The Experts below are selected from a list of 246 Experts worldwide ranked by ideXlab platform
Norbert Langer - One of the best experts on this subject based on the ideXlab platform.
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The Spectroscopic Hertzsprung–Russell Diagram of Hot Massive Stars in the Small Magellanic Cloud
The Astrophysical Journal, 2018Co-Authors: N. Castro, Luca Fossati, M. S. Oey, Norbert LangerAbstract:We present a comprehensive stellar atmosphere analysis of 329 O- and B-type stars in the Small Magellanic Cloud (SMC) from the RIOTS4 survey. Using spectroscopically derived effective temperature (Teff) and surface gravities, we find that classical Be stars appear misplaced to low Teff and high luminosity in the spectroscopic Hertzsprung-Russell Diagram (sHRD). Together with the most luminous stars in our sample, the stellar masses derived from the sHRD for these objects are systematically larger than those obtained from the conventional HRD. This suggests that the well-known, spectroscopic mass-discrepancy problem may be linked to the fact that both groups of stars have outer envelopes that are nearly gravitationally unbound. The non-emission-line stars in our sample mainly appear on the main-sequence, allowing a first estimate of the terminal-age main-sequence (TAMS) in the SMC, which matches the predicted TAMS between 12 and 40$\,$M$_{\odot}$ at SMC metallicity. We further find a large underabundance of stars above $\sim 25\,$M$_{\odot}$ near the ZAMS, reminiscent of such earlier findings in the Milky Way and LMC.
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The spectroscopic Hertzsprung-Russell Diagram of Galactic massive stars
Astronomy & Astrophysics, 2014Co-Authors: N. Castro, Norbert Langer, Luca Fossati, Sergio Simón-díaz, Fabian Schneider, Robert G. IzzardAbstract:The distribution of stars in the Hertzsprung-Russell Diagram narrates their evolutionary history and directly assesses their properties. Placing stars in this Diagram however requires the knowledge of their distances and interstellar extinctions, which are often poorly known for Galactic stars. The spectroscopic Hertzsprung-Russell Diagram (sHRD) tells similar evolutionary tales, but is independent of distance and extinction measurements. Based on spectroscopically derived effective temperatures and gravities of almost 600 stars, we derive for the first time the observational distribution of Galactic massive stars in the sHRD. While biases and statistical limitations in the data prevent detailed quantitative conclusions at this time, we see several clear qualitative trends. By comparing the observational sHRD with different state-of-the-art stellar evolutionary predictions, we conclude that convective core overshooting may be mass-dependent and, at high mass ($\geq 15\,M_\odot$), stronger than previously thought. Furthermore, we find evidence for an empirical upper limit in the sHRD for stars with $T_{\rm{eff}}$ between 10000 and 32000 K and, a strikingly large number of objects below this line. This over-density may be due to inflation expanding envelopes in massive main-sequence stars near the Eddington limit.
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The spectroscopic Hertzsprung-Russell Diagram
Astronomy & Astrophysics, 2014Co-Authors: Norbert Langer, R. P. KudritzkiAbstract:The Hertzsprung-Russell Diagram is an essential diagnostic Diagram for stellar structure and evolution, which has now been in use for more than 100 years. Our spectroscopic Hertzsprung-Russell (sHR) Diagram shows the inverse of the flux-mean gravity versus the effective temperature. Observed stars whose spectra have been quantitatively analyzed can be entered in this Diagram without the knowledge of the stellar distance or absolute brightness. Observed stars can be as conveniently compared to stellar evolution calculations in the sHR Diagram as in the Hertzsprung-Russell Diagram. However, at the same time, our ordinate is proportional to the stellar mass-to-luminosity ratio, which can thus be directly determined. For intermediate- and low-mass star evolution at constant mass, we show that the shape of an evolutionary track in the sHR Diagram is identical to that in the Hertzsprung-Russell Diagram. We also demonstrate that for hot stars, their stellar Eddington factor can be directly read off the sHR Diagram. For stars near their Eddington limit, we argue that a version of the sHR Diagram may be useful where the gravity is exchanged by the effective gravity. We discuss the advantages and limitations of the sHR Diagram, and show that it can be fruitfully applied to Galactic stars, but also to stars with known distance, e.g., in the LMC or in galaxies beyond the Local Group.
N. Castro - One of the best experts on this subject based on the ideXlab platform.
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mapping the core of the tarantula nebula with vlt muse ii the spectroscopic hertzsprung russell Diagram of ob stars in ngc 2070
arXiv: Solar and Stellar Astrophysics, 2021Co-Authors: N. Castro, P A Crowther, C J Evans, J S Vink, J Puls, A Herrero, M R Garcia, F Selman, Martin RothAbstract:We present the spectroscopic analysis of 333 OB-type stars extracted from VLT-MUSE observations of the central 30 x 30 pc of NGC 2070 in the Tarantula Nebula on the Large Magellanic Cloud, the majority of which are analysed for the the first time. The distribution of stars in the spectroscopic Hertzsprung-Russell Diagram (sHRD) shows 281 stars in the main sequence. We find two groups in the main sequence, with estimated ages of 2.1$\pm$0.8 and 6.2$\pm$2 Myr. A subgroup of 52 stars is apparently beyond the main sequence phase, which we consider to be due to emission-type objects and/or significant nebular contamination affecting the analysis. As in previous studies, stellar masses derived from the sHRD are systematically larger than those obtained from the conventional HRD, with the differences being largest for the most massive stars. Additionally, we do not find any trend between the estimated projected rotational velocity and evolution in the sHRD. The projected rotational velocity distribution presents a tail of fast rotators that resembles findings in the wider population of 30 Doradus. We use published spectral types to calibrate the HeI$\lambda$4921/HeII$\lambda$5411 equivalent-width ratio as a classification diagnostic for early-type main sequence stars when the classical blue-visible region is not observed. Our model-atmosphere analyses demonstrate that the resulting calibration is well correlated with effective temperature.
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The Spectroscopic Hertzsprung–Russell Diagram of Hot Massive Stars in the Small Magellanic Cloud
The Astrophysical Journal, 2018Co-Authors: N. Castro, Luca Fossati, M. S. Oey, Norbert LangerAbstract:We present a comprehensive stellar atmosphere analysis of 329 O- and B-type stars in the Small Magellanic Cloud (SMC) from the RIOTS4 survey. Using spectroscopically derived effective temperature (Teff) and surface gravities, we find that classical Be stars appear misplaced to low Teff and high luminosity in the spectroscopic Hertzsprung-Russell Diagram (sHRD). Together with the most luminous stars in our sample, the stellar masses derived from the sHRD for these objects are systematically larger than those obtained from the conventional HRD. This suggests that the well-known, spectroscopic mass-discrepancy problem may be linked to the fact that both groups of stars have outer envelopes that are nearly gravitationally unbound. The non-emission-line stars in our sample mainly appear on the main-sequence, allowing a first estimate of the terminal-age main-sequence (TAMS) in the SMC, which matches the predicted TAMS between 12 and 40$\,$M$_{\odot}$ at SMC metallicity. We further find a large underabundance of stars above $\sim 25\,$M$_{\odot}$ near the ZAMS, reminiscent of such earlier findings in the Milky Way and LMC.
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The spectroscopic Hertzsprung-Russell Diagram of Galactic massive stars
Astronomy & Astrophysics, 2014Co-Authors: N. Castro, Norbert Langer, Luca Fossati, Sergio Simón-díaz, Fabian Schneider, Robert G. IzzardAbstract:The distribution of stars in the Hertzsprung-Russell Diagram narrates their evolutionary history and directly assesses their properties. Placing stars in this Diagram however requires the knowledge of their distances and interstellar extinctions, which are often poorly known for Galactic stars. The spectroscopic Hertzsprung-Russell Diagram (sHRD) tells similar evolutionary tales, but is independent of distance and extinction measurements. Based on spectroscopically derived effective temperatures and gravities of almost 600 stars, we derive for the first time the observational distribution of Galactic massive stars in the sHRD. While biases and statistical limitations in the data prevent detailed quantitative conclusions at this time, we see several clear qualitative trends. By comparing the observational sHRD with different state-of-the-art stellar evolutionary predictions, we conclude that convective core overshooting may be mass-dependent and, at high mass ($\geq 15\,M_\odot$), stronger than previously thought. Furthermore, we find evidence for an empirical upper limit in the sHRD for stars with $T_{\rm{eff}}$ between 10000 and 32000 K and, a strikingly large number of objects below this line. This over-density may be due to inflation expanding envelopes in massive main-sequence stars near the Eddington limit.
R. P. Kudritzki - One of the best experts on this subject based on the ideXlab platform.
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The spectroscopic Hertzsprung-Russell Diagram
Astronomy & Astrophysics, 2014Co-Authors: Norbert Langer, R. P. KudritzkiAbstract:The Hertzsprung-Russell Diagram is an essential diagnostic Diagram for stellar structure and evolution, which has now been in use for more than 100 years. Our spectroscopic Hertzsprung-Russell (sHR) Diagram shows the inverse of the flux-mean gravity versus the effective temperature. Observed stars whose spectra have been quantitatively analyzed can be entered in this Diagram without the knowledge of the stellar distance or absolute brightness. Observed stars can be as conveniently compared to stellar evolution calculations in the sHR Diagram as in the Hertzsprung-Russell Diagram. However, at the same time, our ordinate is proportional to the stellar mass-to-luminosity ratio, which can thus be directly determined. For intermediate- and low-mass star evolution at constant mass, we show that the shape of an evolutionary track in the sHR Diagram is identical to that in the Hertzsprung-Russell Diagram. We also demonstrate that for hot stars, their stellar Eddington factor can be directly read off the sHR Diagram. For stars near their Eddington limit, we argue that a version of the sHR Diagram may be useful where the gravity is exchanged by the effective gravity. We discuss the advantages and limitations of the sHR Diagram, and show that it can be fruitfully applied to Galactic stars, but also to stars with known distance, e.g., in the LMC or in galaxies beyond the Local Group.
Jifeng Liu - One of the best experts on this subject based on the ideXlab platform.
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stellar x ray activity across the hertzsprung russell Diagram i catalogs
The Astrophysical Journal, 2020Co-Authors: Song Wang, Yu Bai, Jifeng LiuAbstract:Stellar magnetic activity provides substantial information on the magnetic dynamo and the coronal heating process. We present a catalog of X-ray activity for about 6000 stars, based on the $Chandra$ and $Gaia$ DR2 data. We also classified more than 3000 stars as young stellar objects, dwarf stars, or giant stars. By using the stars with valid stellar parameters and classifications, we studied the distribution of X-ray luminosity ($L_X$) and the ratio of X-ray-to-bolometric luminosities ($R_X$), the positive relation between $L_X$, $R_X$, and hardness ratio, and the long-term X-ray variation. This catalog can be used to investigate some important scientific topics, including the activity-rotation relation, the comparison between different activity indicators, and the activities of interesting objects (e.g., A-type stars and giants). As an example, we use the catalog to study the activity-rotation relation, and find that the young stellar objects, dwarfs, and giants fall on a single sequence in the relation $R_X$ versus Rossby number, while the giants do not follow the relation $R_X$ versus $P_{\rm rot}^{-2}R^{-4}$ valid for dwarfs.
Eduard I. Vorobyov - One of the best experts on this subject based on the ideXlab platform.
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On the episodic excursions of massive protostars in the Hertzsprung-Russell Diagram
Monthly Notices of the Royal Astronomical Society, 2019Co-Authors: D. M.-a. Meyer, Lionel Haemmerlé, Eduard I. VorobyovAbstract:Massive protostars grow and evolve under the effect of rapid accretion of circumstellar gas and dust, falling at high rates ($\ge 10^{-4}$-$10^{-3}\, \rm M_{\odot}\, \rm yr^{-1}$). This mass infall has been shown, both numerically and observationally, to be episodically interspersed by accretion of dense gaseous clumps migrating through the circumstellar disc to the protostellar surface, causing sudden accretion and luminous bursts. Using numerical gravito-radiation-hydrodynamics and stellar evolution calculations, we demonstrate that, in addition to the known bloating of massive protostars, variable episodic accretion further influences their evolutionary tracks of massive young stellar objects (MYSOs). For each accretion-driven flare, they experience rapid excursions toward more luminous, but colder regions of the Hertzsprung-Russell Diagram. During these excursions, which can occur up to the end of the pre-main-sequence evolution, the photosphere of massive protostars can episodically release much less energetic photons and MYSOs surreptitiously adopt the same spectral type as evolved massive (supergiants) stars. Each of these evolutionary loop brings the young high-mass stars close to the forbidden Hayashi region and might make their surrounding H II regions occasionally fainter, before they recover their quiescent, pre-burst surface properties. We interpret such cold, intermittent pre-main-sequence stellar evolutionary excursions and the dipping variability of HII regions as the signature of the presence of a fragmenting circumstellar accretion disc surrounding the MYSOs. We conjecture that this mechanism might equivalently affect young stars in the intermediate-mass regime.
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Episodic excursions of low-mass protostars on the Hertzsprung-Russell Diagram
Monthly Notices of the Royal Astronomical Society, 2018Co-Authors: Vardan G. Elbakyan, D. M.-a. Meyer, Eduard I. Vorobyov, Christian Rab, Manuel Güdel, Takashi Hosokawa, Harold W. YorkeAbstract:Following our recent work devoted to the effect of accretion on the pre-main-sequence evolution of low-mass stars, we perform a detailed analysis of episodic excursions of low-mass protostars in the Hertzsprung-Russell (H-R) Diagram triggered by strong mass accretion bursts typical of FU Orionis-type objects (FUors). These excursions reveal themselves as sharp increases in the stellar total luminosity and/or effective temperature of the protostar and can last from hundreds to a few thousands of years, depending on the burst strength and characteristics of the protostar. During the excursions, low-mass protostars occupy the same part of the H-R Diagram as young intermediate-mass protostars in the quiescent phase of accretion. Moreover, the time spent by low-mass protostars in these regions is on average a factor of several longer than that spent by the intermediate-mass stars in quiescence. During the excursions, low-mass protostars pass close to the position of most known FUors in the H-R Diagram, but owing to intrinsic ambiguity the model stellar evolutionary tracks are unreliable in determining the FUor properties. We find that the photospheric luminosity in the outburst state may dominate the accretion luminosity already after a few years after the onset of the outburst, meaning that the mass accretion rates of known FUors inferred from the bolometric luminosity may be systematically overestimated, especially in the fading phase.