We present far-ultraviolet (FUV) spectroscopy obtained with HST for 48 blue objects in the core of 47 Tuc. Based on their position in a FUV- optical color-magnitude diagram, these were expected to ...include cataclysmic variables (CVs), blue stragglers (BSs), white dwarfs (WDs), and other exotic objects. For a subset of these sources, we also construct FUV-NIR spectral energy disributions. Based on our analysis of this extensive data set, we report the following main results. (1) We spectroscopically confirm 3 previously known or suspected CVs via the detection of emission lines and find new evidence for dwarf nova eruptions in two of these. (2) Only one other source in our spectroscopic sample exhibits marginal evidence for line emission, but predicted and observed CV numbers still agree to within a factor of about 2-3. (3) We have discovered a hot (image K), low-mass (image M sub(image)) secondary star in a previously known 0.8 day binary system. This exotic object is probably the remnant of a subgiant that has been stripped of its envelope and may represent the 'smoking gun' of a recent dynamical encounter. (4) We have found a helium WD, the second to be optically detected in 47 Tuc, and the first outside a millisecond-pulsar system. (5) We have discovered a BS-WD binary system, the first known in any globular cluster. (6) We have found two additional candidate WD binary systems with putative main sequence and subgiant companions. (7) We estimate the WD binary fraction in the core of 47 Tuc to be image (stat) image (sys). (8) One BS in our sample may exceed twice the cluster turnoff mass, but the uncertainties are large. Taken as a whole, our study illustrates the wide range of stellar exotica that are lurking in the cores of GCs, most of which are likely to have undergone significant dynamical encounters.
ABSTRACT
We present an optical analysis of 55 members of R136, the central cluster in the Tarantula Nebula of the Large Magellanic Cloud. Our sample was observed with STIS aboard the Hubble Space ...Telescope, is complete down to about 40 M⊙, and includes seven very massive stars with masses over 100 M⊙. We performed a spectroscopic analysis to derive their physical properties. Using evolutionary models, we find that the initial mass function of massive stars in R136 is suggestive of being top-heavy with a power-law exponent γ ≈ 2 ± 0.3, but steeper exponents cannot be excluded. The age of R136 lies between 1 and 2 Myr with a median age of around 1.6 Myr. Stars more luminous than log L/L⊙ = 6.3 are helium enriched and their evolution is dominated by mass-loss, but rotational mixing or some other form of mixing could be still required to explain the helium composition at the surface. Stars more massive than 40 M⊙ have larger spectroscopic than evolutionary masses. The slope of the wind–luminosity relation assuming unclumped stellar winds is 2.41 ± 0.13 which is steeper than usually obtained (∼1.8). The ionizing ($\log Q_0\, {\rm ph/s} = 51.4$) and mechanical ($\log L_{\rm SW}\, {\rm erg/s} = 39.1$) output of R136 is dominated by the most massive stars ($\gt 100\, \mathrm{ M}_{\odot }$). R136 contributes around a quarter of the ionizing flux and around a fifth of the mechanical feedback to the overall budget of the Tarantula Nebula. For a census of massive stars of the Tarantula Nebula region, we combined our results with the VLT-FLAMES Tarantula Survey plus other spectroscopic studies. We observe a lack of evolved Wolf–Rayet stars and luminous blue and red supergiants.
I discuss two of the possible sources of biases in the determination of the stellar initial mass function: 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 is relevant for all clusters located beyond the immediate solar neighborhood. For both cases I present results of numerical simulations, and I discuss strategies to correct for their effects. I also present a brief description of a third unrelated bias source.
Abstract
The accuracy in determining the spatial-kinematical parameters of open clusters makes them ideal tracers of the Galactic structure. Young open clusters (YOCs) are the main representatives of ...the clustered star formation mode, which identifies how most of the stars in the Galaxy form. We apply the Kriging technique to a sample of Gaia YOCs within a 3.5 kpc radius around the Sun and log(age) ≤ 7.5, as the age in years, to obtain
Z
(
X
,
Y
) and
V
Z
(
X
,
Y
) maps. Previous work by Alfaro et al. has shown that Kriging can provide reliable results even with small data samples (
N
∼ 100). We approach the 3D spatial and vertical velocity field structure of the Galactic disk defined by YOCs and analyze the hierarchy of the stellar cluster formation, which shows a rich hierarchical structure, displaying complexes embedded within each other. We discuss the fundamental characteristics of the methodology used to perform the mapping and point out the main results obtained in phenomenological terms. Both the 3D spatial distribution and the vertical velocity field reveal a complex disk structure with a high degree of substructures. Their analysis provides clues about the main physical mechanisms that shape the phase space of the clustered star formation in this Galactic area. Warp, corrugations, and high local deviations in
Z
and
V
Z
appear to be intimately connected, in a single but intricate scenario.
ABSTRACT Optical spectroscopic monitoring has been conducted of two O stars in the SMC and one in the LMC, the spectral characteristics of which place them in the Of?p category, which has been ...established in the Galaxy to consist of oblique magnetic rotators. All of these Magellanic stars show systematic spectral variations typical of the Of?p class, further strengthening their magnetic candidacy to the point of virtual certainty. The spectral variations are related to photometric variations derived from Optical Gravitational Lensing Experiment data by Nazé et al. in a parallel study, which yields rotational periods for two of them. Now circular spectropolarimetry is required to measure their fields, and ultraviolet spectroscopy to further characterize their low-metallicity, magnetically confined winds, in support of hydrodynamical analyses.
We show that star formation in the giant H II region NGC 5471 has been ongoing during the past 100 Myr. Using Hubble Space Telescope/Wide-Field Planetary Camera 2 F547M and F675W, ground-based JHK s ..., and GALEX FUV and NUV images, we have conducted a photometric study of the star formation history (SFH) in the massive giant extragalactic H II region NGC 5471 in M101. We perform a photometric study of the color-magnitude diagram (CMD) of the resolved stars and an integrated analysis of the main individual star-forming clusters and of NGC 5471 as a whole. The integrated UV-optical-NIR photometry for the whole region provides two different reference ages, 8 Myr and 60 Myr, revealing a complex SFH, clearly confirmed by the CMD-resolved stellar photometry analysis. The spatial distribution of the stars shows that the star formation in NGC 5471 has proceeded along the whole region during, at least, the last 100 Myr. The current ionizing clusters are enclosed within a large bubble, which is likely to have been produced by the stars that formed in a major event ~20 Myr ago.
ABSTRACT On the basis of the Galactic O Star Spectroscopic Survey (GOSSS), we present a detailed systematic investigation of the O Vz stars. The currently used spectral classification criteria are ...rediscussed, and the Vz phenomenon is recalibrated through the addition of a quantitative criterion based on the equivalent widths of the He iλ4471, He iiλ4542, and He iiλ4686 spectral lines. The GOSSS O Vz and O V populations resulting from the newly adopted spectral classification criteria are comparatively analyzed. The locations of the O Vz stars are probed, showing a concentration of the most extreme cases toward the youngest star-forming regions. The occurrence of the Vz spectral peculiarity in a solar-metallicity environment, as predicted by the fastwind code, is also investigated, confirming the importance of taking into account several processes for the correct interpretation of the phenomenon.
ABSTRACT Extremely broad emission wings at Hβ and H have been found in VLT-FLAMES Tarantula Survey data for five very luminous BA supergiants in or near 30 Doradus in the Large Magellanic Cloud. The ...profiles of both lines are extremely asymmetrical, which we have found to be caused by very broad diffuse interstellar bands (DIBs) in the longward wing of Hβ and the shortward wing of H . These DIBs are well known to interstellar but not to many stellar specialists, so that the asymmetries may be mistaken for intrinsic features. The broad emission wings are generally ascribed to electron scattering, although we note difficulties for that interpretation in some objects. Such profiles are known in some Galactic hyper/supergiants and are also seen in both active and quiescent Luminous Blue Variables (LBVs). No prior or current LBV activity is known in these 30 Dor stars, although a generic relationship to LBVs is not excluded; subject to further observational and theoretical investigation, it is possible that these very luminous supergiants are approaching the LBV stage for the first time. Their locations in the HRD and presumed evolutionary tracks are consistent with that possibility. The available evidence for spectroscopic variations of these objects is reviewed, while recent photometric monitoring does not reveal variability. A search for circumstellar nebulae has been conducted, with an indeterminate result for one of them.
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