arXiv subjects

J. Maíz Apellániz

Publications and source records attributed to J. Maíz Apellániz.

138 records · Page 8Linked to original sources

arXiv2008

IMF biases created by binning and unresolved systems

I discuss two of the possible sources of biases in the determination of the IMF: binning and the existence of unresolved components. The first source is important for clusters with a small number of stars detected in a given mass bin while the second one is relevant for all clusters located beyond the immediate solar neighborhood. For both cases I will present results of numerical simulations and I will discuss strategies to correct for their effects. I also present a brief description of a third unrelated bias source.

astro-ph↗

arXiv2008

Biases on initial mass function determinations. II. Real multiple systems and chance superpositions

When calculating IMFs for young clusters, one has to take into account that (a) most massive stars are born in multiple systems (b) most IMFs are derived from data that cannot resolve such systems, and (c) multiple chance superpositions between members are expected to happen if the cluster is too distant. In this article I use numerical experiments to model the consequences of those phenomena on the observed color-magnitude diagrams and the IMFs derived from them. Real multiple systems affect the observed or apparent massive-star MF slope little but can create a significant population of apparently ultramassive stars. Chance superpositions produce only small biases when the number of superimposed stars is low but, once a certain number threshold is reached, they can affect both the observed slope and the apparent stellar upper mass limit. I apply those experiments to two well known massive young clusters in the Local Group, NGC 3603 and R136. In both cases I show that the observed population of stars with masses above 120 solar masses can be explained by the effects of unresolved objects, mostly real multiple systems for NGC 3603 and a combination of real and chance-alignment multiple systems for R136. Therefore, the case for the reality of a stellar upper mass limit at solar or near-solar metallicities is strengthened, with a possible value even lower than 150 solar masses. An IMF slope somewhat flatter than Salpeter or Kroupa with gamma between -1.6 and -2.0 is derived for the central region of NGC 3603, with a significant contribution to the uncertainty arising from the imprecise knowledge of the distance to the cluster. The IMF at the very center of R136 cannot be measured with the currently available data but the situation could change with new HST observations. (abridged)

astro-ph↗

arXiv2007

The Galactic O Star Catalog V.2.0

The Galactic O star catalog (GOS) is an ambitious project to provide as much information regarding these types of objects as possible. The first version of the catalog (GOS v1) included data for 378 stars with precise spectral classification. It was intended to be complete up to V < 8, but also included many stars fainter than that limit. In this new version, we include a second list with more than 700 stars that have sometimes been classified as O stars. The catalog includes cross-references to other catalogs, spectral classification (various references for each star on the second list), coordinates (obtained in most cases from Tycho-2 data, but some from 2MASS and USNO B1), astrometric distances for some of the nearer stars, Optical and NIR photometry (Tycho-2, Johnson, Stromgren, Cousin and 2MASS), group membership, runaway character, and multiplicity information.

astro-ph↗

arXiv2007

A uniform set of optical/NIR photometric zero points to be used with CHORIZOS

I have recently combined HST/STIS spectrophotometry with existing photometric data to analyze the calibration of three standard optical photometry systems: Tycho-2 B_TV_T, Stromgren uvby, and Johnson UBV. In this contribution I summarize those results, present new ones for 2MASS JHK_s, and combine them with recent literature results to generate a uniform set of zero points for six photometric systems, the above mentioned plus Cousins RI and SDSS ugriz. With the exception of the latter system, the zero points use the new Vega spectrum presented at this meeting by Ralph Bohlin. I also discuss the implementation of these results in CHORIZOS, a Bayesian photometric code that compares multi-filter observational data with spectral energy distributions to solve the inverse problem of finding the models which are compatible with the observations.

astro-ph↗

arXiv2007

The stellar upper mass limit in the solar neighborhood

We are using HST GO programs 10205, 10602, and 10898 to test the stellar upper mass limit in the solar vicinity by attempting to detect optical close companions, thus lowering the calculated evolutionary masses. We have observed with ACS/HRC all the known (as of early 2005) Galactic O2/3/3.5 stars. We also have observations with HST/FGS and ground-based spectroscopy from LCO and CASLEO. Here we discuss our results for Pismis 24 and HD 93129A.

astro-ph↗

arXiv2007

Towards using optical/NIR photometry to measure the temperature of O stars

It has been traditionally stated that it is not possible to use optical/NIR photometry to measure the temperatures of O stars. In this contribution we describe the steps required to overcome the hurdles that have prevented this from happening in the past and we present our preliminary results for the low-extinction case.

astro-ph↗

arXiv2007

Pismis 24-1: The Stellar Upper Mass Limit Preserved

Is there a stellar upper mass limit? Recent statistical work seems to indicate that there is and that it is in the vicinity of 150 solar masses. In this paper we use HST and ground-based data to investigate the brightest members of the cluster Pismis 24, one of which (Pismis 24-1) was previously inferred to have a mass greater than 200 solar masses, in apparent disagreement with that limit. We determine that Pismis 24-1 is composed of at least three objects, the resolved Pismis 24-1SW and the unresolved spectroscopic binary Pismis 24-1NE. The evolutionary zero-age masses of Pismis 24-1SW, the unresolved system Pismis 24-1NE, and the nearby star Pismis 24-17 are all approximately 100 solar masses, very large but under the stellar upper mass limit.

astro-ph↗

arXiv2005

A recalibration of optical photometry: Tycho-2, Strömgren, and Johnson systems

I use HST spectrophotometry to analyze the calibration of 3 optical photometry systems: Tycho-2 B_T V_T, Strömgren u v b y, and Johnson U B V. For Tycho-2, I revisit the analysis of Maíz Apellániz (2005) to include the new recalibration of grating/aperture corrections, vignetting, and charge transfer inefficiency effects produced by the STIS group and to consider the consequences of both random and systematic uncertainties. The new results reaffirm the good quality of both the Tycho-2 photometry and the HST spectrophotometry, but yield a slightly different value for ZP_B_T-V_T. For the Strömgren v, b, and y and the Johnson B and V filters I find that the published sensitivity curves are consistent with the available photometry and spectrophotometry and I derive new values for the associated ZP_b-y, ZP_m_1, and ZP_B-V. The situation is different for the Strömgren u and the Johnson U filters. There I find that the published sensitivity curves yield results that are inconsistent with the available photometry and spectrophotometry, likely caused by an incorrect treatment of atmospheric effects in the short-wavelength end. I reanalyze the data to produce new average sensitivity curves for those two filters and new values for ZP_c_1 and ZP_U-B. The new computation of synthetic U-B and B-V colors uses a single B sensitivity curve, which eliminates the previous unphysical existence of two different definitions. Finally, I find that if one uses values from the literature where uncertainties are not given, reasonable estimates for these are 1-2% for Strömgren b-y, m_1, and c_1 and 2-3% for Johnson B-V and U-B. The use of the results in this article should lead to a significant reduction of systematic errors when comparing synthetic photometry models with real colors and indices.

astro-ph↗

arXiv2005

Deriving stellar properties from photometry: maximizing information content and minimizing biases

I study the importance of the accurate calibration of photometric systems in order to produce meaningful comparisons between the observed colors + magnitudes and model SEDs. Possible sources of errors are discussed and two examples are analyzed. I show that well-calibrated Tycho-2 photometry is stable and precise enough for such comparisons. On the contrary, the available calibrations for Johnson UBV photometry yield relative large systematic errors, which has prompted me to develop a new, more precise calibration. The advantages of multicolor photometry over the standard single-color + magnitude diagrams for the derivation of physical properties of stars (elimination of degeneracies, inclusion of multiple parameters, avoidance of linearizing approximations, possibility of a more precise treatment of errors) are discussed through the use of CHORIZOS, a code developed specifically for this purpose.

astro-ph↗

arXiv2005

IMF biases and how to correct them

We discuss possible sources of biases in the determination of the initial mass function (IMF) introduced by the binning of the data, the uncertainty in the determinations of masses, and the existence of unresolved multiple systems. Those three effects tend to produce IMFs that are flatter than the real one. We analyze the importance of each effect and suggest techniques that minimize or eliminate the biases. We also report the detection of the first astrometric binary system composed of two very-early O-type stars, HD 93129 A.

astro-ph↗

arXiv2005

Numerical biases on IMF determinations created by binning

We detect and quantify significant numerical biases in the determination of the slope of power laws with Salpeter (or similar) indices from uniformly-binned data using chi-square minimization. The biases are caused by the correlation between the number of stars per bin and the assigned weights and are especially important when the number of stars per bin is small. This result implies the existence of systematic errors in the values of IMFs calculated in this way. We propose as an alternative using variable-size bins and dividing the stars evenly among them. Such variable-size bins yield very small biases that are only weakly dependent on the number of stars per bin. Furthermore, we show that they allow for the calculation of reliable IMFs with only a small total number of stars. Therefore, they are a preferred alternative to the standard uniform-size binning.

astro-ph↗

arXiv2004

Self-consistent distance determinations for Lutz-Kelker-limited samples

We present a method designed to correct for Lutz-Kelker effects in distance-limited samples. The method allows for the calculation of distances to individual objects and, at the same time, provides a fit to a parameterized, self-consistent spatial distribution of the population. An example using Hipparcos data is presented and the relevance to Gaia is also discussed.

astro-ph↗