIS database by Phan-Bao et al. (2003). 32 of the dwarf candidates are indeed nearby red dwarfs, with spectral types from M5.5 to M8.5. Out of 11 targets with low proper motion (mu < 0.1$ arc-sec/yr) but a Reduced Proper Motion above an inclusive threshold, 9 are red dwarfs. The 4 contaminants are all reddened F-K main sequence stars, and they could have been eliminated by checking for some well known high latitude molecular clouds. These stars might be of interest as probes of interstellar absorption. For the red dwarfs we derive spectral types and spectroscopic distances, using a new calibration of the PC3 spectral index to absolute magnitudes in the I, J, H and K photometric bands. We confirm 2 new members of the 12 pc volume (2 new M8.5), and one M7.5 NLTT object closer than 10pc. We also show that one quarter of the stars with photometric distances under 30 pc have too small a proper motion for inclusion in the NLTT catalog., Accepted by A&A

This paper reports updated results on our systematic mining of the DENIS database for nearby very cool M-dwarfs (M6V-M8V, 2.0, Comment: Accepted by Astronomy & astrophysics. 17 pages, 8 figures

We have investigated the 2-D stellar distribution in the outer parts of three nearby open clusters: NGC 2287 (=M41), NGC 2516, and NGC 2548 (=M48). Wide-field star counts have been performed in two colours on pairs of digitized ESO and SRC Schmidt plates, allowing us to select likely cluster members in the colour-magnitude diagrams. Cluster tidal extensions were emphasized using a wavelet transform. Taking into account observational biases, namely the galaxy clustering and differential extinction in the Galaxy, we have associated these stellar overdensities with real open cluster structures stretched by the galactic gravitational field. As predicted by theory and simulations, and despite observational limitations, we detected a general elongated (prolate) shape in a direction parallel to the galactic Plane, combined with tidal tails extended perpendicularly to it. This geometry is due both to the static galactic tidal field and the heating up of the stellar system when crossing the Disk. The time varying tidal field will deeply affect the cluster dynamical evolution, and we emphasize the importance of adiabatic heating during the Disk-shocking. In the case of NGC 2548, our dating of the last shocking with the Plane (based on a tidal clump) is consistent with its velocity. During the 10-20 Z-oscillations experienced by a cluster before its dissolution in the Galaxy, crossings through the galactic Disk contribute to at least 15% of the total mass loss. Using recent age estimations published for open clusters, we find a destruction time-scale of about 600 Myr for clusters in the solar neighbourhood., To be published in A&A (accepted July 2001). 10 pages, 10 figures. Version with full resolution figures available at http://www.cai-mama.obspm.fr/pagesPersonnelles/bergond/img/referee.ps.gz

We extend the work developed in previous papers on microlensing with a selection of variable stars. We use the Pixel Method to select variable stars on a set of 2.5 x 10**6 pixel light curves in the LMC Bar presented elsewhere. The previous treatment was done in order to optimise the detection of long timescale variations (larger than a few days) and we further optimise our analysis for the selection of Long Timescale and Long Period Variables (LT&LPV). We choose to perform a selection of variable objects as comprehensive as possible, independent of periodicity and of their position on the colour magnitude diagram. We detail the different thresholds successively applied to the light curves, which allow to produce a catalogue of 632 variable objects. We present a table with the coordinate of each variable, its EROS magnitudes at one epoch and an indicator of blending in both colours, together with a finding chart. A cross-correlation with various catalogues shows that 90% of those variable objects were undetected before, thus enlarging the sample of LT&LPV previously known in this area by a factor of 10. Due to the limitations of both the Pixel Method and the data set, additional data -- namely a longer baseline and near infrared photometry -- are required to further characterise these variable stars, as will be addressed in subsequent papers., 11 pages with 10 figures

We present 30 new nearby (d, Accepted in Astronomy and Astrophysics

Absolute proper motions, their corresponding errors and membership probabilities of 501 stars in the intermediate-age open cluster NGC2548 region are determined from MAMA measurements of 10 photographic plates. The plates have the maximum epoch difference of 82 years and they were taken with the double astrograph at Zo-Se station of Shanghai Observatory, which has an aperture of 40 cm and a plate scale of 30"/mm. The average proper motion precision is 1.18 mas/yr. These proper motions are used to determine the membership probabilities of stars in the region. The number of stars with membership probabilities higher than 0.7 is 165., Comment: 8 pages, 7 figures,accepted for publication in A&A

We present first results of a catalogue of standard double stars for the astrometric calibration of CCD’s. The data were taken from photographically determined precise positions and proper motions of stars in the region of radio sources and open clusters. In this report, we give preliminary results of stars in the region of 3C 273 and NGC 1647.

The EROS and MACHO collaborations have each published upper limits on the amount of planetary mass dark matter in the Galactic Halo obtained from gravitational microlensing searches. In this paper the two limits are combined to give a much stronger constraint on the abundance of low mass MACHOs., 8 pages, 4 figures, submitted to ApJ Letters

Candidate pre-main-sequence stars were observed in the bar of the Large Magellanic Cloud during the search for dark matter in the galactic halo. Seven blue stars of apparent visual magnitude 15 to 17 had irregular photometric variations and hydrogen emission lines in their optical spectra, which suggested that these stars are pre-main-sequence stars of about 10 solar masses. These stars are slightly more massive and definitely more luminous than are Herbig AeBe pre-main-sequence stars in our own galaxy. Continued observations of these very young stars from another galaxy, which are probably at the pre-hydrogen-burning stage, should provide important clues about early stages of star formation.

A new sample of active galaxies was obtained with the ESO Schmidt telescope. We derived quantitative redshifts for 97 emission line galaxies and semi-quantitative photometric and spectro-photometric properties for 62 of them. The results are directly extracted from the Schmidt plates after digitization by the MAMA machine of Observatoire de Paris (C.A.I, INSU Paris).

As a first step in our open cluster programme, a preliminary catalogue of proper motions and photographic B, V magnitudes for stars up to 18 mag within a region centred near Alcyone is presented. The catalogue is based on MAMA measurements of plates taken with the Tautenburg Schmidt telescope. A preliminary survey includes approx. 20,000 stars and covers a total field of about 9 square degrees. For stars brighter than B = 18 mag an accuracy of 0.0S mag and 2.5 mas/year has been estimated for photometric data and proper motions, respectively. The results of the determination of the Pleiades membership up to magnitude 18 are presented.

THE flat rotation curves of spiral galaxies, including our own, indicate that they are surrounded by unseen haloes of ‘dark matter’1,2. In the absence of a massive halo, stars and gas in the outer portions of a galaxy would orbit the centre more slowly, just as the outer planets in the Solar System circle the Sun more slowly than the inner ones. So far, however, there has been no direct observational evidence for the dark matter, or its characteristics. Paczynski3suggested that dark bodies in the halo of our Galaxy can be detected when they act as gravitational ‘microlenses’, amplifying the light from stars in nearby galaxies. The duration of such an event depends on the mass, distance and velocity of the dark object. We have been monitoring the brightness of three million stars in the Large Magellanic Cloud for over three years, and here report the detection of two possible microlensing events. The brightening of the stars was symmetrical in time, achromatic and not repeated during the monitoring period. The timescales of the two events are about thirty days and imply that the masses of the lensing objects lie between a few hundredths and one solar mass. The number of events observed is consistent with the number expected if the halo is dominated by objects with masses in this range. In 1986, Bohdan Paczynski suggested that dark, compact objects in the halo of our Galaxy — the local cousins of similar objects conjectured to contribute dark matter to the haloes of other galaxies — could be detected on Earth, as their gravitation might occasionally amplify the light arriving from more distant stars. In 1993, in surveys of several million stars in the Large Magellanic Cloud, C. Alcock et al. and E. Aubourg et al. observed several candidate events. In total, the two groups detected three stars for which the brightness increased by two magnitudes over an interval of roughly one month.

A programme for the determination of proper motions and photographic magnitudes for stars up to 18m within a 4° by 4° region centered near Alcyone is described. We use MAMA measurements of plates taken with Tautenburg and OCA Schmidt telescopes as well as with the double refractor of Bonn and Carte du Ciel plates. To check the stability of the solution three different, methods of reduction are applied.

Two soft X-ray images of the Chamaeleon I cloud obtained with the ROSAT PSPC are presented. Seventy reliable, and perhaps 19 additional X-ray sources are found. Up to 90% of these sources are certainly or probably identified with T Tauri stars formed in the cloud. Twenty to 35 are probably new “weak” T Tauri (WTT) stars. We summarize a statistical study of the relationships between X-ray emission and other physical properties for a number of Chamaeleon stars. We see no difference in spatial distributions, detection rate and luminosity functions between the WTTS and CTTS.

MAMA (Machine Automatique a Mesurer pour l’Astronomie) is a fast and accurate multichannel microdensitometer developed and operated by INSU and located at the Observatoire de Paris. MAMA processes in a few hours photographic plates up to 14″ × 14″ with a positional accuracy of 1μm (repeatability: 0.2μm) and a photometric accuracy of 2% over a dynamical range of 3 densities. The detector is a RETICON CCPD array with 1024 photodiodes. X and Y coordinates are measured with Heidenhain encoders. Autofocus is achieved through a maximisation of the plate grain noise, and is accurate to better than 4 microns. The plate can be digitised either in a systematic way by lanes 10.24 mm wide, or in a random access mode from a catalogue of preliminary positions. The basic pixel size (and sampling step) is 10μm. Oversampling down to 2μm can be used to digitise spectra; for some applications, pixels of 20, 30… 80 μm can be synthesized in real time.

The thickness of the disc distribution of young objects is shown to be significantly smaller than that of the interstellar gas over the whole Galaxy, as predicted by the Schmidt’s law of star formation. The radial distribution of the density of young objects at z = 0, compared with that of the gas, is also consistent with the Schmidt’s law. The exponent in this law is found to be between 1.5 and 2. A brief discussion of simple models of galactic evolution using the Schmidt’s law is made.

We analyze in detail the pumping of 18 cm OH masers by the overlap of far infra-red lines in HII/OH regions and in circumstellar shells. We present some results of a model of pumping of HII/OH masers by thermal overlap. Pumping by overlap appears to be quite general but very complicated. It can account for the different observed main-line masers. However, other types of pumping, mainly by collisions with H or by near infra-red radiation, are possible if certain conditions are met for the physical and astrophysical parameters.

A recent investigation based upon the OH luminosity distribution (Nguyen-Q-Rieu et al. 1979) has shown that Type I OH sources associated with Mira variables (OH Miras) are weak OH sources and are therefore only detected within ∼1 kpc from the Sun. Type II OH-Miras, which are more intense and rarer than Type I OH-Miras, are probably more distant objects. The group of unidentified Type II OH-IR sources probably consists of Type II OH-Miras of high OH luminosity. The IR colour index, which is usually higher for Type II sources, suggests that they have a colder and denser dust shell.