We derived atmospheric parameters and spectroscopic abundances for C and O for a large sample of stars located in the Hertzsprung gap in the Hertzsprung-Russell Diagram in order to detect chemical peculiarities and get a comprehensive overview of the population of stars in this evolutionary state. We have observed and analyzed high resolution spectra (R = 60 000) of 188 stars in the mass range 2 - 5 Msun with the 2.7 m Harlan J. Smith Telescope at the McDonald Observatory including 28 stars previously identified as Am/Ap stars. We find that the C and O abundances of the majority of stars in the Hertzsprung gap are in accordance with abundances derived for local lower mass dwarfs but detect expected peculiarities for the Am/Ap stars. The C and O abundances of stars with Teff < 6500 K are slightly lower than for the hotter objects but the C/O ratio is constant in the analyzed temperature domain. No indication of an alteration of the C and O abundances of the stars by mixing during the evolution across the Hertzsprung gap could be found before the homogenization of their atmospheres by the first dredge-up.
(Abridged abstract) We explore the formation of ultra-massive (M_{\rm WD} \gtrsim 1.05 M_\sun$), carbon-oxygen core white dwarfs resulting from single stellar evolution. We also study their evolutionary and pulsational properties and compare them with those of the ultra-massive white dwarfs with oxygen-neon cores resulting from carbon burning in single progenitor stars, and with binary merger predictions. We consider two single-star evolution scenarios for the formation of ultra-massive carbon-oxygen core white dwarfs that involve rotation of the degenerate core after core helium burning and r…
Ultra-massive white dwarfs (UMWDs) with masses larger than 1.05Msun are basically believed to harbour oxygen-neon (ONe) cores. Recently, Gaia data reveals an enhancement of UMWDs on Hertzsprung-Russell diagram (HRD), which indicates that extra cooling delay mechanism such as crystallization and elemental sedimentation may exist in the UMWDs. Further studies suggested that a portion of UMWDs should have experienced pretty long cooling delays, implying that they are carbon-oxygen (CO) WDs. However, the formation mechanism of these UMCOWDs is still under debate. In this work, we investigated whet…
Carbon and oxygen isotopic ratios are reported for a sample of 51 SRb- and Lb-type variable asymptotic giant branch stars. Vibration-rotation first- and second-overtone CO lines in 1.5-2.5 mum spectra were measured to derive isotopic ratios for 12C/13C, 16O/17O, and 16O/18O. Comparisons with previous measurements for individual stars and with various samples of evolved stars, as available in the extant literature, are discussed. Using the oxygen isotopic ratios, the masses of the SRb stars can be derived. Combining the masses with Gaia luminosities, the SRb stars are shown to be antecedents of…
Carbon and oxygen are key tracers of the Galactic chemical evolution; in particular, a reported upturn in [C/O] towards decreasing [O/H] in metal-poor halo stars could be a signature of nucleosynthesis by massive Population III stars. We reanalyse carbon, oxygen, and iron abundances in thirty-nine metal-poor turn-off stars. For the first time, we take into account three-dimensional (3D) hydrodynamic effects together with departures from local thermodynamic equilibrium (LTE) when determining both the stellar parameters and the elemental abundances, by deriving effective temperatures from 3D non…
Context. We report the results from the determination of stellar masses, carbon and oxygen abundances in the atmospheres of 107 stars from the CHEPS program. Our stars are drawn from a population with a significantly super-solar metallicity. At least 10 of these stars are known to host orbiting planets. Aims. In this work, we set out to understand the behavior of carbon and oxygen abundance in stars with different spectral classes, metallicities and V sin i , within the metal-rich stellar population. Methods. Masses of these stars were determined using the data from Gaia DR2 release. The oxyge…
Stark Broadening of Carbon and Oxygen Lines in Hot DQ White Dwarf Stars: Recent Results and Applications
传统上，人们发现白矮星的表面成分主要是氢或氦。然而，最近发现了一个新的家族，即所谓的热DQ白矮星，其表面成分以碳和氧为主，几乎没有氢和氦（Dufour et al. 2007，2008，2010）。要推导出这些物体的精确大气参数（如有效温度和表面重力），就需要对谱线剖面进行详细的建模。斯塔克加宽参数在这方面至关重要。我们提出了初步的结果，从我们的新一代模型大气，包括最新的斯塔克加宽计算CII线，并讨论的影响，以及未来的工作，仍然需要做的。
Type Ia supernovae (SNe Ia) originate from the thermonuclear explosion of carbon-oxygen white dwarfs (CO WDs), giving rise to luminous optical transients. A relatively common variety of subluminous SNe Ia events, referred to as SNe Iax, are believed to arise from the failed detonation of a CO WD. In this paper, we explore failed detonation SNe Ia in the context of the double-degenerate channel of merging white dwarfs. In particular, we have carried out the first fully three-dimensional simulation of the merger of a ONe WD with a CO WD. While the hot, tidally-disrupted carbon-rich disk material…
On the origin of carbon- and oxygen-rich hot subdwarfs: an sdB merger scenario
Institute for Frontier in Astronomy and Astrophysics, Beijing Normal University（前沿天文学与天体物理研究所，北京师范大学）；School of Physics and Astronomy, Beijing Normal University（物理与天文学院，北京师范大学）
Carbon, nitrogen, and oxygen are the fourth, sixth, and third most abundant elements in the Sun. Their abundances remain hotly debated due to the so-called solar modelling problem that has persisted for almost $20$ years. We revisit this issue by presenting a homogeneous analysis of $408$ molecular lines across $12$ diagnostic groups, observed in the solar intensity spectrum. Using a realistic 3D radiative-hydrodynamic model solar photosphere and LTE (local thermodynamic equilibrium) line formation, we find $\logε_{C} = 8.47\pm0.02$, $\logε_{N} = 7.89\pm0.04$, and $\logε_{O} = 8.70\pm0.04$. Th…
Motivated by the possible existence of other universes, this paper considers the evolution of massive stars with different values for the fundamental constants. We focus on variations in the triple alpha resonance energy and study its effects on the resulting abundances of $^{12}$C, $^{16}$O, and larger nuclei. In our universe, the $0^{+}$ energy level of carbon supports a resonant nuclear reaction that dominates carbon synthesis in stellar cores and accounts for the observed cosmic abundances. Here we define $Δ{E}_R$ to be the change in this resonant energy level, and show how different value…
The carbon-oxygen white dwarf (CO WD) + He star channel is one of the promising ways for producing type Ia supernovae (SNe Ia) with short delay times. Recent studies found that carbon under the He-shell can be ignited if the mass-accretion rate of CO WD is higher than a critical rate (about 2 * 10^-6 Msun/yr), triggering an inwardly propagating carbon flame. Previous studies usually supposed that the off-centre carbon flame would reach the centre, resulting in the formation of an oxygen-neon (ONe) WD that will collapse into a neutron star. However, the process of off-centre carbon burning is n…
We have computed a grid of hydrostatic spherical COMARCS models for C stars covering metallicities from [Z/H]=0 to -2 and values of the carbon excess [C-O] from 6.41 to 9.15, plus some temperature sequences, where the amount of oxygen and nitrogen is increased relative to a scaled solar element mixture. Such abundance variations may appear during the late stages of stellar evolution. Our study covers changes of [O/Z] and [N/Z] going up to +0.5. Based on the atmospheric structures we have calculated synthetic spectra and photometry for all of the models in a consistent way. The sequences with c…
Abundances of lithium, carbon, and oxygen have been derived using spectral synthesis for a sample of 249 bright F, G, and K Northern Hemisphere dwarf stars from the high-resolution spectra acquired with the VUES spectrograph at the Moletai Astronomical Observatory of Vilnius University. The sample stars have metallicities, effective temperatures, and ages between -0.7 and 0.4 dex; 5000 and 6900 K; 1 and 12 Gyr, accordingly. We confirm a so far unexplained lithium abundance decrease at supersolar metallicities - $A$(Li) in our sample stars, which drop by 0.7 dex in the [Fe/H] range from +0.10 t…
The abundances of carbon, oxygen, and iron in late-type stars are important parameters in exoplanetary and stellar physics, as well as key tracers of stellar populations and Galactic chemical evolution. We carried out three-dimensional (3D) non-LTE radiative transfer calculations for CI and OI, and 3D LTE radiative transfer calculations for FeII, across the STAGGER-grid of 3D hydrodynamic model atmospheres. The absolute 3D non-LTE versus 1D LTE abundance corrections can be as severe as $-0.3$ dex for CI lines in low-metallicity F dwarfs, and $-0.6$ dex for OI lines in high-metallicity F dwarfs…
Oxygen is the most abundant "metal" element in stars and in the cosmos. But determining oxygen abundances in stars has proven challenging, because of the shortage of detectable atomic oxygen lines in their optical spectra as well as observational and theoretical complications with these lines (e.g., blends, 3D, non-LTE). Nonetheless, Ting et al. (2017) were recently able to demonstrate that oxygen abundances can be determined from low-resolution (R$\simeq$2000) optical spectra. Here we investigate the physical processes that enable such a measurement for cool stars, such as K-giants. We show t…
We explore the effects of the residual $\mathrm{^{12}C}$ present in oxygen-neon white dwarfs (ONe WDs) on their evolution towards accretion-induced collapse (AIC). We produce a set of ONe WD models using MESA and illustrate how the amount and location of the residual carbon depends on the initial mass of the star and assumptions about rotation and convective overshooting. We find a wide range of possible $\mathrm{^{12}C}$ mass fractions roughly ranging from 0.1 to 10 per cent. Convection and thermohaline mixing that occurs as the ONe WDs cool leads to nearly homogeneous interior compositions b…
Exploring the Milky Way stellar disk. Carbon, nitrogen, oxygen, sulphur, potassium, and copper abundances for 714 F- and G-type dwarf stars in the solar neighbourhood
The double-degenerate model, involving the merger of double carbon-oxygen white dwarfs (CO WDs), is one of the two classic models for the progenitors of type Ia supernovae (SNe Ia). Previous studies suggested that off-centre carbon burning would occur if the mass-accretion rate (Macc) is relatively high during the merging process, leading to the formation of oxygen-neon (ONe) cores that may collapse into neutron stars. However, the off-centre carbon burning is still incompletely understood, especially when the inwardly propagating burning wave reaches the centre. In this paper, we aim to inves…
The carbon-oxygen white dwarf (CO WD) + He star channel has been thought to be one of the promising scnarios to produce young type Ia supernovae (SNe Ia). Previous studies found that if the mass-accretion rate is greater than a critical value, the He-accreting CO WD will undergo inwardly propagating (off-centre) carbon ignition when it increases its mass close to the Chandrasekhar limit. The inwardly propagating carbon flame was supposed to reach the centre by previous works, leading to the production of an oxygen-neon (ONe) WD that may collapse into a neutron star but not an SN Ia. However, i…
Although multidimensional simulations have investigated the processes of double WD mergers, post-merger evolution only focused on the carbon-oxygen (CO) WD or helium (He) WD merger remnants. In this work, we investigate for the first time the evolution of the remnants stemmed from the merger of oxygen-neon (ONe) WDs with CO WDs. Our simulation results indicate that the merger remnants can evolve to hydrogen- and helium-deficient giants with maximum radius of about 300Rsun. Our models show evidence that merger remnants more massive than 1.95Msun can ignite Ne before significant mass-loss ensues…
Ultra-massive white dwarfs ($\rm M_{WD} \gtrsim 1.05\, M_{\odot}$) are considered powerful tools to study type Ia supernovae explosions, merger events, the occurrence of physical processes in the Super Asymptotic Giant Branch (SAGB) phase, and the existence of high magnetic fields. Traditionally, ultra-massive white dwarfs are expected to harbour oxygen-neon (ONe) cores. However, new observations and recent theoretical studies suggest that the progenitors of some ultra-massive white dwarfs can avoid carbon burning, leading to the formation of ultra-massive white dwarfs harbouring carbon-oxygen…
CONTEXT: [ABRIDGED]. For the Milky Way bulge, there are currently essentially no measurements of carbon in un-evolved stars, hampering our abilities to properly compare Galactic chemical evolution models to observational data for this still enigmatic stellar population. AIMS: We aim to determine carbon abundances for our sample of 91 microlensed bulge dwarf and subgiant stars. Together with new determinations for oxygen this forms the first statistically significant sample of bulge stars that have C and O abundances measured, and for which the C abundances have not been altered by the nuclear …
We construct evolutionary models of the remnant of the merger of two carbon-oxygen (CO) core white dwarfs (WDs). With total masses in the range $1-2 {\rm M_\odot}$, these remnants may either leave behind a single massive WD or undergo a merger-induced collapse to a neutron star (NS). On the way to their final fate, these objects generally experience a $\sim 10$ kyr luminous giant phase, which may be extended if sufficient helium remains to set up a stable shell-burning configuration. The uncertain, but likely significant, mass loss rate during this phase influences the final remnant mass and f…
Detections of SiO masers from the Bulge Asymmetries and Dynamical Evolution (BAaDE) survey more tightly define the region where Oxygen-rich (O) Asymptotic Giant Branch (AGB) stars reside in multiple infrared (IR) color-color diagrams. Using MSX and 2MASS data along with radio spectra from the BAaDE survey we find that three main populations were observed in the BAaDE survey: O-rich AGB stars of which about $73\%$ host SiO masers, Carbon-rich (C) AGB stars which do not host these masers, and a small contaminating set of possible Young Stellar Objects (YSOs). The distinction between YSOs and AGB…
Type Ia supernovae from violent mergers of carbon-oxygen white dwarfs: polarisation signatures
两颗碳氧白矮星的剧烈合并被认为是某些Ia型超新星的可能祖先。然而，有人认为，这种模型产生的强烈的喷出物不对称性可能与Ia型超新星爆炸中通常观察到的低偏振度不一致。在这里，我们通过对Pakmor等人（2012）提出的模型进行分光偏振分析来验证这一说法，该模型是在1.1 M $_{\odot}$和0.9 M $_{\odot}$碳氧白矮星双星系统合并期间触发的爆炸。由于喷出物的不对称性，偏振信号随视角显著变化。我们发现，在赤道平面的观察员的偏振水平是适度的（$\lesssim$ 1%），并显示出明确的证据，作为一个主导轴，作为一个后果的喷出物对称的轨道平面。相反，平面外的取向与更高程度的偏振和偏离主轴相关联。虽然这里研究的特定模型与SN 2004dt等高度极化事件有很好的匹配，但它很难再现正常Ia型超新星中常见的低极化水平。具体来说，我们发现，元素分布的显着不对称性导致了丰富的强偏振功能，没有观察到在大多数目前可用的光谱偏振数据的Ia型超新星。未来的研究将绘制出合并情景的参数空间，以调查替代模型是否可以提供与观测更好的一致性。
Molecular diagnostics for the mid-infrared emission of planet-forming disks. Carbon and oxygen elemental abundances
Kapteyn Astronomical Institute, Rijksuniversiteit Groningen（格罗宁根大学卡普坦天文研究所）；Leiden Observatory, Leiden University（莱顿大学莱顿天文台）；Max-Planck Institut für Extraterrestrische Physik (MPE)（马克斯·普朗克地外物理研究所）；Space Research Institute, Austrian Academy of Sciences（奥地利科学院空间研究所）；University Observatory, Faculty of Physics, Ludwig-Maximilians-Universität München（慕尼黑大学物理学院大学天文台）；Department of Physics and Astronomy, University of Exeter（埃克塞特大学物理与天文学系）；Department of Astronomy, University of Michigan（密歇根大学天文学系）；Université Paris-Saclay, CNRS, Institut d’Astrophysique Spatiale（巴黎萨克雷大学、法国国家科学研究中心、空间天体物理学研究所）
