Néstor Sánchez

Crucial aspects of the initial mass function (I) The statistical correlation between the total mass of an ensemble of stars and its most massive star

Context. Our understanding of stellar systems depends on the adopted interpretation of the initial mass function, IMF φ(m). Unfortunately, there is not a common interpretation of the IMF, which leads to different methodologies and diverging analysis of observational data. Aims. We study the correlation between the most massive star that a cluster would host, mmax, and its total mass into stars, M ,a s an example where different views of the IMF lead to different results. Methods. We assume that the IMF is a probability distribution function and analyze the mmax −M correlation within this context. We also examine the meaning of the equation used to derive a theoretical M− ˆ mmax relationship, N× m up ˆ mmax φ(m )d m = 1 with N the total number of stars in the system, according to different interpretations of the IMF. Results. We find that only a probabilistic interpretation of the IMF, where stellar masses are identically independent distributed random variables, provides a self-consistent result. Neither M nor the total number of stars in the cluster, N, can be used as IMF scaling factors. In addition, ˆ mmax is a characteristic maximum stellar mass in the cluster, but not the actual maximum stellar mass. A �M� − ˆ mmax correlation is a natural result of a probabilistic interpretation of the IMF; however, the distribution of observational data in the N (or M) −mmax plane includes a dependence on the distribution of the total number of stars, N (and M), in the system, ΦN (N), which is not usually taken into consideration. Conclusions. We conclude that a random sampling IMF is not in contradiction to a possible mmax −M physical law. However, such a law cannot be obtained from IMF algebraic manipulation or included analytically in the IMF functional form. The possible physical information that would be obtained from the N (or M) −mmax correlation is closely linked with the ΦM(M )a ndΦN (N) distributions; hence it depends on the star formation process and the assumed definition of stellar cluster.

World Space Observatory-Ultraviolet: ISSIS, the imaging instrument ✩

The Imaging and Slitless Spectroscopy Instrument (ISSIS) will be flown as part of the science instrumentation in the World Space Observatory-Ultraviolet (WSO-UV). ISSIS will be the first UV imager to operate in a high Earth orbit from a 2 m class space telescope. In this contribution, the science driving the ISSIS design and the main characteristics of this instrument are presented.

A GALEX-BASED SEARCH FOR THE SPARSE YOUNG STELLAR POPULATION IN THE TAURUS-AURIGAE STAR FORMING REGION

In this work, we identify 63 bona fide new candidates to T Tauri stars (TTSs) in the Taurus-Auriga region using as baseline its ultraviolet excess. The initial data set has been defined from the GALEX all sky survey (AIS). The GALEX satellite obtained images in the near ultraviolet (NUV) and far ultraviolet (FUV) bands where the TTSs show a prominent excess, compared with main sequence or giants stars. GALEX AIS surveyed the Taurus-Auriga molecular complex, as well as, a fraction of the California Nebula and the Perseus complex; bright sources and the dark clouds themselves are avoided. nThe properties of the TTSs in the ultraviolet (GALEX), optical (UCAC4) and infrared (2MASS) have been defined using as qualification sample the TTSs observed with the International Ultraviolet Explorer. The candidates have been identified by means of a mixed ultraviolet-optical-infrared excess set of colors; it is found that the color-color diagram FUV-NUV versus J-K is ideally suited for this purpose. From an initial sample of 163,313 bona-fide NUV sources, a final list with 63 new candidates to TTSs in the region has been produced. The search procedure has been validated by its ability to detect all known TTSs in the area surveyed: 31 TTSs. nAlso, it is shown that the weak-lined TTSs are located in a well defined stripe in the FUV-NUV versus J-K diagram. Moreover, we provide in this work a list of TTSs photometric standards for future GALEX-based studies of the young stellar population in star forming regions.

THE DEPENDENCE OF PRESTELLAR CORE MASS DISTRIBUTIONS ON THE STRUCTURE OF THE PARENTAL CLOUD

The mass distribution of prestellar cores is obtained for clouds with arbitrary internal mass distributions using a selection criterion based on the thermal and turbulent Jeans mass and applied hierarchically from small to large scales. We have checked this methodology comparing our results for a lognormal density PDF with the theoretical CMF derived by Hennebelle and Chabrier, namely a power-law at large scales and a log-normal cutoff at low scales, but our method can be applied to any mass distributions representing a star-forming cloud. This methodology enables us to connect the parental cloud structure with the mass distribution of the cores and their spatial distribution, providing an efficient tool for investigating the physical properties of the molecular clouds that give rise to the prestellar core distributions observed. Simulated fBm clouds with the Hurst exponent close to the value H=1/3 give the best agreement with the theoretical CMF derived by Hennebelle and Chabrier and Chabriers system IMF. Likewise, the spatial distribution of the cores derived from our methodology show a surface density of companions compatible with those observed in Trapezium and Ophiucus star-forming regions. This method also allows us to analyze the properties of the mass distribution of cores for different realizations. We found that the variations in the number of cores formed in different realizations of fBm clouds (with the same Hurst exponent) are much larger than the expected root N statistical fluctuations, increasing with H.

ISSIS: the imaging and slitless spectroscopy instrument for surveys in the World Space Observatory-Ultraviolet telescope

ISSIS is the Imaging and Slitless Spectroscopy Instrument for the World Space Observatory - Ultraviolet (WSO-UV), a 170 cm space telescope to be launched in late 2015. ISSIS is a multipurpose instrument designed to carry out high resolution and high sensitivity imaging and slitless spectroscopy in the ultraviolet range. ISSIS has two acquisition channels: the Far Ultraviolet Channel (FUV) covering the 1150-1750 Å wavelength range and the Near Ultraviolet Channel (NUV) in the 1850-3200 Å range. Both channels are equipped with Multi Channel Plate detectors to guarantee high sensitivity and high rejection of lower energy radiation. ISSIS will be the first UV imager into a high altitude Earth orbit and it will provide unique information on star formation, accretion physics, astronomical engines and planets.

Young stellar object candidates toward the Orion region selected from GALEX

We analyze 359 ultraviolet tiles from the All Sky Imaging Survey of the space mission GALEX covering roughly 400 square degrees toward the Orion star-forming region. There is a total of 1,555,174 ultraviolet sources that were cross-matched with others catalogs (2MASS, UCAC4, SDSS, DENIS, CMC15 and WISE) to produce a list of 290,717 reliable sources with a wide range of photometric information. Using different color selection criteria we identify 111 Young Stellar Object candidates showing both ultraviolet and infrared excesses, of which 81 are new identifications. We discuss the spatial distribution, the spectral energy distributions and other physical properties of these stars. Their properties are, in general, compatible with those expected for T Tauri stars. This population of TTS candidates is widely dispersed around the Orion molecular cloud.

Crucial aspects of the initial mass function - II. The inference of total quantities from partial information on a cluster

Context. In a probabilistic framework of the interpretation of the initial mass function (IMF), the IMF cannot be arbitrarily normalized to the total mass, M, or number of stars, N, of the system. Hence, the inference of M and N when partial information about the studied system is available must be revised (i.e., the contribution to the total quantity cannot be obtained by simple algebraic manipulations of the IMF). Aims. We study how to include constraints in the IMF to make inferences about different quantities characterizing stellar systems. It is expected that including any particular piece of information about a system would constrain the range of possible solutions. However, different pieces of information might be irrelevant depending on the quantity to be inferred. In this work we want to characterize the relevance of the priors in the possible inferences. Methods. Assuming that the IMF is a probability distribution function, we derive the sampling distributions of M and N of the system constrained to different types of information available. Results. We show that the value of M that would be inferred must be described as a probability distribution ΦM[M;ma,Na, ΦN (N)] that depends on the completeness limit of the data, ma, the number of stars observed down to this limit, Na, and the prior hypothesis made on the distribution of the total number of stars in clusters, ΦN (N).

Variation of the ultraviolet extinction law across the Taurus-Auriga star-forming complex. A GALEX based study

The Taurus-Auriga molecular complex (TMC) is the main laboratory for the study of low-mass star formation. The density and properties of interstellar dust are expected to vary across the TMC. These variations trace important processes such as dust nucleation or the magnetic field coupling with the cloud. In this paper, we show how the combination of near ultraviolet (NUV) and infrared (IR) photometry can be used to derive the strength of the 2175u2009A bump and thus any enhancement in the abundance of small dust grains and polycyclic aromatic hydrocarbons in the dust grains size distribution. This technique is applied to the envelope of the TMC, mapped by the GALEXAll Sky Survey (AIS). Ultraviolet and IR photometric data have been retrieved from the GALEX-AIS and the 2MASS catalogues. NUV and K-band star counts have been used to identify the areas in the cloud envelope where the 2175u2009A bump is weaker than in the diffuse interstellar medium namely, the low column density extensions of L1495, L1498 and L1524 in Taurus, L1545, L1548, L1519, L1513 in Auriga and L1482-83 in the California region. This finding agrees with previous results on dust evolution derived from Spitzer data and suggests that dust grains begin to decouple from the environmental Galactic magnetic field already in the envelope.

MOSAIX: a tool to built large mosaics from GALEX images

Large sky surveys are providing a huge amount of information for studies of the interstellar medium, the galactic structure or the cosmic web. Setting into a common frame information coming from different wavelengths, over large fields of view, is needed for this kind of research. GALEX is the only nearly all-sky survey at ultraviolet wavelengths and contains fundamental information for all types of studies. GALEX field of view is circular embedded in a squared matrix of 3840×3840 pixels. This fact makes it hard to get GALEX images properly overlapped with the existing astronomical tools such as Aladin or Montage. We developed our own software for this purpose. In this article, we describe this software and makes it available to the community.