AT 2018lqh and the Nature of the Emerging Population of Day-scale Duration Optical Transients

Abstract We report on the discovery of AT 2018lqh (ZTF 18abfzgpl)—a rapidly evolving extragalactic transient in a star-forming host at 242 Mpc. The transient g-band light curve’s duration above a half-maximum light is about 2.1 days, where 0.4/1.7 days are spent on the rise/decay, respectively. The estimated bolometric light curve of this object peaked at about 7 × 1042erg s−1—roughly 7 times brighter than the neutron star (NS)–NS merger event AT 2017gfo. We show that this event can be explained by an explosion with a fast (v ∼ 0.08 c) low-mass (≈0.07 M ⊙) ejecta, composed mostly of radioactive elements. For example, ejecta dominated by 56Ni with a timescale of t 0 ≅ 1.6 days for the ejecta to become optically thin for γ-rays fits the data well. Such a scenario requires burning at densities that are typically found in the envelopes of neutron stars or the cores of white dwarfs. A combination of circumstellar material (CSM) interaction power at early times and shock cooling at late times is consistent with the photometric observations, but the observed spectrum of the event may pose some challenges for this scenario. We argue that the observations are not consistent with a shock breakout from a stellar envelope, while a model involving a low-mass ejecta ramming into low-mass CSM cannot explain both the early- and late-time observations.

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The Formation of Binary Stars

Proceedings of the International Astronomical Union ◽

10.1017/s1743921307004279 ◽

2006 ◽

Vol 2 (S240) ◽

pp. 337-346

Author(s):

C.J. Clarke

Keyword(s):

Binary Stars ◽

Binary Star ◽

Vital Role ◽

Physical Processes ◽

Multiple Systems ◽

Star Forming ◽

Circumstellar Material ◽

Low Mass ◽

Body Level ◽

Hydrodynamical Simulations

AbstractI argue that binary star statistics offer the best observational constraints on current hydrodynamical simulations of star forming clusters. In these simulations, clusters form hierarchically from the bottom up, and dynamical interactions, mediated by the presence of circumstellar material, play a vital role at the lowest (few body) level of the hierarchy. Such a scenario produces a rich array of complex multiple systems whose properties are in many respects consistent with observations. I however highlight two areas of current disagreement: the simulations over-produce low mass single stars and under-produce binaries with low mass ratios. It is currently unclear to what extent these shortcomings reflect numerical issues and to what extent the omission of relevant physical processes. I conclude with a theorist's wish list for observational diagnostics that would most meaningfully constrain future modeling efforts.

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Gravitational Collapse versus Thermonuclear Explosion of Degenerate Stellar Cores

International Astronomical Union Colloquium ◽

10.1017/s0252921100026373 ◽

1994 ◽

Vol 147 ◽

pp. 186-213

Author(s):

J. Isern ◽

R. Canal

Keyword(s):

Neutron Star ◽

White Dwarfs ◽

Light Curves ◽

Type Ia Supernovae ◽

Radioactive Elements ◽

Border Line ◽

Thermonuclear Explosion ◽

Type Ia ◽

Γ Rays ◽

Ia Supernovae

AbstractIn this paper we review the behavior of growing stellar degenerate cores. It is shown that ONeMg white dwarfs and cold CO white dwarfs can collapse to form a neutron star. This collapse is completely silent since the total amount of radioactive elements that are expelled is very small and a burst of γ-rays is never produced. In the case of an explosion (always carbonoxygen cores), the outcome fits quite well the observed properties of Type Ia supernovae. Nevertheless, the light curves and the velocities measured at maximum are very homogeneous and the diversity introduced by igniting at different densities is not enough to account for the most extreme cases observed. It is also shown that a promising way out of this problem could be the He-induced detonation of white dwarfs with different masses. Finally, we outline that the location of the border line which separetes explosion from collapse strongly depends on the input physics adopted.

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Very Low-Luminosity Objects in Star-Forming Regions

Highlights of Astronomy ◽

10.1017/s1539299600021626 ◽

1998 ◽

Vol 11 (1) ◽

pp. 423-424

Author(s):

Motohide Tamura ◽

Yoichi Itoh ◽

Yumiko Oasa ◽

Alan Tokunaga ◽

Koji Sugitani

Keyword(s):

Brown Dwarfs ◽

Mass Function ◽

T Tauri Stars ◽

Initial Mass Function ◽

Formation Mechanisms ◽

Star Forming ◽

Star Forming Regions ◽

T Tauri ◽

Low Mass ◽

Stellar Sources

Abstract In order to tackle the problems of low-mass end of the initial mass function (IMF) in star-forming regions and the formation mechanisms of brown dwarfs, we have conducted deep infrared surveys of nearby molecular clouds. We have found a significant population of very low-luminosity sources with IR excesses in the Taurus cloud and the Chamaeleon cloud core regions whose extinction corrected J magnitudes are 3 to 8 mag fainter than those of typical T Tauri stars in the same cloud. Some of them are associated with even fainter companions. Follow-up IR spectroscopy has confirmed for the selected sources that their photospheric temperature is around 2000 to 3000 K. Thus, these very low-luminosity young stellar sources are most likely very low-mass T Tauri stars, and some of them might even be young brown dwarfs.

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The study of multipeaked type-I X-ray bursts in the neutron star low-mass X-ray binary 4U 1636 − 536 with RXTE

Monthly Notices of the Royal Astronomical Society ◽

10.1093/mnras/staa3539 ◽

2020 ◽

Vol 501 (1) ◽

pp. 168-178

Author(s):

Chen Li ◽

Guobao Zhang ◽

Mariano Méndez ◽

Jiancheng Wang ◽

Ming Lyu

Keyword(s):

Neutron Star ◽

Flux Ratio ◽

Light Curves ◽

Type I ◽

X Ray ◽

Soft State ◽

Peak Flux ◽

Low Mass ◽

Two Peaks ◽

Second Peak

ABSTRACT We have found and analysed 16 multipeaked type-I bursts from the neutron-star low-mass X-ray binary 4U 1636 − 53 with the Rossi X-ray Timing Explorer (RXTE). One of the bursts is a rare quadruple-peaked burst that was not previously reported. All 16 bursts show a multipeaked structure not only in the X-ray light curves but also in the bolometric light curves. Most of the multipeaked bursts appear in observations during the transition from the hard to the soft state in the colour–colour diagram. We find an anticorrelation between the second peak flux and the separation time between two peaks. We also find that in the double-peaked bursts the peak-flux ratio and the temperature of the thermal component in the pre-burst spectra are correlated. This indicates that the double-peaked structure in the light curve of the bursts may be affected by enhanced accretion rate in the disc, or increased temperature of the neutron star.

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A Machine Learning Approach For Classifying Low-mass X-ray Binaries Based On Their Compact Object Nature

Monthly Notices of the Royal Astronomical Society ◽

10.1093/mnras/staa3899 ◽

2020 ◽

Author(s):

R Pattnaik ◽

K Sharma ◽

K Alabarta ◽

D Altamirano ◽

M Chakraborty ◽

...

Keyword(s):

Machine Learning ◽

Black Hole ◽

Neutron Star ◽

Binary Systems ◽

Compact Object ◽

X Ray ◽

Average Accuracy ◽

Machine Learning Approach ◽

Low Mass ◽

X Ray Binaries

Abstract Low Mass X-ray binaries (LMXBs) are binary systems where one of the components is either a black hole or a neutron star and the other is a less massive star. It is challenging to unambiguously determine whether a LMXB hosts a black hole or a neutron star. In the last few decades, multiple observational works have tried, with different levels of success, to address this problem. In this paper, we explore the use of machine learning to tackle this observational challenge. We train a random forest classifier to identify the type of compact object using the energy spectrum in the energy range 5-25 keV obtained from the Rossi X-ray Timing Explorer archive. We report an average accuracy of 87±13% in classifying the spectra of LMXB sources. We further use the trained model for predicting the classes for LMXB systems with unknown or ambiguous classification. With the ever-increasing volume of astronomical data in the X-ray domain from present and upcoming missions (e.g., SWIFT, XMM-Newton, XARM, ATHENA, NICER), such methods can be extremely useful for faster and robust classification of X-ray sources and can also be deployed as part of the data reduction pipeline.

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Nuclear burning in collapsar accretion discs

Monthly Notices of the Royal Astronomical Society ◽

10.1093/mnras/staa3002 ◽

2020 ◽

Vol 499 (3) ◽

pp. 4097-4113 ◽

Cited By ~ 1

Author(s):

Yossef Zenati ◽

Daniel M Siegel ◽

Brian D Metzger ◽

Hagai B Perets

Keyword(s):

Angular Momentum ◽

Gamma Ray ◽

Nuclear Reactions ◽

Reaction Network ◽

Accretion Discs ◽

Late Time ◽

Rotating Stars ◽

Stellar Envelope ◽

Nuclear Burning ◽

R Process

ABSTRACT The core collapse of massive, rapidly-rotating stars are thought to be the progenitors of long-duration gamma-ray bursts (GRB) and their associated hyperenergetic supernovae (SNe). At early times after the collapse, relatively low angular momentum material from the infalling stellar envelope will circularize into an accretion disc located just outside the black hole horizon, resulting in high accretion rates necessary to power a GRB jet. Temperatures in the disc mid-plane at these small radii are sufficiently high to dissociate nuclei, while outflows from the disc can be neutron-rich and may synthesize r-process nuclei. However, at later times, and for high progenitor angular momentum, the outer layers of the stellar envelope can circularize at larger radii ≳ 107 cm, where nuclear reactions can take place in the disc mid-plane (e.g. 4He + 16O → 20Ne + γ). Here we explore the effects of nuclear burning on collapsar accretion discs and their outflows by means of hydrodynamical α-viscosity torus simulations coupled to a 19-isotope nuclear reaction network, which are designed to mimic the late infall epochs in collapsar evolution when the viscous time of the torus has become comparable to the envelope fall-back time. Our results address several key questions, such as the conditions for quiescent burning and accretion versus detonation and the generation of 56Ni in disc outflows, which we show could contribute significantly to powering GRB SNe. Being located in the slowest, innermost layers of the ejecta, the latter could provide the radioactive heating source necessary to make the spectral signatures of r-process elements visible in late-time GRB-SNe spectra.

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The effect of photoionizing feedback on the shaping of hierarchically-forming stellar clusters

Monthly Notices of the Royal Astronomical Society ◽

10.1093/mnras/staa2921 ◽

2020 ◽

Vol 499 (1) ◽

pp. 668-680

Author(s):

Alejandro González-Samaniego ◽

Enrique Vazquez-Semadeni

Keyword(s):

Star Formation ◽

Early Stage ◽

Stellar Clusters ◽

Low Mass Stars ◽

Transfer Scheme ◽

Star Forming ◽

Secondary Star ◽

Low Mass ◽

Hydrodynamical Simulations ◽

Formation Efficiency

ABSTRACT We use two hydrodynamical simulations (with and without photoionizing feedback) of the self-consistent evolution of molecular clouds (MCs) undergoing global hierarchical collapse (GHC), to study the effect of the feedback on the structural and kinematic properties of the gas and the stellar clusters formed in the clouds. During this early stage, the evolution of the two simulations is very similar (implying that the feedback from low-mass stars does not affect the cloud-scale evolution significantly) and the star-forming region accretes faster than it can convert gas into stars, causing the instantaneous measured star formation efficiency (SFE) to remain low even in the absence of significant feedback. Afterwards, the ionizing feedback first destroys the filamentary supply to star-forming hubs and ultimately removes the gas from it, thus first reducing the star formation (SF) and finally halting it. The ionizing feedback also affects the initial kinematics and spatial distribution of the forming stars because the gas being dispersed continues to form stars, which inherit its motion. In the non-feedback simulation, the groups remain highly compact and do not mix, while in the run with feedback, the gas dispersal causes each group to expand, and the cluster expansion thus consists of the combined expansion of the groups. Most secondary star-forming sites around the main hub are also present in the non-feedback run, implying a primordial rather than triggered nature. We do find one example of a peripheral star-forming site that appears only in the feedback run, thus having a triggered origin. However, this appears to be the exception rather than the rule, although this may be an artefact of our simplified radiative transfer scheme.

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The Circumstellar Environment of Embedded Massive Stars

Highlights of Astronomy ◽

10.1017/s1539299600013083 ◽

2002 ◽

Vol 12 ◽

pp. 143-145 ◽

Cited By ~ 1

Author(s):

Lee G. Mundy ◽

Friedrich Wyrowski ◽

Sarah Watt

Keyword(s):

Massive Star ◽

Massive Stars ◽

Low Mass Stars ◽

Submillimeter Wavelength ◽

Star Forming ◽

Star Forming Regions ◽

Low Mass ◽

Circumstellar Environments ◽

Near Future

Millimeter and submillimeter wavelength images of massive star-forming regions are uncovering the natal material distribution and revealing the complexities of their circumstellar environments on size scales from parsecs to 100’s of AU. Progress in these areas has been slower than for low-mass stars because massive stars are more distant, and because they are gregarious siblings with different evolutionary stages that can co-exist even within a core. Nevertheless, observational goals for the near future include the characterization of an early evolutionary sequence for massive stars, determination if the accretion process and formation sequence for massive stars is similar to that of low-mass stars, and understanding of the role of triggering events in massive star formation.

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