arXiv subjects

Y. Götberg

Publications and source records attributed to Y. Götberg.

22 records · Page 2Linked to original sources

arXiv2019

The young massive SMC cluster NGC 330 seen by MUSE. I. Observations and stellar content

A majority of massive stars are part of binary systems, a large fraction of which will inevitably interact during their lives. Binary-interaction products (BiPs), i.e. stars affected by such interaction, are expected to be commonly present in stellar populations. BiPs are thus a crucial ingredient in the understanding of stellar evolution. We aim to identify and characterize a statistically significant sample of BiPs by studying clusters of 10-40 Myr, an age at which binary population models predict the abundance of BiPs to be highest. One example of such a cluster is NGC 330 in the Small Magellanic Cloud. Using MUSE WFM-AO observations of NGC 330, we resolve the dense cluster core for the first time and are able to extract spectra of its entire massive star population. We develop an automated spectral classification scheme based on the equivalent widths of spectral lines in the red part of the spectrum. We characterize the massive star content of the core of NGC 330 which contains more than 200 B stars, 2 O stars, 6 A-type supergiants and 11 red supergiants. We find a lower limit on the Be star fraction of $32 \pm 3\%$ in the whole sample. It increases to at least $46 \pm 10\%$ when only considering stars brighter than V=17 mag. We estimate an age of the cluster core between 35 and 40 Myr and a total cluster mass of $88^{+17}_{-18} \times 10^3 M_{\odot}$. We find that the population in the cluster core is different than the population in the outskirts: while the stellar content in the core appears to be older than the stars in the outskirts, the Be star fraction and the observed binary fraction are significantly higher. Furthermore, we detect several BiP candidates that will be subject of future studies.

astro-ph.SR↗

arXiv2019

The impact of stars stripped in binaries on the integrated spectra of stellar populations

Stars stripped of their envelopes from interaction with a binary companion emit a significant fraction of their radiation as ionizing photons. They are potentially important stellar sources of ionizing radiation, however, they are still often neglected in spectral synthesis simulations or simulations of stellar feedback. We modeled the radiative contribution from stripped stars by using detailed evolutionary and spectral models. We estimated their impact on the integrated spectra and specifically on the emission rates of HI-, HeI-, and HeII-ionizing photons from stellar populations. We find that stripped stars have the largest impact on the ionizing spectrum of a population in which star formation halted several Myr ago. In such stellar populations, stripped stars dominate the emission of ionizing photons, mimicking a younger stellar population in which massive stars are still present. Our models also suggest that stripped stars have harder ionizing spectra than massive stars. The additional ionizing radiation affects observable properties that are related to the ionizing emission from stellar populations. In co-eval stellar populations, the ionizing radiation from stripped stars increases the ionization parameter and the production efficiency of HI-ionizing photons. They also cause high values for these parameters for about ten times longer than what is predicted for massive stars. The hard ionizing radiation from stripped stars likely introduces a characteristic ionization structure of the nebula, which leads to the emission of highly ionized elements such as O$^{2+}$ and C$^{3+}$. We, therefore, expect that the presence of stripped stars affects the location in the BPT diagram and the diagnostic ratio of OIII to OII nebular emission lines. Our models are publicly available through CDS database and on the Starburst99 website.

astro-ph.GA↗

arXiv2019

Massive runaways and walkaway stars

Anticipating the kinematic constraints from the Gaia mission, we perform an extensive numerical study of the evolution of massive binary systems to predict the peculiar velocities that stars obtain when their companion collapses and disrupts the system. Our aim is to (1) identify which predictions are robust against model uncertainties and assess their implications, (2) investigate which physical processes leave a clear imprint and may therefore be constrained observationally and (3) provide a suite of publicly available model predictions. We find that $22_{-8}^{+26}$% of all massive binary systems merge prior to the first core collapse in the system. Of the remainder, $86_{-9}^{+11}$% become unbound because of the core-collapse. Remarkably, this rarely produce runaway stars (i.e., stars with velocities above 30 km/s). These are outnumbered by more than an order of magnitude by slower unbound companions, or "walkaway stars". This is a robust outcome of our simulations and is due to the reversal of the mass ratio prior to the explosion and widening of the orbit, as we show analytically and numerically. We estimate a $10^{+5}_{-8}$% of massive stars to be walkaways and only $0.5^{+1.0}_{-0.4}$% to be runaways, nearly all of which have accreted mass from their companion. Our findings are consistent with earlier studies, however the low runaway fraction we find is in tension with observed fractions 10%. If Gaia confirms these high fractions of massive runaway stars resulting from binaries, it would imply that we are currently missing physics in the binary models. Finally, we show that high end of the mass distributions of runaway stars is very sensitive to the assumed black hole natal kicks and propose this as a potentially stringent test for the explosion mechanism. We discuss companions remaining bound which can evolve into X-ray and gravitational wave sources.

astro-ph.SR↗

arXiv2018

Spectral models for binary products: Unifying Subdwarfs and Wolf-Rayet stars as a sequence of stripped-envelope stars

Stars stripped of their envelope through interaction in a binary are generally not considered when accounting for ionizing radiation from stellar populations, despite the expectation that stripped stars emit hard ionizing radiation, form frequently and live 10-100 times longer than single massive stars. We compute the first grid of spectral models specially made for stars stripped in binaries for a range of progenitor masses (2-20$M_{\odot}$) and metallicities covering a wide range. For stripped stars with masses between 0.3-7$M_{\odot}$, we find high effective temperatures (20-100 kK, increasing with mass), small radii (0.2-1$R_{\odot}$) and high bolometric luminosities, comparable to that of their progenitor before stripping. The spectra show a continuous sequence that naturally bridges subdwarf-type stars and Wolf-Rayet like spectra. For intermediate masses we find hybrid spectral classes showing a mixture of absorption and emission lines. These appear for stars with mass loss rates of $10^{-8}-10^{-6}M_{\odot}$/yr, which have semi-transparent atmospheres. At low metallicity, substantial H-rich layers are left at the surface and we predict spectra that resemble O-type stars instead. We obtain spectra undistinguishable from subdwarfs for stripped stars with masses up to 1.7$M_{\odot}$, which questions whether the widely adopted canonical value of 0.47$M_{\odot}$ is uniformly valid. Increasing the observed sample of stripped stars will provide necessary tests for the physics of interaction, internal mixing and stellar winds. We investigate the feasibility to detect stripped stars next to an optically bright companion and recommend searches for their UV excess and possible emission lines, most notably HeII4686 in the optical and HeII1640 in the UV. Our models are publicly available for further investigations or inclusion in spectral synthesis simulations.

astro-ph.SR↗