Gemini GMOS spectroscopy of HeII nebulae in M 33

Kehrig, C.; Oey, M. S.; Crowther, P. A.; Fogel, Jeffrey K. J.; Pellegrini, E.; Schnurr, O.; Massey, Philip; Roth, Kathy

doi:10.1051/0004-6361/201015493

Cited by 54 publications

(70 citation statements)

References 94 publications

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“…As with the BHGs, LPV is present and observed in the Hβ line of J013341.28+302237.2 and the He i lines of J013416.07+303642.1 6 . Discovery and subsequent follow up spectra of both stars extend the baseline of observations to ≥17 yr for the former (Monteverde et al 1996, Ma07) and ≥18 yr for the latter (Spiller 1992;Corral 1996;Kehrig et al 2011), but provide no evidence for (LBV driven) secular evolution during these intervals. For completeness we note that Urbaneja et al (2005) prefer a classification of B1 Ia + for J013341.28+302237.2 (=[HS80] 110A) which they find to be a highly luminous object with a high mass loss rate in comparison to Galactic examples (Clark et al 2012); we return to this point in Sect.…”

Section: P Cygni Starsmentioning

confidence: 68%

On the nature of candidate luminous blue variables in M 33

Clark

Castro

García

et al. 2012

A&A

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Context. Luminous blue variables (LBVs) are a class of highly unstable stars that have been proposed to play a critical role in massive stellar evolution as well as being the progenitors of some of the most luminous supernovae known. However the physical processes underlying their characteristic instabilities are currently unknown. Aims. In order to provide observational constraints on this behaviour we have initiated a pilot study of the population of (candidate) LBVs in the Local Group galaxy M 33. Methods. To accomplish this we have obtained new spectra of 18 examples within M 33. These provide a baseline of ≥4 yr with respect to previous observations, which is well suited to identifying LBV outbursts. We also employed existing multi-epoch optical and mid-IR surveys of M 33 to further constrain the variability of the sample and search for the presence of dusty ejecta. Results. Combining the datasets reveals that spectroscopic and photometric variability appears common, although in the majority of cases further observations will be needed to distinguish between an origin for this behavour in short lived stochastic wind structure and low level photospheric pulsations or coherent long term LBV excursions. Of the known LBVs we report a hitherto unidentified excursion of M 33 Var C between 2001-5, while the transition of the WNLh star B517 to a cooler B supergiant phase between 1993−2010 implies an LBV classification. Proof-of-concept quantitative model atmosphere analysis is provided for Romano's star; the resultant stellar parameters being consistent with the finding that the LBV excursions of this star are accompanied by changes in bolometric luminosity. The combination of temperature and luminosity of two stars, the BHG [HS80] 110A and the cool hypergiant B324, appear to be in violation of the empirical Humphreys-Davidson limit. Mid-IR observations demonstrate that a number of candidates appear associated with hot circumstellar dust, although no objects as extreme as η Car are identified. The combined dataset suggests that the criteria employed to identify candidate LBVs results in a heterogeneous sample, also containing stars demonstrating the B[e] phenomenon. Of these, a subset of optically faint, low luminosity stars associated with hot dust are of particular interest since they appear similar to the likely progenitor of SN 2008S and the 2008 NGC 300 transient (albeit suffering less intrinsic extinction). Conclusions. The results of such a multiwavelength observational approach, employing multiplexing spectrographs and supplemented with quantitative model atmosphere analysis, appears to show considerable promise in both identifying and characterising the physical properties of LBVs as well as other short lived phases of massive stellar evolution.

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Section: P Cygni Starsmentioning

confidence: 68%

On the nature of candidate luminous blue variables in M 33

Clark

Castro

García

et al. 2012

A&A

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“…This temperature is higher than measured in other H  regions with He  emission, making S3 the hottest H  region currently known in the Local Group (see Kehrig et al 2011, for an overview of known He  nebulae). This electron temperature is indicative for both the high temperature of the ionizing source, and the low-metallicity environment.…”

Section: Nebular Spectrummentioning

confidence: 75%

On the nature of WO stars: a quantitative analysis of the WO3 star DR1 in IC 1613

Tramper

Gräfener

Hartoog

et al. 2013

A&A

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Context. Oxygen sequence Wolf-Rayet (WO) stars are thought to represent the final evolutionary stage of the most massive stars. The characteristic strong O  emission possibly originates from an enhanced oxygen abundance in the stellar wind. Alternatively, the O  emission can be caused by the high temperature of these stars, in which case the WO stars are the high-temperature extension of the more common carbon sequence Wolf-Rayet (WC) stars. Aims. By constraining the physical properties and evolutionary status of DR1, a WO star in the low-metallicity Local Group dwarf galaxy IC 1613 and one of only two objects of its class known in a SMC-like metallicity environment, we aim to investigate the nature of WO stars and their evolutionary connection with WC stars. Methods. We use the non-local thermodynamic equilibrium atmosphere code  to model the observed spectrum of DR1 and to derive its stellar and wind parameters. We compare our values with other studies of WC and WO stars, as well as with the predictions of evolutionary models. We also model the surrounding nebula using the photo-ionization code . Results. The oxygen and carbon abundances that we obtain are comparable to values found for WC stars. The temperature and luminosity are, however, higher than those of WC stars. DR1 is embedded in the hottest known H  region in the Local Group. The nebular properties can be consistently reproduced by  models adopting DR1 as central ionizing source. Conclusions. Comparison of the abundances and temperature of DR1 with core helium-burning models show that DR1 is currently well into the second half of helium burning. If the properties of DR1 are representative for the WO class, it would imply that WO stars are the high-temperature and high-luminosity extension of the WC stars, and do not necessarily represent a later evolutionary stage.

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“…Although no convincing explanation for the origin of the He II emission has been found so far, these objects have indeed similar properties to our narrow He II emitters, and we can not exclude that they are all powered by the same mechanism. The fact that these objects are quite rare in the local Universe (Kehrig et al 2011;Shirazi & Brinchmann 2012) and more frequent in our sample at high redshift (3-5% depending on the possible He II class, see below) may simply reflect the evolution of the star-formation rate density of the Universe (that is at its peak around the median redshift of our galaxies) and the evolution of the average metallicity of the Universe.…”

Section: Discussionmentioning

confidence: 87%

“…For many of these regions the spectra show the features typical of W-R stars, which are thought to be the source of the strong ionizing continuum (see Sect. 4.2), but some are not clearly associated with W-R stars, nor with O-B stars (Kehrig et al 2011).…”

Section: Peculiar Stellar Populationsmentioning

confidence: 91%

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He II emitters in the VIMOS VLT Deep Survey: Population III star formation or peculiar stellar populations in galaxies at 2 <z< 4.6?

Cassata

Fèvre

Charlot

et al. 2013

A&A

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Aims. The aim of this work is to identify He II emitters at 2 < z < 4.6 and to constrain the source of the hard ionizing continuum that powers the He II emission. Methods. We assembled a sample of 277 galaxies with a highly reliable spectroscopic redshift at 2 < z < 4.6 from the VIMOS-VLT Deep Survey (VVDS) Deep and Ultra-Deep data, and we identified 39 He II λ1640 emitters. We studied their spectral properties, measuring the fluxes, equivalent widths (EW), and full width at half maximum (FWHM) for most relevant lines, including He II λ1640, Lyα line, Si II λ1527, and C IV λ1549. Results. About 10% of galaxies at z ∼ 3 and i AB ≤ 24.75 show He II in emission, with rest frame equivalent widths EW 0 ∼ 1-7 Å, equally distributed between galaxies with Lyα in emission or in absorption. We find 11 (3.9% of the global population) reliable He II emitters with unresolved He II lines (FWHM 0 < 1200 km s −1 ), 13 (4.6% of the global population) reliable emitters with broad He II emission (FWHM 0 > 1200 km s −1 ), 3 active galactic nuclei (AGN), and an additional 12 possible He II emitters. The properties of the individual broad emitters are in agreement with expectations from a Wolf-Rayet (W-R) model. Instead, the properties of the narrow emitters are not compatible with this model, nor with predictions of gravitational cooling radiation produced by gas accretion, unless this is severely underestimated by current models by more than two orders of magnitude. Rather, we find that the EW of the narrow He II line emitters are in agreement with expectations for a Population III (PopIII) star formation, if the episode of star formation is continuous, and we calculate that a PopIII star formation rate (SFR) of 0.1-10 M yr −1 alone is enough to sustain the observed He II flux. Conclusions. We conclude that narrow He II emitters are powered either by the ionizing flux from a stellar population rare at z ∼ 0 but much more common at z ∼ 3, or by PopIII star formation. As proposed by Tornatore and collaborators, incomplete interstellar medium mixing may leave some small pockets of pristine gas at the periphery of galaxies from which PopIII may form, even down to z ∼ 2 or lower. If this interpretation is correct, we measure at z ∼ 3 a star formation rate density in PopIII stars of 10 −6 M yr −1 Mpc −3 , higher than, but qualitatively comparable to the value predicted by Tornatore and collaborators.

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Gemini GMOS spectroscopy of HeII nebulae in M 33

Cited by 54 publications

References 94 publications

On the nature of candidate luminous blue variables in M 33

On the nature of candidate luminous blue variables in M 33

On the nature of WO stars: a quantitative analysis of the WO3 star DR1 in IC 1613

He II emitters in the VIMOS VLT Deep Survey: Population III star formation or peculiar stellar populations in galaxies at 2 <z< 4.6?

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