Final Evolution and Delayed Explosions of Spinning White Dwarfs in Single Degenerate Models for Type Ia Supernovae

We study the occurrence of delayed SNe~Ia in the single degenerate (SD) scenario. We assume that a massive carbon-oxygen (CO) white dwarf (WD) accretes matter coming from a companion star, making it to spin at the critical rate. We assume uniform rotation due to magnetic field coupling. The carbon ignition mass for non-rotating WDs is M_{ig}^{NR} \approx 1.38 M_{\odot}; while for the case of uniformly rotating WDs it is a few percent larger (M_{ig}^{R} \approx 1.43 M_{\odot}). When accretion rate decreases, the WD begins to lose angular momentum, shrinks, and spins up; however, it does not overflow its critical rotation rate, avoiding mass shedding. Thus, angular momentum losses can lead the CO WD interior to compression and carbon ignition, which would induce an SN~Ia. The delay, largely due to the angular momentum losses timescale, may be large enough to allow the companion star to evolve to a He WD, becoming undetectable at the moment of explosion. This scenario supports the occurrence of delayed SNe~Ia if the final CO WD mass is 1.38 M_{\odot} < M < 1.43 M_{\odot}. We also find that if the delay is longer than ~3 Gyr, the WD would become too cold to explode, rather undergoing collapse.Comment: 6 pages, 5 figures, published in the Astrophysical Journal Letters, 809, L6 (2015), added some corrections for errat

New evolutionary calculations for the born again scenario

We present evolutionary calculations to describe the born-again scenario for post-AGB remnant stars of 0.5842 and 0.5885 M⊙. Results are based on a detailed treatment of the physical processes responsible for the chemical abundance changes. We considered two theories of convection: the standard mixing length theory (MLT) and the double-diffusive GNA convection. The latter accounts for the effect of the chemical gradient (∇μ) in the mixing processes and in the transport of energy. We also explore the dependence of born-again evolution on some physical hypotheses, such as the effect of the existence of non-zero chemical gradients, the prescription for the velocity of the convective elements and the size of the overshooting zones. Attention is paid to the behavior of the born-again times and to the chemical evolution during the ingestion of protons. We find that in our calculations born again times are dependent on time resolution. In particular when the minimum allowed time step is below 5 × 10-5 yr we obtain, with the standard mixing length theory, born again times of 5-10 yr. This is true without altering the prescription for the efficiency of convective mixing during the proton ingestion. On the other hand we find that the inclusion of chemical gradients in the calculation of the mixing velocity tends to increase the born again times by about a factor of two. In addition we find that proton ingestion can be altered if the occurrence of overshooting is modified by the ∇μ-barrier at the H-He interface, significantly changing born again times.Facultad de Ciencias Astronómicas y Geofísica

The runaway binary LP 400-22 is leaving the Galaxy

We present optical spectroscopy, astrometry, radio and X-ray observations of the runaway binary LP 400-22. We refine the orbital parameters of the system based on our new radial velocity observations. Our parallax data indicate that LP 400-22 is significantly more distant (3σ lower limit of 840pc) than initially predicted. LP 400-22 has a tangential velocity in excessof 830 km s-1; it is unbound to the Galaxy. Our radio and X-ray observations fail to detect a recycled millisecond pulsar companion, indicating that LP 400-22 is a double white dwarf system. This essentially rules out a supernova runaway ejection mechanism. Based on its orbit, a Galactic Centre origin is also unlikely. However, its orbit intersects the locations of several globular clusters; dynamical interactions between LP 400-22 and other binary stars or a central black hole in a dense cluster could explain the origin of this unusual binary.Instituto de Astrofísica de La Plat

New evolutionary calculations for the born again scenario

We present evolutionary calculations to describe the born-again scenario for post-AGB remnant stars of 0.5842 and 0.5885 M⊙. Results are based on a detailed treatment of the physical processes responsible for the chemical abundance changes. We considered two theories of convection: the standard mixing length theory (MLT) and the double-diffusive GNA convection. The latter accounts for the effect of the chemical gradient (∇μ) in the mixing processes and in the transport of energy. We also explore the dependence of born-again evolution on some physical hypotheses, such as the effect of the existence of non-zero chemical gradients, the prescription for the velocity of the convective elements and the size of the overshooting zones. Attention is paid to the behavior of the born-again times and to the chemical evolution during the ingestion of protons. We find that in our calculations born again times are dependent on time resolution. In particular when the minimum allowed time step is below 5 × 10-5 yr we obtain, with the standard mixing length theory, born again times of 5-10 yr. This is true without altering the prescription for the efficiency of convective mixing during the proton ingestion. On the other hand we find that the inclusion of chemical gradients in the calculation of the mixing velocity tends to increase the born again times by about a factor of two. In addition we find that proton ingestion can be altered if the occurrence of overshooting is modified by the ∇μ-barrier at the H-He interface, significantly changing born again times.Facultad de Ciencias Astronómicas y Geofísica

Evolutionary and pulsational properties of low-mass white dwarf stars with oxygen cores resulting from close binary evolution

The present work is designed to explore the evolutionary and pulsational properties of low-mass white dwarfs with carbon/oxygen cores. In particular, we follow the evolution of a 0.33-M⊙ white dwarf remnant in a self-consistent way with the predictions of nuclear burning, element diffusion and the history of the white dwarf progenitor. Attention is focused on the occurrence of hydrogen shell flashes induced by diffusion processes during cooling phases. The evolutionary stages prior to the white dwarf formation are also fully accounted for by computing the conservative binary evolution of an initially 2.5-M⊙ Population I star with a 1.25-M ⊙ companion, and with period Pi = 3d. Evolution is followed down to the domain of the ZZ Ceti stars on the white dwarf cooling branch. We find that chemical diffusion induces the occurrence of an additional hydrogen thermonuclear flash, which leads to stellar models with thin hydrogen envelopes. As a result, a fast cooling is encountered at advanced stages of evolution. In addition, we explore the adiabatic pulsational properties of the resulting white dwarf models. As compared with their helium-core counterparts, low-mass oxygen-core white dwarfs are characterized by a pulsational spectrum much more featured, an aspect which could eventually be used for distinguishing both types of stars, if low-mass white dwarfs were in fact found to pulsate as ZZ Ceti-type variables. Finally, we perform a non-adiabatic pulsational analysis on the resulting carbon/oxygen low-mass white dwarf models.Facultad de Ciencias Astronómicas y GeofísicasInstituto de Astrofísica de La Plat

The embedded clusters DBS 77, 78, 102, and 160-161, and their link with the interstellar medium

Aims. We report on a study of the global properties of some embedded clusters that are found in the fourth quadrant of the Milky Way. We aim to clarify some issues that relate to their location in the Galaxy and their stellar formation processes. Methods. We performed BVI photometric observations in the region of DBS 77, 78, 102, 160, and 161 clusters, and infrared spectroscopy in the DBS 77 region. These were complemented with JHK data from the VVV survey combined with the 2MASS catalogue, and used mid-infrared information from the GLIMPSE catalogue. We also searched for HI data from the SGPS and PMN radio surveys, and previous spectroscopic stellar classifications. The spectroscopic and photometric information enabled us to estimate the spectral classification of the brightest stars for each of the studied regions. On the other hand, we used radio data to investigate the interstellar material parameters and the continuum sources that are probably associated with the respective stellar components. Results. We estimated the basic physical parameters of the clusters (reddening, distance, age, and initial mass function). We searched for HII regions located near to the studied clusters and we analyzed the possible links between them. In the particular case of the DBS 160-161 clusters, we identified the HI bubble B332.5-0.1-42 that is located around them. We found that the mechanical energy injected in to the interstellar medium by the more massive stars of this couple of clusters was enough to generate the bubble.Facultad de Ciencias Astronómicas y GeofísicasInstituto de Astrofísica de La PlataInstituto Argentino de Radioastronomí

Evolutionary and pulsational properties of low-mass white dwarf stars with oxygen cores resulting from close binary evolution

The present work is designed to explore the evolutionary and pulsational properties of low-mass white dwarfs with carbon/oxygen cores. In particular, we follow the evolution of a 0.33-M⊙ white dwarf remnant in a self-consistent way with the predictions of nuclear burning, element diffusion and the history of the white dwarf progenitor. Attention is focused on the occurrence of hydrogen shell flashes induced by diffusion processes during cooling phases. The evolutionary stages prior to the white dwarf formation are also fully accounted for by computing the conservative binary evolution of an initially 2.5-M⊙ Population I star with a 1.25-M ⊙ companion, and with period Pi = 3d. Evolution is followed down to the domain of the ZZ Ceti stars on the white dwarf cooling branch. We find that chemical diffusion induces the occurrence of an additional hydrogen thermonuclear flash, which leads to stellar models with thin hydrogen envelopes. As a result, a fast cooling is encountered at advanced stages of evolution. In addition, we explore the adiabatic pulsational properties of the resulting white dwarf models. As compared with their helium-core counterparts, low-mass oxygen-core white dwarfs are characterized by a pulsational spectrum much more featured, an aspect which could eventually be used for distinguishing both types of stars, if low-mass white dwarfs were in fact found to pulsate as ZZ Ceti-type variables. Finally, we perform a non-adiabatic pulsational analysis on the resulting carbon/oxygen low-mass white dwarf models.Facultad de Ciencias Astronómicas y GeofísicasInstituto de Astrofísica de La Plat

Vinculacion entre varios cúmulos estelares y estructuras del medio interestelar

Se estudiaron los cúmulos inmersos DBS 77, 78, 102, 160 y 161 localizados en el plano Galáctico en el cuarto cuadrante de la Vía Láctea y el medio interestelar (MIE) circundante. Se analizó fotometría UBVIc (SOAR) y espectroscopía infrarroja (NTT, ESO). Estos datos fueron complementados con las bandas JHK (VVV+2MASS), H I en 21 cm (SGPS), 1.4 GHz (ATCA) y 4.85 GHz (PMN). Se realizó un análisis multibanda y clasificación espectral de las estrellas brillantes de cada zona. Se identificaron también, estructuras del MIE posiblemente vinculadas con los cúmulos. Finalmente, se obtuvieron valores preliminares para los parámetros fundamentales de los cúmulos estudiados y de las estructuras del MIE. Se estudió la vinculación entre ambosWe study the embedded clusters DBS77, 78, 102, 160, and 161 located in the Galactic plane in the fourth quadrant of the Milky Way and the surrounding interstellar medium (ISM). We analyzed UBVIc photometry (SOAR) and infrared spectroscopy (NTT, ESO). We complemented these data with JHK (VVV2MASS), HI 21 cm bands (SGPS), 1.4 GHz (ATCA), and 4.85 GHz (PMN). We did multiband analysis and spectral classification of the brightest stars in each area. We also identified the ISM structures possibly related to the clusters. Finally, we obtained the main parameters of the studied clusters, the structures of the ISM and the link between them.Instituto de Astrofísica de La Plat

Vinculacion entre varios cúmulos estelares y estructuras del medio interestelar

Se estudiaron los cúmulos inmersos DBS 77, 78, 102, 160 y 161 localizados en el plano Galáctico en el cuarto cuadrante de la Vía Láctea y el medio interestelar (MIE) circundante. Se analizó fotometría UBVIc (SOAR) y espectroscopía infrarroja (NTT, ESO). Estos datos fueron complementados con las bandas JHK (VVV+2MASS), H I en 21 cm (SGPS), 1.4 GHz (ATCA) y 4.85 GHz (PMN). Se realizó un análisis multibanda y clasificación espectral de las estrellas brillantes de cada zona. Se identificaron también, estructuras del MIE posiblemente vinculadas con los cúmulos. Finalmente, se obtuvieron valores preliminares para los parámetros fundamentales de los cúmulos estudiados y de las estructuras del MIE. Se estudió la vinculación entre ambosWe study the embedded clusters DBS77, 78, 102, 160, and 161 located in the Galactic plane in the fourth quadrant of the Milky Way and the surrounding interstellar medium (ISM). We analyzed UBVIc photometry (SOAR) and infrared spectroscopy (NTT, ESO). We complemented these data with JHK (VVV2MASS), HI 21 cm bands (SGPS), 1.4 GHz (ATCA), and 4.85 GHz (PMN). We did multiband analysis and spectral classification of the brightest stars in each area. We also identified the ISM structures possibly related to the clusters. Finally, we obtained the main parameters of the studied clusters, the structures of the ISM and the link between them.Instituto de Astrofísica de La Plat

Vinculacion entre varios cúmulos estelares y estructuras del medio interestelar

Se estudiaron los cúmulos inmersos DBS 77, 78, 102, 160 y 161 localizados en el plano Galáctico en el cuarto cuadrante de la Vía Láctea y el medio interestelar (MIE) circundante. Se analizó fotometría UBVIc (SOAR) y espectroscopía infrarroja (NTT, ESO). Estos datos fueron complementados con las bandas JHK (VVV+2MASS), H I en 21 cm (SGPS), 1.4 GHz (ATCA) y 4.85 GHz (PMN). Se realizó un análisis multibanda y clasificación espectral de las estrellas brillantes de cada zona. Se identificaron también, estructuras del MIE posiblemente vinculadas con los cúmulos. Finalmente, se obtuvieron valores preliminares para los parámetros fundamentales de los cúmulos estudiados y de las estructuras del MIE. Se estudió la vinculación entre ambosWe study the embedded clusters DBS77, 78, 102, 160, and 161 located in the Galactic plane in the fourth quadrant of the Milky Way and the surrounding interstellar medium (ISM). We analyzed UBVIc photometry (SOAR) and infrared spectroscopy (NTT, ESO). We complemented these data with JHK (VVV2MASS), HI 21 cm bands (SGPS), 1.4 GHz (ATCA), and 4.85 GHz (PMN). We did multiband analysis and spectral classification of the brightest stars in each area. We also identified the ISM structures possibly related to the clusters. Finally, we obtained the main parameters of the studied clusters, the structures of the ISM and the link between them.Instituto de Astrofísica de La Plat