The IACOB project

Abstract

Context. The IACOB and OWN surveys are two ambitious, complementary observational projects which have made available a large multi-epoch spectroscopic database of optical high resolution spectra of Galactic massive O-type stars. Aims. Our aim is to study the full sample of (more than 350) O stars surveyed by the IACOB and OWN projects. As a first step towards this aim, we have performed the quantitative spectroscopic analysis of a subsample of 128 stars included in the modern grid of O-type standards for spectral classification. The sample comprises stars with spectral types in the range O3–O9.7 and covers all luminosity classes. Methods. We used the semi-automatized IACOB-BROAD and IACOB-GBAT/FASTWIND tools to determine the complete set of spectroscopic parameters that can be obtained from the optical spectrum of O-type stars. A quality flag was assigned to the outcome of the IACOB-GBAT/FASTWIND analysis for each star, based on a visual evaluation of how the synthetic spectrum of the best fitting FASTWIND model reproduces the observed spectrum. We also benefitted from the multi-epoch character of the IACOB and OWN surveys to perform a spectroscopic variability study of the complete sample, providing two different flags for each star accounting for spectroscopic binarity as well as variability of the main wind diagnostic lines. Results. We obtain – for the first time in a homogeneous and complete manner – the full set of spectroscopic parameters of the “anchors” of the spectral classification system in the O star domain. We provide a general overview of the stellar and wind parameters of this reference sample, as well as updated recipes for the SpT–Teff and SpT–log g calibrations for Galactic O-type stars. We also propose a distance-independent test for the wind-momentum luminosity relationship. We evaluate the reliability of our semi-automatized analysis strategy using a subsample of ~40 stars extensively studied in the literature, and find a fairly good agreement between our derived effective temperatures and gravities and those obtained by means of more traditional “by-eye” techniques and different stellar atmosphere codes. The overall agreement between the synthetic spectra associated with the IACOB-GBAT/FASTWIND best fitting models and the observed spectra is good for most of the analyzed targets, but 46 stars out of the 128 present a particular behavior of the wind diagnostic lines that cannot be reproduced by our grid of spherically symmetric unclumped models. These are potential targets of interest for more detailed investigations of clumpy winds and/or the existence of additional circumstellar emitting components contaminating the wind diagnostic lines (e.g., disks, magnetospheres). Last, our spectroscopic variability study has led to the detection of clear or likely signatures of spectroscopic binarity in 27% of the stars and small amplitude radial velocity variations in the photospheric lines of another 30%. Additionally, 31% of the investigated stars show variability in the wind diagnostic lines.

Angular Sizes and Effective Temperatures of O-type Stars from Optical Interferometry with the CHARA Array

“…Both spectroscopic observations and our interferometric results arrive at relatively high effective temperatures, 40.0 kK and 35.5 kK, respectively, that are well above the 31.9 kK temperature associated with its classification of O9 V (Martins et al 2005). It has been noted in other studies Holgado et al 2018) that the T eff values from the calibrations in Martins et al (2005) are too low for O9 V stars. However, the extended minimum valley in the χ 2 ν diagram (Fig.…”

“…In Section 4, we gather distance estimates in order to transform the angular diameters into linear radii, and we compare the angular size derived from interferometry with that from model fits of the continuum flux for published values of effective temperature. Bouret et al (2008) · · · · · · · · · 29.5, 3.25 · · · · · · CMFGEN Herrero et al (2002) · · · · · · · · · · · · · · · 35.5, 3.95 FASTWIND Holgado et al (2018) · · · 29.4, 2.90 35.2, 3.50 · · · · · · 35.2, 3.90 IACOB- GBAT Marcolino et al (2009) · · · · · · · · · · · · 32.0, 3.60 · · · TLUSTY Markova et al (2004) 34.0, 3.35 31.0, 3.19 33.6, 3.56 · · · · · · · · · FASTWIND Martins et al (2012) · · · · · · · · · 29.5, 3.25 · · · · · · CMFGEN Martins et al (2015) 34.0, 3.60 29.5, 3.25 35.0, 3.75 · · · 31.0, 3.60 35.0, 4.05 CMFGEN Martins et al (2017) 33.5, 3.50 · · · 35.0, 3.75 · · · · · · · · · CMFGEN Mokiem et al (2005) · · · · · · · · · · · · 32.1, 3.62 36.0, 4.03 FASTWIND Najarro et al (2011) · · · 28.9, 3.01 34.5, 3.70 · · · · · · · · · CMFGEN Puls et al (2006) 35.0, 3.50 29.0, 3.00 33.6, 3.56 · · · · · · · · · FASTWIND Repolust et al (2004) 34.0, 3.50 29.0, 2.97 · · · · · · 32.0, 3.65 · · · FASTWIND Repolust et al (2005) · · · 29.0, 2.88 · · · · · · 33.5, 3.85 · · · FASTWIND Simón- Díaz et al (2006) · · · · · · · · · · · · · · · 36.0, 3.90 FASTWIND (Hummel et al 2013);d -Hummel et al (2013); e -Typical values for a B0 III star (Hummel et al 2013).…”

Effective temperature of ionizing stars in extragalactic H iiregions – II. Nebular parameter relationships based on CALIFA data

“…2 shows the values of the a and b coefficients as a function of Z/Z ⊙ and log U. In this figure, the values of the coefficients are calculated interpolating the relations given in It is worth to mention that the expected maximum effective temperature of a young stellar cluster is ∼ 50 kK (e.g., Martins et al 2005;Simón-Díaz et al 2014;Tramper et al 2014;Walborn et al 2014;Wright et al 2015;Crowther et al 2016;Martins & Palacios 2017;Holgado et al 2018) Meanwhile, the T eff − R relation can be applied only for T eff < = 40 kK because, for higher T eff values, small variations of R produce extremely large uncertainties in T eff estimations (Dors et al 2017). Kennicutt et al (2000) derived a calibration between T eff and the He IIλ5876/Hβ and He IIλ6678/Hβ emission-line ratios, and they also pointed out a similar difficulty in deriving effective temperature values higher than ∼40kK.…”

Toward Early-type Eclipsing Binaries as Extragalactic Milestones. II. NLTE Spectral Analysis and Stellar Parameters of the Detached O-type System OGLE-LMC-ECL-06782 in the LMC

“…Stellar winds modify the outer atmospheric layers of hot massive stars through their outflow velocity field, which affects the formation of stronger spectral lines through Doppler shifts and a modified more extended mass density stratification, which causes additional wind emission (see Kudritzki & Puls 2000 for a review). It is, therefore, important to constrain the strengths of stellar wind line emission, which in the case of optical hydrogen and helium lines depends on the parameter Q ∼Ṁ (R v ∞ ) −3/2 , whereṀ and v ∞ are the stellar wind mass-loss rate and the terminal velocity of the wind outflow, respectively (see Puls et al 2005;Holgado et al 2018). Terminal velocities are usually constrained through the observation of UV resonance lines of highly ionized metal lines, which show so-called P-Cygni profiles.…”