arXiv subjects

Hanna Sai

Publications and source records attributed to Hanna Sai.

At least 19 recordsLinked to original sources

arXiv2026

Spectral Dataset of Stripped-Envelope Supernovae from the Tsinghua Supernova Group

The extent of envelope stripping in the progenitor stars is directly reflected in the diversity of spectral features observed in stripped-envelope supernovae (SESNe). Through extensive spectral observation and analysis, we aim to clarify the statistical differences between the subclasses of SESNe. The Tsinghua Supernova group obtained 249 optical spectra of 62 SESNe during the years from 2010 to 2020, covering phases from $-$16 to over 190 days relative to maximum light. Most spectra were obtained during the photospheric phases after the supernova explosion. For each spectrum, the pseudo-equivalent widths (pEWs) and blueshift velocities of principal lines were measured. We further investigated the common spectral features by analysing their velocity and strength correlations across all subtypes. We identify the feature near 6200~Å in SNe Ib as H$\mathrmα$ through comparison with SNe IIb and Ic, which resolves inconsistent literature interpretations. Our finding reveals prevalent residual hydrogen in SNe Ib, further supporting a continuous stripping sequence from SNe IIb to Ib. We observe a trend in increasing velocity among different subtypes of stripped-envelope SNe, with SNe IIb exhibiting the lowest line velocities, followed by Ib, Ic, and Ic-BL. Typically, the O~I lines in SNe Ic/Ic-BL are stronger than those seen in SNe IIb/Ib. In nebular phases, the [Ca II] emission dominates over [O I] in SNe IIb/Ib while [O I] is stronger in SNe Ic, including the He-rich SN 2016coi. This spectral dichotomy implies that progenitors of SNe Ic (BL) have more massive CO cores and hence higher initial masses.

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arXiv2025

Supernovae at Distances < 40 Mpc: II. Supernova Rate in the Local Universe

Context.This is the second paper of a series aiming to determine the birth rates of supernovae in the local Universe. Aims. In this paper, we aim to estimate the SN rates in the local universe and fit the delay-time distribution of SNe Ia to put constraints on their progenitor scenarios. Methods.We performed a Monte-Carlo simulation to estimate the volumetric rates with the nearby SN sample introduced in Paper I of the series. The rate evolution of core-collapse SNe well traces the evolution of cosmic star formation history; while that of SNe Ia involves the convolution of cosmic star-formation history and a two-component delay-time distribution including a power law and a Gaussian component. Results.The volumetric rates of type Ia, Ibc and II SNe are derived as $0.325\pm0.040^{+0.016}_{-0.010}$, $0.160\pm0.028^{+0.044}_{-0.014}$, and $0.528\pm0.051^{+0.162}_{-0.013}$ (in unit of $10^{-4} yr^{-1} Mpc^{-3} h^3_{70}$), respectively. The rate of CCSNe is consistent with previous estimates. The newly derived local SN Ia rate is larger than existing results given at redshifts 0.01 < z < 0.1, favoring an increased rate from the universe at z ~ 0.1 to the local universe. A two-component model can well fit the rate variation, with the power law component accounting for the rate evolution at larger redshifts and the Gaussian component with a delay time of 12.63$\pm$0.38 Gyr accounting for the local rate evolution. This delayed component with such a longer delay time suggests that the progenitors of these SNe Ia were formed at around 1 Gyr after the birth of the universe, which could only be explained by a double-degenerate progenitor scenario. This is evidenced by the comparison with the PTF sample of SNe Ia at z = 0.073, which reveals that the increase in SN Ia rate at z < 0.01 is primarily due to the SNe Ia of massive E and S0 galaxies with old stellar populations.

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arXiv2025

Supernovae at Distances < 40 Mpc: I.Catalogues and fractions of Supernovae in a Complete Sample

Context.This is the first paper of a series aiming to determine the fractions and birth rates of various types of supernovae (SNe) in the local Universe. Aims. In this paper, we aim to construct a complete sample of SNe in the nearby universe and provide more precise measurement of subtype fractions. Methods.We carefully selected our SN sample at a distance of < 40 Mpc mainly from wide-field surveys conducted over the years from 2016 to 2023. Results.The sample contains a total of 211 SNe, including 109 SNe II, 69 SNe Ia, and 33 SNe Ibc. With the aid of sufficient spectra, we can obtain relatively accurate subtype classifications for all SNe in this sample. After corrections for the Malmquist bias, this volume-limited sample gives fractions of SNe Ia, SNe Ibc, and SNe II as $30.4^{+3.7}_{-11.5}\%$, $16.3^{+3.7}_{-7.4}\%$, and $53.3^{+9.5}_{-18.7}\%$, respectively.In the SN Ia sample, the fraction of the 91T-like subtype becomes relatively low (~5.4\%), while that of the 02cx-like subtype shows a moderate increase (~6.8\%). In the SN Ibc sample, we find significant fractions of broadlined SNe Ic (~18.0\%) and SNe Ibn (~8.8\%). The fraction of 87A-like subtype is determined as ~2.3\% for the first time, indicating rare explosions from blue supergiant stars. We find that SNe Ia show a double peak number distribution in S0- and Sc-type host galaxies, which may serve as a straightforward evidence for the presence of "prompt" and "delayed" progenitor components giving rise to SN Ia explosions. Several subtypes of SNe such as 02cx-like SNe Ia, broadlined SNe Ic, SNe IIn (and perhaps SNe Ibn) are found to occur preferentially in less massive spiral galaxies, favoring their associations with young stellar progenitors. Moreover, the 02cx-like subtype shows a trend of exploding in the outer skirt of their hosts, suggestive of metal-poor progenitors.

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arXiv2024

A spectral data release for 104 Type II Supernovae from the Tsinghua Supernova Group

We present 206 unpublished optical spectra of 104 type II supernovae obtained by the Xinglong 2.16m telescope and Lijiang 2.4m telescope during the period from 2011 to 2018, spanning the phases from about 1 to 200 days after the SN explosion. The spectral line identifications, evolution of line velocities and pseudo equivalent widths, as well as correlations between some important spectral parameters are presented. Our sample displays a large range in expansion velocities. For instance, the Fe~{\sc ii} $5169$ velocities measured from spectra at $t\sim 50$ days after the explosion vary from ${\rm 2000\ km\ s^{-1}}$ to ${\rm 5500\ km\ s^{-1}}$, with an average value of ${\rm 3872 \pm 949\ km\ s^{-1}}$. Power-law functions can be used to fit the velocity evolution, with the power-law exponent quantifying the velocity decline rate. We found an anticorrelation existing between H$β$ velocity at mid-plateau phase and its velocity decay exponent, SNe II with higher velocities tending to have smaller velocity decay rate. Moreover, we noticed that the velocity decay rate inferred from the Balmer lines (i.e., H$α$ and H$β$) have moderate correlations with the ratio of absorption to emission for H$α$ (a/e). In our sample, two objects show possibly flash-ionized features at early phases. Besides, we noticed that multiple high-velocity components may exist on the blue side of hydrogen lines of SN 2013ab, possibly suggesting that these features arise from complex line forming region. All our spectra can be found in WISeREP and Zenodo.

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arXiv2024

Newly Formed Dust within the Circumstellar Environment of SNIa-CSM 2018evt

Dust associated with various stellar sources in galaxies at all cosmic epochs remains a controversial topic, particularly whether supernovae (SNe) play an important role in dust production. We report evidence of dust formation in the cold, dense shell behind the ejecta-circumstellar medium (CSM) interaction in the Type Ia-CSM SN 2018evt three years after the explosion, characterized by a rise in the mid-infrared (MIR) emission accompanied by an accelerated decline in the optical radiation of the SN. Such a dust-formation picture is also corroborated by the concurrent evolution of the profiles of the Ha emission line. Our model suggests enhanced CSM dust concentration at increasing distances from the SN as compared to what can be expected from the density profile of the mass loss from a steady stellar wind. By the time of the last MIR observations at day +1041, a total amount of 1.2+-0.2x10^{-2} Msun of new dust has been formed by SN 2018evt, making SN 2018evt one of the most prolific dust factories among SNe with evidence of dust formation. The unprecedented witness of the intense production procedure of dust may shed light on the perceptions of dust formation in cosmic history.

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arXiv2023

SN 2022vqz: A Peculiar Subluminous Type Ia Supernova with Prominent Early Excess Emission

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

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arXiv2023

Implications for the Explosion Mechanism of Type Ia Supernovae from their Late-time Spectra

Late-time spectra of Type Ia supernovae (SNe Ia) are important in clarifying the physics of their explosions, as they provide key clues to the inner structure of the exploding white dwarfs. We examined late-time optical spectra of 36 SNe Ia, including five from our own project (SNe 2019np, 2019ein, 2021hpr, 2021wuf, and 2022hrs), with phase coverage of $\sim 200$ to $\sim 400$ days after maximum light. At this late phase, the outer ejecta have become transparent and the features of inner iron-group elements emerge in the spectra. Based on multicomponent Gaussian fits and reasonable choices for the pseudocontinuum around Ni and Fe emission features, we get reliable estimates of the Ni to Fe ratio, which is sensitive to the explosion models of SNe Ia. Our results show that the majority (about 67%) of our SNe Ia are more consistent with the sub-Chandrasekhar-mass (i.e., double-detonation) model, although they could be affected by evolutionary or ionisation effects. Moreover, we find that the Si II $λ$6355 velocity measured around the time of maximum light tends to increase with the Ni to Fe ratio for the subsample with either redshifted or blueshifted nebular velocities, suggesting that progenitor metallicity might play an important role in accounting for the observed velocity diversity of SNe Ia.

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arXiv2023

SN 2016ije: An SN 2002es-like Type Ia Supernova Exploded in a Metal-poor and Low-surface Brightness Galaxy

We have conducted photometric and spectroscopic observations of the peculiar Type Ia supernova (SN Ia) 2016ije that was discovered through the Tsinghua-NAOC Transient Survey. This peculiar object exploded in the outskirts of a metal-poor, low-surface brightness galaxy (i.e., $M_{g}$ = $-$14.5 mag). Our photometric analysis reveals that SN 2016ije is subluminous ($M_{B,\rm{max}}$ = $-$17.65$\pm$0.06 mag) but exhibits relatively broad light curves ($Δm_{15}(B)$ = 1.35$\pm$0.14 mag), similar to the behavior of SN 2002es. Our analysis of the bolometric light curve indicates that only 0.14$\pm$0.04 $M_{\odot}$ of $^{56}$Ni was synthesized in the explosion of SN 2016ije, which suggests a less energetic thermonuclear explosion when compared to normal SNe Ia, and this left a considerable amount of unburned materials in the ejecta. Spectroscopically, SN 2016ije resembles other SN 2002es-like SNe Ia, except that the ejecta velocity inferred from its carbon absorption line ($\sim$ 4500 km s$^{-1}$) is much lower than that from silicon lines ($\sim$ 8300 km s$^{-1}$) at around the maximum light. Additionally, most of the absorption lines are broader than other 02es-like SNe Ia. These peculiarities suggest the presence of significant unburned carbon in the inner region and a wide line-forming region along the line of sight. These characteristics suggest that SN 2016ije might originate from the violent merger of a white dwarf binary system, when viewed near an orientation along the iron-group-element cavity caused by the companion star.

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arXiv2023

SN 2018hna: Adding a Piece to the Puzzles of the Explosion of Blue Supergiants

We present extensive optical/ultraviolet observations and modelling analysis for the nearby SN 1987A-like peculiar Type II supernova (SN) 2018hna. Both photometry and spectroscopy covered phases extending to $>$500 days after the explosion, making it one of the best-observed SN II of this subtype. SN 2018hna is obviously bluer than SN 1987A during the photospheric phase, suggesting higher photospheric temperature, which may account for weaker BaII $\mathrmλ$6142 lines in its spectra. Analysis of early-time temperature evolution suggests a radius of $\sim$45 $\mathrm{R_{\odot}}$ for the progenitor of SN 2018hna, consistent with a blue supergiant (BSG). By fitting the bolometric light curve with hydrodynamical models, we find that SN 2018hna has an ejecta mass of $\sim$(13.7--17.7) $\mathrm{M_{\odot}}$, a kinetic energy of $\sim$ (1.0--1.2) $\times 10^{51}$ erg, and a $^{56}$Ni mass of about 0.05 $\mathrm{M_{\odot}}$. Moreover, based on standard stellar evolution and the oxygen mass (0.44--0.73 $\mathrm{M_{\odot}}$) deduced from nebular [OI] lines, the progenitor of SN 2018hna is expected to have an initial main-sequence mass $<$16 $\mathrm{M_{\odot}}$. In principle, such a relatively low-mass star cannot end as a BSG just before core-collapse, except some unique mechanisms are involved, such as rapid rotation, restricted semiconvection, etc. On the other hand, binary scenario may be more favourable, like in the case of SN 1987A. While the much lower oxygen mass inferred for SN~2018hna may imply that its progenitor system also had much lower initial masses than that of SN 1987A.

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arXiv2022

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.

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arXiv2022

SN 2019ein: A Type Ia Supernova Likely Originated from a Sub-Chandrasekhar-Mass Explosion

We present extensive optical photometric and spectroscopic observations for the nearby Type Ia supernova (SN Ia) 2019ein, spanning the phases from $\sim 3$ days to $\sim 330$ days after the explosion. This SN Ia is characterized by extremely fast expansion at early times, with initial velocities of Si II and Ca II being above ~ 25,000--30,000 km/s. After experiencing an unusually rapid velocity decay, the ejecta velocity dropped to ~ 13,000 km/s around maximum light. Photometrically, SN 2019ein has a moderate post-peak decline rate ($Δm_{15}(B) = 1.35 \pm 0.01$ mag), while being fainter than normal SNe Ia by about 40% (with $M^{\rm max}_{B} \approx -18.71 \pm 0.15$ mag). The nickel mass synthesized in the explosion is estimated to be 0.27--0.31 $M_{\odot}$ from the bolometric light curve. Given such a low nickel mass and a relatively high photospheric velocity, we propose that SN 2019ein likely had a sub-Chandrasekhar-mass white dwarf (WD) progenitor, $M_{\rm WD} \lesssim 1.22 M_{\odot}$. In this case, the explosion could have been triggered by a double-detonation mechanism, for which 1- and 2-dimensional models with WD mass $M_{\rm WD} \approx 1 M_\odot$ and a helium shell of 0.01 $M_{\odot}$ can reasonably produce the observed bolometric light curve and spectra. The predicted asymmetry as a result of double detonation is also favored by the redshifted Fe II and Ni II lines observed in the nebular-phase spectrum. Possible diversity in origin of high velocity SNe Ia is also discussed.

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arXiv2022

Multiwavelength observations of Swift J0243.6+6124 from 2017 to 2022

We have obtained optical spectroscopy and photometry data during four years after the event. The long-term photometric light-curve and the equivalent widths of the Halpha and He I 6678 lines were used to monitor the state of the Be star disk. The Halpha line profiles show evidence for V/R variability that was accounted for by fitting the Halpha spectral line profile with two Gaussian functions. We divided our data into three phases according to the intensity of the X-ray, optical, and infrared emission. Phase I covers the rise and decay of the giant X-ray outburst that took place in October to November 2017. We interpret phase II as the dissipation of the Be star equatorial disk and phase III as its recovery. The timescale of a complete formation and dissipation process is about 1250 days. The epoch when the dissipation process stopped and the reformation period began is estimated to be around MJD 58530. We find a delay of about 100 to 200 days between the minimum of the optical or infrared intensity and the strength of the Halpha line after the X-ray outburst, which may indicate that the dissipation of the disk begins from the inner parts. The motion of the density perturbation inside the disk is prograde, with a V/R quasi-period of about four years. The source shows a positive correlation in the (B-V) color index versus V-band magnitude diagram, which implies that the system is seen at a small or moderate inclination angle.

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arXiv2022

Observations of the Very Young Type Ia Supernova 2019np with Early-excess Emission

Early-time radiative signals from type Ia supernovae (SNe Ia) can provide important constraints on the explosion mechanism and the progenitor system. We present observations and analysis of SN 2019np, a nearby SN Ia discovered within 1-2 days after the explosion. Follow-up observations were conducted in optical, ultraviolet, and near-infrared bands, covering the phases from $\sim-$16.7 days to $\sim$+367.8 days relative to its $B-$band peak luminosity. The photometric and spectral evolutions of SN 2019np resembles the average behavior of normal SNe Ia. The absolute B-band peak magnitude and the post-peak decline rate are $M_{\rm max}(B)=-19.52 \pm 0.47$mag and $Δm_{\rm15}(B) =1.04 \pm 0.04$mag, respectively. No Hydrogen line has been detected in the near-infrared and nebular-phase spectra of SN 2019np. Assuming that the $^{56}$Ni powering the light curve is centrally located, we find that the bolometric light curve of SN 2019np shows a flux excess up to 5.0% in the early phase compared to the radiative diffusion model. Such an extra radiation perhaps suggests the presence of an additional energy source beyond the radioactive decay of central nickel. Comparing the observed color evolution with that predicted by different models such as interactions of SN ejecta with circumstellar matter (CSM)/companion star, a double-detonation explosion from a sub-Chandrasekhar mass white dwarf (WD), and surface $^{56}$Ni mixing, the latter one is favored.

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arXiv2022

SN 2019va: A Type IIP Supernova with Large Influence of Nickel-56 Decay on the Plateau-phase Light Curve

We present multi-band photometric and spectroscopic observations of the type II supernova, (SN) 2019va, which shows an unusually flat plateau-phase evolution in its V-band light curve. Its pseudo-bolometric light curve even shows a weak brightening towards the end of the plateau phase. These uncommon features are related to the influence of 56Ni decay on the light curve during the plateau phase, when the SN emission is usually dominated by cooling of the envelope. The inferred 56Ni mass of SN 2019va is 0.088+/-0.018 solar mass, which is significantly larger than most SNe II. To estimate the influence of 56Ni decay on the plateau-phase light curve, we calculate the ratio (dubbed as eta_Ni) between the integrated time-weighted energy from 56Ni decay and that from envelope cooling within the plateau phase, obtaining a value of 0.8 for SN 2019va, which is the second largest value among SNe II that have been measured. After removing the influence of 56Ni decay on the plateau-phase light curve, we found that the progenitor/explosion parameters derived for SN 2019va are more reasonable. In addition, SN 2019va is found to have weaker metal lines in its spectra compared to other SNe IIP at similar epochs, implying a low-metallicity progenitor, which is consistent with the metal-poor environment inferred from the host-galaxy spectrum. We further discuss the possible reasons that might lead to SN 2019va-like events.

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arXiv2022

SN 2012ij: A low-luminosity type Ia supernova and evidence for continuous distribution from 91bg-like explosion to normal ones

In this paper, we present photometric and spectroscopic observations of a subluminous type Ia supernova (SN Ia) 2012ij, which has an absolute $B$-band peak magnitude $M_{B,\rm{max}}$ = $-$17.95 $\pm$ 0.15 mag. The $B$-band light curve exhibits a fast post-peak decline with $Δm_{15}(B)$ = 1.86 $\pm$ 0.05 mag. All the $R$ and $I$/$i$-band light curves show a weak secondary peak/shoulder feature at about 3 weeks after the peak, like some transitional subclass of SNe Ia, which could result from an incomplete merger of near-infrared (NIR) double peaks. The spectra are characterized by Ti~{\sc ii} and strong Si~{\sc ii} $λ$5972 absorption features that are usually seen in low-luminosity objects like SN 1999by. The NIR spectrum before maximum light reveals weak carbon absorption features, implying the existence of unburned materials. We compare the observed properties of SN 2012ij with those predicted by the sub-Chandrasekhar-mass and the Chandrasekhar-mass delayed-detonation models, and find that both optical and NIR spectral properties can be explained to some extent by these two models. By comparing the secondary maximum features in $I$ and $i$ bands, we suggest that SN 2012ij is a transitional object linking normal SNe Ia to typical 91bg-like ones. From the published sample of SNe Ia from the $Carnegie~Supernova~Project~II$ (CSP-II), we estimate that the fraction of SN 2012ij-like SNe Ia is not lower than $\sim$ 2%.

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arXiv2021

SN 2018hfm : A Low-Energy Type II Supernova with Prominent Signatures of Circumstellar Interaction and Dust Formation

We present multiband optical photometric and spectroscopic observations of an unusual Type II supernova, SN 2018hfm, which exploded in the nearby (d = 34.67 Mpc) dwarf galaxy PGC 1297331 with a very low star-formation rate (0.0270 M yr-1) and a subsolar metallicity environment(~ 0.5 Z). The V-band light curve of SN 2018hfm reaches a peak with value of -18.69+/-0.64 mag, followed by a fast decline(4.42+/-0.13 mag (100d)-1). After about 50 days, it is found to experience a large flux drop (~3.0 mag in V), and then enters into an unusually faint tail, which indicates a relatively small amount of 56Ni synthesized during the explosion. From the bolometric light curve, SN 2018hfm is estimated to have low ejecta mass (~1.3M) and low explosion energy(~10 50 erg) compared with typical SNe II. The photospheric spectra of SN 2018hfm are similar to those of other SNe II, with P Cygni profiles of the Balmer series and metal lines, while at late phases the spectra are characterised by box-like profiles of HU emission, suggesting significant interaction between the SN ejecta and circumstellar matter. These box-like emission features are found to show increasing asymmetry with time, with the red-side component becoming gradually weaker, indicating that dust is continuously formed in the ejecta. Based on the dust-estimation tool damocles, we find that the dust increases from ~10 -6M to 10 -4 - 10 -3 M between +66.7 d and +389.4 d after explosion.

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arXiv2021

Minute-cadence Observations of the LAMOST Fields with the TMTS: I. Methodology of Detecting Short-period Variables and Results from the first-year Survey

Tsinghua University-Ma Huateng Telescopes for Survey (TMTS), located at Xinglong Station of NAOC, has a field of view upto 18 deg^2. The TMTS has started to monitor the LAMOST sky areas since 2020, with the uninterrupted observations lasting for about 6 hours on average for each sky area and a cadence of about 1 minute. Here we introduce the data analysis and preliminary scientific results for the first-year observations, which covered 188 LAMOST plates ( about 1970 deg^2). These observations have generated over 4.9 million uninterrupted light curves, with at least 100 epochs for each of them. These light curves correspond to 4.26 million Gaia-DR2 sources, among which 285 thousand sources are found to have multi-epoch spectra from the LAMOST. By analysing these light curves with the Lomb-Scargle periodograms, we identify more than 3700 periodic variable star candidates with periods below 7.5 hours, primarily consisting of eclipsing binaries and Delta Scuti stars. Those short-period binaries will provide important constraints on theories of binary evolution and possible sources for space gravitational wave experiments in the future. Moreover, we also identified 42 flare stars by searching rapidly-evolving signals in the light curves. The densely-sampled light curves from the TMTS allow us to better quantify the shapes and durations for these flares.

astro-ph.IM↗

arXiv2021

SN 2017fgc: A Fast-Expanding Type Ia Supernova Exploded in Massive Shell Galaxy NGC 474

We present extensive optical photometric and spectroscopic observations of the high-velocity (HV) Type Ia supernova (SN Ia) 2017fgc, covering the phase from $\sim$ 12 d before to $\sim 389$ d after maximum brightness. SN 2017fgc is similar to normal SNe Ia, with an absolute peak magnitude of $M_{\rm max}^{B} \approx$ $-19.32 \pm 0.13$ mag and a post-peak decline of $Δm_{15}(B)$ = $1.05 \pm 0.07$ mag. Its peak bolometric luminosity is derived as $1.32 \pm 0.13) \times 10^{43} $erg s$^{-1}$, corresponding to a $^{56}$Ni mass of $0.51 \pm 0.03 M_{\odot}$. The light curves of SN 2017fgc are found to exhibit excess emission in the $UBV$ bands in the early nebular phase and pronounced secondary shoulder/maximum features in the $RrIi$ bands. Its spectral evolution is similar to that of HV SNe Ia, with a maximum-light Si II velocity of $15,000 \pm 150 $km s$^{-1}$ and a post-peak velocity gradient of $\sim$ $120 \pm 10 $km s$^{-1} $d$^{-1}$. The Fe II and Mg II lines blended near 4300 Å and the Fe II, Si II, and Fe III lines blended near 4800 Å are obviously stronger than those of normal SNe Ia. Inspecting a large sample reveals that the strength of the two blends in the spectra, and the secondary peak in the $i/r$-band light curves, are found to be positively correlated with the maximum-light Si II velocity. Such correlations indicate that HV SNe~Ia may experience more complete burning in the ejecta and/or that their progenitors have higher metallicity. Examining the birthplace environment of SN 2017fgc suggests that it likely arose from a stellar environment with young and high-metallicity populations.

astro-ph.HE↗