arXiv subjects

P. Tisserand

Publications and source records attributed to P. Tisserand.

At least 37 records · Page 2Linked to original sources

arXiv2013

Space based microlensing planet searches

The discovery of extra-solar planets is arguably the most exciting development in astrophysics during the past 15 years, rivalled only by the detection of dark energy. Two projects unite the communities of exoplanet scientists and cosmologists: the proposed ESA M class mission EUCLID and the large space mission WFIRST, top ranked by the Astronomy 2010 Decadal Survey report. The later states that: "Space-based microlensing is the optimal approach to providing a true statistical census of planetary systems in the Galaxy, over a range of likely semi-major axes". They also add: "This census, combined with that made by the Kepler mission, will determine how common Earth-like planets are over a wide range of orbital parameters". We will present a status report of the results obtained by microlensing on exoplanets and the new objectives of the next generation of ground based wide field imager networks. We will finally discuss the fantastic prospect offered by space based microlensing at the horizon 2020-2025.

astro-ph.EP↗

arXiv2013

Main Sequence Star Populations in the Virgo Overdensity Region

We present deep CMDs for two Subaru Suprime-Cam fields in the Virgo Stellar Stream(VSS)/Virgo Overdensity(VOD) and compare them to a field centred on the highest concentration of Sagittarius (Sgr) Tidal Stream stars in the leading arm, Branch A of the bifurcation. A prominent population of MS stars is detected in all three fields and can be traced as faint as g~24 mag. Using theoretical isochrone fitting we derive an age of 9.1(+1.0;-1.1)Gyr, a median abundance of [Fe/H]=-0.70 (+0.15; -0.20)dex and d_helio of 30.9+-3.0kpc for the MS of the Sgr Stream Branch A. The dominant main sequence populations in the two VSS/VOD fields (Lsun ~265 deg, Bsun ~13 deg) are located at a mean distance of 23.3+-1.6kpc and have an age ~8.2Gyr and an abundance [Fe/H]=-0.67(+0.16;-0.12)dex similar to the Sgr Stream stars. These parameters are also in good agreement with the age of the main population in the Sagittarius dwarf (8.0+-1.5Gyr). They also agree with the peak in the metallicity distribution of 2-3Gyr old M-giants, [Fe/H] ~ -0.6dex, in the Sgr leading arm north. We then compare the results from the VSS/VOD fields with the Sgr Tidal Stream model by Law & Majewski based on a triaxial Galactic halo that is calibrated with SDSS Sgr A-branch and 2MASS M-giant stars. We find that the most prominent feature in the CMDs, the MS population at 23kpc, is not explained by the model. Instead the model predicts in these directions a low density filamentary structure of Sgr debris stars at ~9kpc and a slightly higher concentration of Sgr stars spread from 42-53kpc. At best there is only marginal evidence for the presence of these populations in our data. Our findings then suggest that while there are probably some Sgr debris stars present, the dominant stellar population in the VOD originates from a different halo structure that has almost identical age and metallicity as some sections of the Sgr tidal stream.

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arXiv2013

The ongoing pursuit of R Coronae Borealis stars: ASAS-3 survey strikes again

R Coronae Borealis stars (RCBs) are rare, hydrogen-deficient, carbon-rich supergiant variable stars that are likely the evolved merger products of pairs of CO and He white dwarfs. Only 55 RCB stars are known in our galaxy and their distribution on the sky is weighted heavily by microlensing survey field positions. A less-biased wide-area survey would provide the ability to test competing evolutionary scenarios, understand the population or populations that produce RCBs and constraint their formation rate. The ASAS-3 survey monitored the sky south of declination +28 deg since 2000 to a limiting magnitude of V=14. We searched ASAS-3 for RCB variables using a number of different methods to ensure that the probability of RCB detection was as high as possible and to reduce selection biases based on luminosity, temperature, dust production activity and shell brightness. Candidates whose light curves were visually inspected were pre-selected based on their infrared excesses due to warm dust in their circumstellar shells using the WISE and/or 2MASS catalogues, and criteria on light curve variability. We then acquired spectra of 104 stars to determine their real nature using the SSO/WiFeS spectrograph. We report 21 newly-discovered RCB stars and 2 new DY Per stars. Two previously suspected RCB candidates were also spectroscopically confirmed. Our methods allowed us to extend our detection efficiency to fainter magnitudes that would not have been easily accessible to discovery techniques based only on light curve variability. The overall detection efficiency is about 90% for RCBs with maximum light brighter than V~13. This growing sample is of great value to constrain the peculiar and disparate atmosphere composition of RCBs. Most importantly, we show that the spatial distribution and apparent magnitudes of Galactic RCB stars is consistent with RCBs being part of the Galactic bulge population.

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arXiv2012

Preliminary results for RR Lyrae stars and Classical Cepheids from the Vista Magellanic Cloud (VMC) Survey

The Vista Magellanic Cloud (VMC, PI M.R. Cioni) survey is collecting $K_S$-band time series photometry of the system formed by the two Magellanic Clouds (MC) and the "bridge" that connects them. These data are used to build $K_S$-band light curves of the MC RR Lyrae stars and Classical Cepheids and determine absolute distances and the 3D geometry of the whole system using the $K$-band period luminosity ($PLK_S$), the period - luminosity - color ($PLC$) and the Wesenhiet relations applicable to these types of variables. As an example of the survey potential we present results from the VMC observations of two fields centered respectively on the South Ecliptic Pole and the 30 Doradus star forming region of the Large Magellanic Cloud. The VMC $K_S$-band light curves of the RR Lyrae stars in these two regions have very good photometric quality with typical errors for the individual data points in the range of $\sim$ 0.02 to 0.05 mag. The Cepheids have excellent light curves (typical errors of $\sim$ 0.01 mag). The average $K_S$ magnitudes derived for both types of variables were used to derive $PLK_S$ relations that are in general good agreement within the errors with the literature data, and show a smaller scatter than previous studies.

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arXiv2011

Long Period Variables in the Large Magellanic Cloud from the EROS-2 survey

Context. The EROS-2 survey has produced a database of millions of time series from stars monitored for more than six years, allowing to classify some of their sources into different variable star types. Among these, Long Period Variables (LPVs), known to follow sequences in the period-luminosity diagram, include long secondary period variables whose variability origin is still a matter of debate. Aims.We use the 856 864 variable stars available from the Large Magellanic Cloud (LMC) in the EROS-2 database to detect, classify and characterize LPVs. Methods. Our method to extract LPVs is based on the statistical Abbe test. It investigates the regularity of the light curve with respect to the survey duration in order to extract candidates with long-term variability. The period search is done by Deeming, Lomb-Scargle and generalized Lomb-Scargle methods, combined with Fourier series fit. Color-magnitude, period-magnitude and period-amplitude diagrams are used to characterize our candidates. Results. We present a catalog of 43 551 LPV candidates for the Large Magellanic Cloud. For each of them, we provide up to five periods, mean magnitude in EROS-2, 2MASS and Spitzer bands, BE-RE color, RE amplitude and spectral type.We use infrared data to make the distinction between RGB, O-rich, C-rich and extreme AGB stars. Properties of our LPV candidates are investigated by analyzing period-luminosity and period-amplitude diagrams.

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arXiv2011

The OGLE View of Microlensing towards the Magellanic Clouds. IV. OGLE-III SMC Data and Final Conclusions on MACHOs

In this fourth part of the series presenting the Optical Gravitational Lensing Experiment (OGLE) microlensing studies of the dark matter halo compact objects (MACHOs) we describe results of the OGLE-III monitoring of the Small Magellanic Cloud (SMC). Three sound candidates for microlensing events were found and yielded the optical depth tau_SMC-OIII=1.30+-1.01 10^{-7}, consistent with the expected contribution from Galactic disk and SMC self-lensing. We report that event OGLE-SMC-03 is the most likely a thick disk lens candidate, the first of such type found towards the SMC. In this paper we also combined all OGLE Large and Small Magellanic Cloud microlensing results in order to refine the conclusions on MACHOs. All but one of OGLE events are most likely caused by the lensing by known populations of stars, therefore we concluded that there is no need for introducing any special dark matter compact objects in order to explain the observed events rates. Potential black hole event indicates that similar lenses can contribute only about 2 per cent to the total mass of the halo, which is still in agreement with the expected number of such objects.

astro-ph.GA↗

arXiv2011

Oxygen- and carbon-rich variable red giant populations in the Magellanic Clouds from EROS, OGLE, MACHO, and 2MASS photometry

The carbon-to-oxygen (C/O) ratio of asymptotic giant branch (AGB) stars constitutes an important index of evolutionary and environment/metallicity factor. We develop a method for mass C/O classification of AGBs in photometric surveys without using periods. For this purpose we rely on the slopes in the tracks of individual stars in the colour-magnitude diagram. We demonstrate that our method enables the separation of C-rich and O-rich AGB stars with little confusion. For the Magellanic Clouds we demonstrate that this method works for several photometric surveys and filter combinations. As we rely on no period identification, our results are relatively insensitive to the phase coverage, aliasing, and time-sampling problems that plague period analyses. For a subsample of our stars, we verify our C/O classification against published C/O catalogues. With our method we are able to produce C/O maps of the entire Magellanic Clouds. Our purely photometric method for classification of C- and O-rich AGBs constitutes a method of choice for large, near-infrared photometric surveys. Because our method depends on the slope of colour-magnitude variation but not on magnitude zero point, it remains applicable to objects with unknown distances.

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arXiv2011

New R Coronae Borealis stars discovered in OGLE-III Galactic Bulge fields from their mid- and near- infrared properties

An R Coronae Borealis (RCB) star is a rare type of supergiant star that is increasingly thought to be the evolved merger product of two white dwarfs. Recently, many of them have been found distributed in a thin disk structure embedded inside the Galactic Bulge. This unexpected high density can give us more insight into the nature and age of RCB stars. We applied and tested successfully a new technique to find RCB stars based on the particular infrared emission. We demonstrated that RCB stars can now be found without the need of a light curve analysis, and therefore outside optically monitored fields. The selection of RCB candidates was based on their near-infrared excess and on particular mid-infrared emission of RCB shells, using photometric data from the 2MASS and Spitzer/GLIMPSE surveys. The OGLE light curves of all RCB candidates were then inspected visually and the ones presenting large and fast declines were followed-up spectroscopically . We discovered two new R Coronae Borealis stars, but also propose four new candidates. We stress that all of the 7 known RCB stars located in both Spitzer/GLIMPSE and OGLE-III fields were re-discovered, which indicates the high efficiency of our analysis. The proposed new technique to find RCB stars has been successful. It can now be extented to larger area, specially where the instellar extinction is too high to have been monitored by microlensing surveys, i.e the inner part of the Galactic Bulge.

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arXiv2010

Herschel Observations of a Newly Discovered UX Ori Star in the Large Magellanic Cloud

The LMC star, SSTISAGE1C J050756.44-703453.9, was first noticed during a survey of EROS-2 lightcurves for stars with large irregular brightness variations typical of the R Coronae Borealis (RCB) class. However, the visible spectrum showing emission lines including the Balmer and Paschen series as well as many Fe II lines is emphatically not that of an RCB star. This star has all of the characteristics of a typical UX Ori star. It has a spectral type of approximately A2 and has excited an H II region in its vicinity. However, if it is an LMC member, then it is very luminous for a Herbig Ae/Be star. It shows irregular drops in brightness of up to 2 mag, and displays the reddening and "blueing" typical of this class of stars. Its spectrum, showing a combination of emission and absorption lines, is typical of a UX Ori star that is in a decline caused by obscuration from the circumstellar dust. SSTISAGE1C J050756.44-703453.9 has a strong IR excess and significant emission is present out to 500 micron. Monte Carlo radiative transfer modeling of the SED requires that SSTISAGE1C J050756.44-703453.9 has both a dusty disk as well as a large extended diffuse envelope to fit both the mid- and far-IR dust emission. This star is a new member of the UX Ori subclass of the Herbig Ae/Be stars and only the second such star to be discovered in the LMC.

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arXiv2009

New Magellanic Cloud R Coronae Borealis and DY Per type stars from the EROS-2 database: the connection between RCBs, DYPers and ordinary carbon stars

R Coronae Borealis stars (RCB) are a rare type of evolved carbon-rich supergiant stars that are increasingly thought to result from the merger of two white dwarfs, called the Double degenerate scenario. This scenario is also studied as a source, at higher mass, of type Ia Supernovae (SnIa) explosions. Therefore a better understanding of RCBs composition would help to constrain simulations of such events. We searched for and studied RCB stars in the EROS Magellanic Clouds database. We also extended our research to DY Per type stars (DYPers) that are expected to be cooler RCBs (T~3500 K) and much more numerous than their hotter counterparts. The light curves of ~70 millions stars have been analysed to search for the main signature of RCBs and DYPers: a large drop in luminosity. Follow-up optical spectroscopy was used to confirm each photometric candidate found. We have discovered and confirmed 6 new Magellanic Cloud RCB stars and 7 new DYPers, but also listed new candidates: 3 RCBs and 14 DYPers. We estimated a range of Magellanic RCB shell temperatures between 360 and 600 K. We confirm the wide range of absolute luminosity known for RCB stars, M_V~-5.2 to -2.6. Our study further shows that mid-infrared surveys are ideal to search for RCB stars, since they have thinner and cooler circumstellar shells than classical post-AGB stars. In addition, by increasing the number of known DYPers by ~400%, we have been able to shed light on the similarities in the spectral energy distribution between DYPers and ordinary carbon stars. We also observed that DYPer circumstellar shells are fainter and hotter than those of RCBs. This suggests that DYPers may simply be ordinary carbon stars with ejection events, but more abundance analysis is necessary to give a status on a possible evolutionnary connexion between RCBs and DYPers.

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arXiv2009

The EROS2 search for microlensing events towards the spiral arms: the complete seven season results

The EROS-2 project has been designed to search for microlensing events towards any dense stellar field. The densest parts of the Galactic spiral arms have been monitored to maximize the microlensing signal expected from the stars of the Galactic disk and bulge. 12.9 million stars have been monitored during 7 seasons towards 4 directions in the Galactic plane, away from the Galactic center. A total of 27 microlensing event candidates have been found. Estimates of the optical depths from the 22 best events are provided. A first order interpretation shows that simple Galactic models with a standard disk and an elongated bulge are in agreement with our observations. We find that the average microlensing optical depth towards the complete EROS-cataloged stars of the spiral arms is $\barτ =0.51\pm .13\times 10^{-6}$, a number that is stable when the selection criteria are moderately varied. As the EROS catalog is almost complete up to $I_C=18.5$, the optical depth estimated for the sub-sample of bright target stars with $I_C<18.5$ ($\barτ=0.39\pm >.11\times 10^{-6}$) is easier to interpret. The set of microlensing events that we have observed is consistent with a simple Galactic model. A more precise interpretation would require either a better knowledge of the distance distribution of the target stars, or a simulation based on a Galactic model. For this purpose, we define and discuss the concept of optical depth for a given catalog or for a limiting magnitude.

astro-ph.GA↗

arXiv2009

The beat Cepheids in the Magellanic Clouds: an analysis from the EROS-2 database

A number of microlensing dark-matter surveys have produced tens of millions of light curves of individual background stars. These data provide an unprecedented opportunity for systematic studies of whole classes of variable stars and their host galaxies. We aim to use the EROS-2 survey of the Magellanic Clouds to detect and study the population of beat Cepheids (BCs) in both Clouds. BCs pulsating simultaneously in the first overtone and fundamental modes (FO/F) or in the second and first overtone modes (SO/FO) are of particular interest. Using special software designed to search for periodic variables, we have scanned the EROS-2 data base for variables in the typical period range of Cepheids. Metallicities of FO/F objects were then calculated from linear nonadiabatic convective stellar models. We identify 74 FO/F BCs in the LMC and 41 in the SMC, and 173 and 129 SO/FO pulsators in the LMC and SMC, respectively; 185 of these stars are new discoveries. For nearly all the FO/F objects we determine minimum, mean, and maximum values of the metallicity. The EROS data have expanded the samples of known BCs in the LMC by 31%, in the SMC by 110%. The FO/F objects provide independent measures of metallicities in these galaxies. The mean value of metallicity is 0.0045 in the LMC and 0.0018 in the SMC.

astro-ph.SR↗

arXiv2008

Discovery of a peculiar Cepheid-like star towards the northern edge of the Small Magellanic Cloud

For seven years, the EROS-2 project obtained a mass of photometric data on variable stars. We present a peculiar Cepheid-like star, in the direction of the Small Magellanic Cloud, which demonstrates unusual photometric behaviour over a short time interval. We report on data of the photometry acquired by the MARLY telescope and spectroscopy from the EFOSC instrument for this star, called EROS2 J005135-714459(sm0060n13842), which resembles the unusual Cepheid HR 7308. The light curve of our target is analysed using the Analysis of Variance method to determine a pulsational period of 5.5675 days. A fit of time-dependent Fourier coefficients is performed and a search for proper motion is conducted. The light curve exhibits a previously unobserved and spectacular change in both mean magnitude and amplitude, which has no clear theoretical explanation. Our analysis of the spectrum implies a radial velocity of 104 km s$^{-1}$ and a metallicity of -0.4$\pm$0.2 dex. In the direction of right ascension, we measure a proper motion of 17.4$\pm$6.0 mas yr$^{-1}$ using EROS astrometry, which is compatible with data from the NOMAD catalogue. The nature of EROS2 J005135-714459(sm0060n13842) remains unclear. For this star, we may have detected a non-zero proper motion for this star, which would imply that it is a foreground object. Its radial velocity, pulsational characteristics, and photometric data, however, suggest that it is instead a Cepheid-like object located in the SMC. In such a case, it would present a challenge to conventional Cepheid models.

astro-ph↗

arXiv2008

R Coronae Borealis stars in the Galactic Bulge discovered by EROS-2

Rare types of variable star may give unique insight into short-lived stages of stellar evolution. The systematic monitoring of millions of stars and advanced light curve analysis techniques of microlensing surveys make them ideal for discovering also such rare variable stars. One example is the R Coronae Borealis (RCB) stars, a rare type of evolved carbon-rich supergiant. We have conducted a systematic search of the EROS-2 database for the Galactic catalogue Bulge and spiral arms to find Galactic RCB stars. The light curves of $\sim$100 million stars, monitored for 6.7 years (from July 1996 to February 2003), have been analysed to search for the main signature of RCB stars, large and rapid drops in luminosity. Follow-up spectroscopy has been used to confirm the photometric candidates. We have discovered 14 new RCB stars, all in the direction of the Galactic Bulge, bringing the total number of confirmed Galactic RCB stars to about 51. After reddening correction, the colours and absolute magnitudes of at least 9 of the stars are similar to those of Magellanic RCB stars. This suggests that these stars are in fact located in the Galactic Bulge, making them the first RCB stars discovered in the Bulge. The localisation of the 5 remaining RCBs is more uncertain: 4 are either located behind the Bulge at an estimated maximum distance of 14 kpc or have an unusual thick circumstellar shell; the other is a DY Per RCB which may be located in the Bulge, even if it is fainter than the known Magellanic DY Per. From the small scale height found using the 9 new Bulge RCBs, $61<h^{RCB}_{Bulge}<246$ pc (95% C.L.), we conclude that the RCB stars follow a disk-like distribution inside the Bulge.

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arXiv2007

Limits on the Macho Content of the Galactic Halo from the EROS-2 Survey of the Magellanic Clouds

The EROS-2 project was designed to test the hypothesis that massive compact halo objects (the so-called ``machos'') could be a major component of the dark matter halo of the Milky Way galaxy. To this end, EROS-2 monitored over 6.7 years $33\times10^6$ stars in the Magellanic clouds for microlensing events caused by such objects. In this work, we use only a subsample of $7\times10^6$ bright stars spread over $84 °^2$ of the LMC and $9 °^2$ of the SMC. The strategy of using only bright stars helps to discriminate against background events due to variable stars and allows a simple determination of the effects of source confusion (blending). The use of a large solid angle makes the survey relatively insensitive to effects that could make the optical depth strongly direction dependent. Using this sample of bright stars, only one candidate event was found, whereas $\sim39$ events would have been expected if the Halo were entirely populated by objects of mass $M\sim0.4M_{\odot}$. Combined with the results of EROS-1, this implies that the optical depth toward the Large Magellanic Cloud (\object{LMC}) due to such lenses is $τ<0.36\times10^{-7}$ (95%CL), corresponding to a fraction of the halo mass of less than 8%. This optical depth is considerably less than that measured by the MACHO collaboration in the central region of the LMC. More generally, machos in the mass range $0.6\times10^{-7}M_\odot<M<15M_{\odot}$ are ruled out as the primary occupants of the Milky Way Halo.

astro-ph↗

arXiv2007

SkyMapper and the Southern Sky Survey

This paper presents the design and science goals for the SkyMapper telescope. SkyMapper is a 1.3m telescope featuring a 5.7 square degree field-of-view Cassegrain imager commissioned for the Australian National University's Research School of Astronomy and Astrophysics. It is located at Siding Spring Observatory, Coonabarabran, NSW, Australia and will see first light in late 2007. The imager possesses 16kx16k 0.5 arcsec pixels. The primary scientific goal of the facility is to perform the Southern Sky Survey, a six colour and multi-epoch (4 hour, 1 day, 1 week, 1 month, 1 year sampling) photometric survey of the southerly 2pi steradians to g~23 mag. The survey will provide photometry to better than 3% global accuracy and astrometry to better than 50 mas. Data will be supplied to the community as part of the Virtual Observatory effort. The survey will take five years to complete.

astro-ph↗

arXiv2006

Galactic Bulge Microlensing Optical Depth from EROS-2

We present a new EROS-2 measurement of the microlensing optical depth toward the Galactic Bulge. Light curves of $5.6\times 10^{6}$ clump-giant stars distributed over $66 °^2$ of the Bulge were monitored during seven Bulge seasons. 120 events were found with apparent amplifications greater than 1.6 and Einstein radius crossing times in the range $5 {\rm d}<t_\e <400 {\rm d}$. This is the largest existing sample of clump-giant events and the first to include northern Galactic fields. In the Galactic latitude range $1.4\degr<|b|<7.0\degr$, we find $τ/10^{-6}=(1.62 \pm 0.23)\exp[-a(|b|-3 {\rm deg})]$ with $a=(0.43 \pm0.16)°^{-1}$. These results are in good agreement with our previous measurement, with recent measurements of the MACHO and OGLE-II groups, and with predictions of Bulge models.

astro-ph↗

arXiv2004

Microlensing towards LMC and M31

(Short version) The nature and the location of the lenses discovered in the microlensing surveys done so far towards the LMC remain unclear. This contribution is comprised of two distinct parts. In the first part, motivated by these questions, we compute the optical depth for the different intervening populations an the number of expected events for self-lensing, using a recently drawn coherent picture of the geometrical structure and dynamics of the LMC disk. In the second part (section 5), a preliminary account of the final results from the EROS-2 programme is presented. Based on the analysis of 33 million LMC and SMC stars followed during 6.7 years, strict limits on the macho content of the galactic halo are presented; they cover the range of macho masses between 0.0001 and 100 solar mass. The limits are better than 20% (resp. 5%) of the standard halo for masses between 0.0002 and 10 (resp. 0.001 to 0.1) solar mass. This is presently the data set with the largest sensitivity to halo machos.

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