Time varying Na I D absorption in ILRTs as a probe of circumstellar material

Intermediate-Luminosity Red Transients (ILRTs) are a class of observed transient posited to arise from the production of an electron-capture supernova from a super-asymptotic giant branch star within a dusty cocoon. In this paper, we present a systematic analysis of narrow Na I D absorption as a means of probing the circumstellar environment of these events. We find a wide diversity of evolution in ILRTs in terms of line strength, time-scale, and shape. We present a simple toy model designed to predict this evolution as arising from ejecta from a central supernova passing through a circumstellar environment wherein Na II is recombining to Na I over time. We find that while our toy model can qualitatively explain the evolution of a number of ILRTs, the majority of our sample undergoes evolution more complex than predicted. The success of using the Na I D doublet as a diagnostic tool for studying circumstellar material will rely on the availability of regular high-resolution spectral observations of multiple ILRTs, and more detailed spectral modelling will be required to produce models capable of explaining the diverse range of behaviours exhibited by ILRTs. In addition, the strength of the Na I D absorption feature has been used as a means of estimating the extinction of sources, and we suggest that the variability visible in ILRTs would prevent such methods from being used for this class of transient, and any others showing evidence of variabilityComment: 14 pages, 10 figures, submitted to MNRA

The Retrospective Analysis of Posterior Short-Segment Pedicle Instrumentation without Fusion for Thoracolumbar Burst Fracture with Neurological Deficit

This study aims to investigate the efficacy of posterior short-segment pedicle instrumentation without fusion in curing thoracolumbar burst fracture. All of the 53 patients were treated with short-segment pedicle instrumentation and laminectomy without fusion, and the restoration of retropulsed bone fragments was conducted by a novel custom-designed repositor (RRBF). The mean operation time and blood loss during surgery were analyzed; the radiological index and neurological status were compared before and after the operation. The mean operation time was 93 min (range: 62–110 min) and the mean intraoperative blood loss was 452 mL in all cases. The average canal encroachment was 50.04% and 10.92% prior to the surgery and at last followup, respectively (P<0.01). The preoperative kyphotic angle was 17.2 degree (±6.87 degrees), whereas it decreased to 8.42 degree (±4.99 degrees) at last followup (P<0.01). Besides, the mean vertebral body height increased from 40.15% (±9.40%) before surgery to 72.34% (±12.32%) at last followup (P<0.01). 45 patients showed 1-2 grades improvement in Frankel’s scale at last followup. This technique allows for satisfactory canal clearance and restoration of vertebral body height and kyphotic angle, and it may promote the recovery of neurological function. However, further research is still necessary to confirm the efficacy of this treatment

Minute-cadence Observations of the LAMOST Fields with the TMTS: III. Statistic Study of the Flare Stars from the First Two Years

Tsinghua University-Ma Huateng Telescopes for Survey (TMTS) aims to detect fast-evolving transients in the Universe, which has led to the discovery of thousands of short-period variables and eclipsing binaries since 2020. In this paper, we present the observed properties of 125 flare stars identified by the TMTS within the first two years, with an attempt to constrain their eruption physics. As expected, most of these flares were recorded in late-type red stars with $G_{\rm BP}-G_{\rm RP}$ > 2.0 mag, however, the flares associated with bluer stars tend to be on average more energetic and have broader profiles. The peak flux (F_peak) of the flare is found to depend strongly on the equivalent duration (ED) of the energy release, i.e., $F_{{\rm peak}} \propto {\rm ED}^{0.72\pm0.04}$, which is consistent with results derived from the Kepler and Evryscope samples. This relation is likely related to the magnetic loop emission, while -- for the more popular non-thermal electron heating model -- a specific time evolution may be required to generate this relation. We notice that flares produced by hotter stars have a flatter $F_{{\rm peak}} \propto {\rm ED}$ relation compared to that from cooler stars. This is related to the statistical discrepancy in light-curve shape of flare events with different colors. In spectra from LAMOST, we find that flare stars have apparently stronger H alpha emission than inactive stars, especially at the low temperature end, suggesting that chromospheric activity plays an important role in producing flares. On the other hand, the subclass having frequent flares are found to show H alpha emission of similar strength in their spectra to that recorded with only a single flare but similar effective temperature, implying that the chromospheric activity may not be the only trigger for eruptions.Comment: 17 pages, 15 figures, 2 tables, refereed version. For associated data files, see https://cdsarc.cds.unistra.fr/viz-bin/cat/J/MNRAS/523/219

SN 2022vqz: A Peculiar SN 2002es-like Type Ia Supernova with Prominent Early Excess Emission

We present extensive photometric and spectroscopic observations of a peculiar type Ia supernova (SN Ia) 2022vqz. It shares many similarities with the SN 2002es-like SNe Ia, such as low luminosity (i.e., $M_{B,\rm max}=-18.11\pm0.16$ mag) and moderate post-peak decline rate (i.e., $\Delta m_{15,B}=1.33\pm0.11$ mag). The nickel mass synthesized in the explosion is estimated as $0.20\pm0.04~{\rm M}_\odot$ from the bolometric light curve, which is obviously lower than normal SNe Ia. SN 2022vqz is also characterized by a slow expanding ejecta, with Si II velocities persisting around 7000 km s$^{-1}$ since 16 days before the peak, which is unique among all known SNe Ia. While all these properties imply a less energetic thermonuclear explosion that should leave considerable amount of unburnt materials, however, absent signature of unburnt carbon in the spectra of SN 2022vqz is puzzling. A prominent early peak is clearly detected in the $c$- and $o$-band light curves of ATLAS and in the $gr$-band data of ZTF within days after the explosion. Possible mechanisms for the early peak are discussed, including sub-Chandrasekhar mass double detonation model and interaction of SN ejecta with circumstellar material (CSM). We found both models face some difficulties in replicating all aspects of the observed data. As an alternative, we propose a hybrid CONe white dwarf as progenitor of SN 2022vqz which can simultaneously reconcile the tension between low ejecta velocity and absence of carbon. We further discuss the diversity of 02es-like objects and possible origins of different scenarios.Comment: 24 pages, 12 figures, submitted to MNRA

An 18.9-minute Blue Large-Amplitude Pulsator Crossing the 'Hertzsprung Gap' of Hot Subdwarfs

Blue large-amplitude pulsators (BLAPs) represent a new and rare class of hot pulsating stars with unusually large amplitudes and short periods. Up to now, only 24 confirmed BLAPs have been identified from more than one billion monitored stars, including a group with pulsation period longer than $\sim 20$ min (classical BLAPs, hereafter) and the other group with pulsation period below $\sim 8$ min. The evolutionary path that could give rise to such kinds of stellar configurations is unclear. Here we report on a comprehensive study of the peculiar BLAP discovered by the Tsinghua University - Ma Huateng Telescopes for Survey (TMTS), TMTS J035143.63+584504.2 (TMTS-BLAP-1). This new BLAP has an 18.9 min pulsation period and is similar to the BLAPs with a low surface gravity and an extended helium-enriched envelope, suggesting that it is a low-gravity BLAP at the shortest-period end. In particular, the long-term monitoring data reveal that this pulsating star has an unusually large rate of period change, P_dot/P=2.2e-6/yr. Such a significant and positive value challenges its origins from both helium-core pre-white-dwarfs and core helium-burning subdwarfs, but is consistent with that derived from shell helium-burning subdwarfs. The particular pulsation period and unusual rate of period change indicate that TMTS-BLAP-1 is at a short-lived (~10^6 yr) phase of shell-helium ignition before the stable shell-helium burning; in other words, TMTS-BLAP-1 is going through a "Hertzsprung gap" of hot subdwarfs.Comment: 26 pages, 12 figures, 4 tables, published on Nature Astronomy, URL: https://www.nature.com/articles/s41550-022-01783-

SN 2021csp -- the explosion of a stripped envelope star within a H and He-poor circumstellar medium

We present observations of SN 2021csp, a unique supernova (SN) which displays evidence for interaction with H- and He- poor circumstellar material (CSM) at early times. Using high-cadence spectroscopy taken over the first week after explosion, we show that the spectra of SN 2021csp are dominated by C III lines with a velocity of 1800 km s$^{-1}$. We associate this emission with CSM lost by the progenitor prior to explosion. Subsequently, the SN displays narrow He lines before metamorphosing into a broad-lined Type Ic SN. We model the bolometric light curve of SN 2021csp, and show that it is consistent with the energetic ($4\times10^{51}$ erg) explosion of a stripped star, producing 0.4 M$_\odot$ of 56Ni within a $\sim$1 M$_\odot$ shell of CSM extending out to 400 R$_\odot$...

Intermediate Luminosity Optical Transients

Beyond supernovae (SNe), very few cosmic explosions can release an amount of kinetic energy of the order of 10ˆ{51} erg (1 foe). In past years, modern all-sky surveys discovered numerous peculiar transients releasing much lower energies. With the label of Intermediate Luminosity Optical Transients or Gap Transients, we refer to objects fainter than typical SNe but brighter than classical novae (i.e., they lay in the magnitude range -10< Mv < -15 mag). In this poorly populated luminosity range (the “gap”), we find several types of stellar transients, including faint supernovae, giant eruptions of massive stars including luminous blue variables (LBVs), intermediate-luminosity red transients (ILRTs), and luminous red novae (LRNe). These gap transients may originate from various physical mechanisms, and sometimes the classification is a tricky task. A major goal of this research work is characterising the observational properties and correlating the physical parameters of gap transients, shedding light on their nature. In this thesis project, I studied in detail a recent faint and red transient, AT 2017be, classified as an ILRT, and show that an electron-capture supernova (EC SN) is the most likely scenario to explain the observed outburst. In-depth analysis on a large ILRT sample favours the same explosion mechanism for all of them. Finally, I studied an unprecedented object, AT 2018hso, that reveals transitional observational properties between ILRTs and LRNe, making its precise classification dubious. However, follow-up observations support it to be a LRN, hence most likely a transient produced by a stellar merging event.Oltre alle supernove (SNe), pochissime esplosioni cosmiche sono in grado di rilasciare una quantità di energia cinetica dell’ordine di 1051 erg (1 foe). Negli anni passati, moderne “all-sky surveys” hanno permesso di scoprire numerosi transienti peculiari che rilasciavano energie molto più modeste. Con il nome di transienti ottici di luminosità intermedia o transienti nel “gap”, ci riferiamo ad oggetti che sono più deboli delle tipiche SNe ma più luminosi delle classiche novae (cio`e hanno magnitudine nell’intervallo -10< MV < -15 mag). In questo intervallo di luminosità scarsamente popolato (il “gap”), troviamo diversi tipi di transienti stellari, tra cui supernove deboli, eruzioni giganti di stelle massicce incluse le Variabili Luminose Blu (LBV), i transienti rossi di luminosità intermedia (ILRTs) e le novae rosse luminose (LRNe). Questi transienti di “gap” possono essere prodotti da diversi meccanismi fisici, e talvolta la loro classificazione è un compito arduo. Uno degli obiettivi principali di questo studio è caratterizzare le proprietà osservative e correlare i parametri fisici dei transienti di “gap”, svelandone la natura. In questo lavoro di tesi, ho studiato in dettaglio un recente evento transiente debole e rosso, AT 2017be, classificato come ILRT, e ho mostrato come una supernova prodotta da cattura elettronica (EC SN) sia lo scenario più probabile per spiegare l’evento eruttivo osservato. Un’analisi approfondita su un grande campione di ILRTs favorisce lo stesso meccanismo di esplosione per tutti questi transienti. Infine, ho studiato un oggetto senza precedenti, AT 2018hso, che rivela proprietà osservative intermedie tra quelle degli ILRT e le LRNe, e che rendono una sua precisa classificazione incerta. Tuttavia, le osservazioni di follow-up supportano la tesi che sia un LRN, quindi probabilmente un transiente prodotto da un evento di coalescenza stellare

Intermediate Luminosity Optical Transients

Beyond supernovae (SNe), very few cosmic explosions can release an amount of kinetic energy of the order of 10ˆ{51} erg (1 foe). In past years, modern all-sky surveys discovered numerous peculiar transients releasing much lower energies. With the label of Intermediate Luminosity Optical Transients or Gap Transients, we refer to objects fainter than typical SNe but brighter than classical novae (i.e., they lay in the magnitude range -10< Mv < -15 mag). In this poorly populated luminosity range (the “gap”), we find several types of stellar transients, including faint supernovae, giant eruptions of massive stars including luminous blue variables (LBVs), intermediate-luminosity red transients (ILRTs), and luminous red novae (LRNe). These gap transients may originate from various physical mechanisms, and sometimes the classification is a tricky task. A major goal of this research work is characterising the observational properties and correlating the physical parameters of gap transients, shedding light on their nature. In this thesis project, I studied in detail a recent faint and red transient, AT 2017be, classified as an ILRT, and show that an electron-capture supernova (EC SN) is the most likely scenario to explain the observed outburst. In-depth analysis on a large ILRT sample favours the same explosion mechanism for all of them. Finally, I studied an unprecedented object, AT 2018hso, that reveals transitional observational properties between ILRTs and LRNe, making its precise classification dubious. However, follow-up observations support it to be a LRN, hence most likely a transient produced by a stellar merging event