We present a physical characterization of Tsinghua University-Ma Huateng Telescope for Survey (TMTS) J00063798+3104160 (J0006), a rapidly rotating, ultramassive white dwarf (WD) identified in high-cadence light curves from the TMTS. A coherent 23 minute periodicity is detected in TMTS, Transiting Exoplanet Survey Satellite, and Zwicky Transient Facility photometry. A time series of low-resolution spectra with the Keck-I 10 m telescope reveals broad, shallow hydrogen absorption features indicative of an extreme magnetic field and shows no evidence for radial-velocity variations. Atmospheric modeling yields a magnetic field strength of ∼250 MG, while Gaia astrometry and photometry imply a mass of 1.06 ± 0.01 M _⊙ . A significant infrared excess is detected in the Wide-field Infrared Survey Explorer W1 band and is well fitted by a 550 K blackbody, likely arising from residual material of a merger. We interpret the 23 minute photometric modulation as the rotation period of an isolated, massive WD formed likely through the merger of a double WD binary. With one of the shortest rotation periods known among candidate merger remnants and with constraints from a deep Einstein Probe X-ray nondetection, J0006 provides a rare and important observational window into the poorly explored intermediate stages of postmerger evolution.

Aims. We present optical, ultraviolet, and X-ray observations of supernova (SN) 2024iss, a Type IIb SN that shows a prominent double-peaked light curve. Methods. We modeled the first peak with a semianalytical shock-cooling model and the X-ray emission with a free-free model. We also compared the envelope radius and mass-loss rate with those of other Type IIb SNe to explore the relationships between the progenitor envelope and the circumstellar material. Results. The shock-cooling peak in the V-band light curve reached MV = −17.33 ± 0.26 mag, while the 56Ni-powered second peak attained MV = −17.43 ± 0.26 mag. Early spectra show a photospheric velocity of approximately 19 400 km s−1 at 3.82 days from the Hα P Cygni profile. The Balmer lines persist for at least more than 87 days after the explosion, which is characteristic of hydrogen-rich ejecta. Modeling the first light-curve peak with the shock-cooling model suggests an extended hydrogen envelope with a mass of 0.11 ± 0.04 M⊙ and a radius of 244 ± 43 R⊙. Fitting the second light-curve peak with an Arnett-like model indicates a typical 56Ni mass of 0.117 ± 0.013 M⊙ and a relatively low ejecta mass of 1.27 ± 0.34 M⊙. X-ray observations revealed bright thermal bremsstrahlung emission and indicate a mass-loss rate of 1.6 × 10−5 M⊙ yr−1, which is similar to that of SN 1993J. Conclusions. Supernova 2024iss occupies a transitional position between the two subclasses of extended and compact Type IIb SNe. Its envelope radius and preexplosion mass-loss rate appear to be consistent with the correlation observed in the broader sample. The observational properties of SN 2024iss are compatible with a binary-interaction scenario being the dominant mechanism for envelope stripping.

The R2Pub, built by Beijing Planetarium, is a state-of-the-art 60 cm equatorial binocular telescope located at the Daocheng Site (with an altitude of 4700 m) of Yunnan Observatories in China. This paper provides an overview of the R2Pub telescope system, discusses its design and capabilities, and presents an evaluation of its performance for astronomical surveys. R2Pub is a prime-focus binocular system, with each tube covering a field of view of about 18 deg2. This system is designed to detect various transients in local universe, including variables, eclipsing binaries, supernovae, gamma-ray bursts afterglow, tidal disruption events, Active Galactic Nuclei, and other unknown transients, which are ideal targets for both time-domain astronomy research and science population. The entire R2Pub system has completed the construction and installation of all observatory infrastructure, including the dome, equatorial mount, optical tube, and associated components, and has now entered the commissioning phase. The high-altitude location, good seeing, and dark background sky light at Daocheng site ensure optimal observational conditions for time-domain astronomy. Performance testing during the commissioning phase has demonstrated that the R2Pub system can achieve a 5 sigma limiting magnitude of approximately 18.7 mag in the Pan-STARRS r ' band for 60 s exposures. The ongoing observations from R2Pub is expected to contribute significantly to the study of time-variable phenomena in the universe and greatly improve the public outreach in astronomy. In particular, the binocular telescope systems capable of simultaneous dual-band observations can obtain the instantaneous color information of transient sources, enabling more accurate characterization of their physical properties and evolution, and providing a significant advantage in rapid distinguishing different classes of variables and transients.

Context. The extent of envelope stripping in progenitor stars is directly reflected in the diversity of spectral features that are observed in stripped-envelope supernovae (SESNe). Aims. Through extensive spectral observation and analysis, we aim to clarify the statistical differences between the subclasses of SESNe. Methods. The Tsinghua supernova group obtained 249 optical spectra of 62 SESNe 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 and blueshift velocities of the principal lines were measured. We further investigated the common spectral features by analyzing their velocity and strength correlations across all subtypes. Results. We identified the feature near 6200 Å in SNe Ib as Hα through a comparison with SNe IIb and Ic. This resolves inconsistent interpretations in the literature. Our finding reveals prevalent residual hydrogen in SNe Ib, further supporting a continuous stripping sequence from SNe IIb to Ib. The velocity among different subtypes of stripped-envelope SNe increases, 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 [O I] in SNe IIb/Ib, while [O I] is stronger in SNe Ic, including in 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.

Context. This is the second paper of a series aiming to determine the birth rates of supernovae (SNe) in the local Universe. Aims. We aimed to estimate the SN rates in the local Universe and fit the delay-time distribution of type Ia SNe (SNe Ia) to put constraints on their progenitor scenarios. Methods. We performed a Monte Carlo simulation to estimate volumetric rates using the nearby SN sample introduced in Paper I. The rate evolution of core-collapse (CC) SNe closely follows the evolution of the cosmic star formation history, while the rate evolution of SNe Ia involves the convolution of the 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 +/- 0.040(-0.010)(+0.016), 0.160 +/- 0.028(-0.014)(+0.044), and 0.528 +/- 0.051(-0.013)(+0.162) (in units of 10(-4)yr(-1) Mpc(-3) h(70)(3)), respectively. The rate of CCSNe (0.688 +/- 0.078(-0.027)(+0.0206)) is consistent with previous estimates, which trace the star formation history. Conversely, 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 similar to 0.1 to the local Universe at z < 0.01. A two-component model effectively reproduces 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 +/- 0.38 Gyr accounting for the local rate evolution. This delayed component, with its exceptionally long delay time, suggests that the progenitors of these SNe Ia were formed around 1 Gyr after the birth of the Universe, which could only be explained by a double-degenerate progenitor scenario. Comparison with the Palomar Transient Factory (PTF) sample of SNe Ia at z = 0.073 and the morphology of their host galaxies, reveals that the increased SN Ia rate at z < 0.01 is primarily due to the SNe Ia of massive E and S0 galaxies with old stellar populations. Based on the above results, we estimate the Galactic SN rate as 3.08 +/- 1.29 per century.

While delta Scuti stars are the most numerous class of kappa-mechanism pulsators in the instability strip, the short periods and small peak-to-peak amplitudes have left them understudied and underutilized. Recently, large-scale time-domain surveys have significantly increased the number of identified delta Scuti stars. Notably, the Tsinghua University-Ma Huateng Telescopes for Survey (TMTS), with its high-cadence observations at 1-minute intervals, has identified thousands of delta Scuti stars, greatly expanding the sample of these short-period pulsating variables. Using the delta Scuti stars from the TMTS catalogs of Periodic Variable Stars, we cross-matched the dataset with Pan-STARRS1, 2MASS, and WISE to obtain photometric measurements across optical and infrared bands. Parallax data, used as Bayesian priors, were retrieved from Gaia DR3, and line-of-sight dust extinction priors were estimated from a three-dimensional dust map. Using PyMC, we performed a simultaneous determination of the 11-band P-L relations of delta Scuti stars, which not only yields precise measurements of these relations, but also greatly improves constraints on the distance moduli and color excesses, as evidenced by the reduced uncertainties in the posterior distributions. Furthermore, our methodology enables an independent estimation of the color excess through the P-L relations, offering a potential complement to existing 3-D dust maps. Moreover, by cross-matching with LAMOST DR7, we investigated the influence of metallicity on the P-L relations. Our analysis reveals that incorporating metallicity might reduce the intrinsic scatter at longer wavelengths. However, this result does not achieve 3 sigma significance, leaving open the possibility that the observed reduction is attributable to statistical fluctuations.

Context. The dynamical method provides an efficient way to discover post-common-envelope binaries (PCEBs) with faint white dwarfs (WDs), thanks to the development of time-domain survey projects. As close binary systems undergo a common-envelope phase, they offer unique opportunities to study the astrophysical processes associated with binary evolution. Aims. We perform a comprehensive analysis of the PCEB system TMTS J15530469+4457458 (J1553), discovered by the Tsinghua University-Ma Huateng Telescopes for Survey, to explore its physical origin and evolutionary fate. Methods. This system is characterized by double-peaked Balmer emission lines, and we applied a cross-correlation function to derive its radial velocity (RV) from a series of phase-resolved Keck spectra. The physical parameters of this binary were obtained by fitting the light curves and RV simultaneously. The locations of the Balmer lines were inferred from Doppler tomography, and a MESA simulation was performed to explore the evolution of this system. Results. Analyses using the cross-correlation function suggest that this system is a single-lined spectroscopic binary and only one star is optically visible. Further analysis through Doppler tomography indicates that J1553 is a detached binary without an accretion disk. Under such a configuration, the simultaneous light-curve and RV fitting reveal that this system contains an unseen WD with mass M-A = 0.56 +/- 0.09 M-circle dot, and an M4 dwarf with mass M-B = 0.37 +/- 0.02 M-circle dot and radius R-B = 0.403(-0.015)(+0.014). The extra prominent Balmer emission lines seen in the spectra can trace the motion of the WD; these lines are likely formed near the WD surface as a result of wind accretion. According to the MESA simulation, J1553 could have evolved from a binary consisting of a 2.0-4.0 M-circle dot zero-age-main-sequence star and an M dwarf with an initial orbital period P-i approximate to 201-476 d, and the system has undergone a common-envelope (CE) phase. After about 3.3x10(6) yr, J1553 should evolve into a cataclysmic variable, with a transient state as a supersoft X-ray source at the beginning. J1553 is an excellent system for studying wind accretion, CE ejection physics, and binary evolution theory.

With the development of wide-field surveys, a large amount of data on short-period W UMa contact binaries have been obtained. Continuous and uninterrupted light curves as well as high-resolution spectroscopic data are crucial in determining the absolute physical parameters. Targets with both TMTS light curves and LAMOST medium-resolution spectra were selected. The absolute physical parameters were inferred with the W-D code for 10 systems, all of them are W-type shallow or medium contact binaries. The O'Connell effect observed in the light curves can be explained by adding a spot on the primary or secondary component in the models. According to O − C analysis, the orbital periods exhibit a long-term increasing or decreasing trend, among which J0132, J1300, and J1402 show periodic variations that may be attributed to the presence of a third body or magnetic activity cycles. Spectral subtraction analysis revealed that the equivalent width of H α indicates strong magnetic activity in J0047, J0305, J0638, and J1402. Among the 10 selected binary systems, except for J0132 and J0913, the more massive components are found to be main-sequence stars while the less massive components have evolved off the main sequence. In J0132, both components are in the main sequence, whereas both components of J0913 lie above the terminal-age main sequence. Based on the relationship between orbital angular momentum and total mass for these two systems, as well as their low fill-out factors, it is possible that these two systems are newly formed contact binaries, having recently evolved from the detached configuration.

Context. This is the first paper in 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 measurements of subtype fractions. Methods. We carefully selected our SN sample at a distance of less than 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 obtained relatively accurate subtype classifications for all SNe in this sample. After corrections for the Malmquist bias, this volumelimited sample yielded fractions of SNe Ia, SNe Ibc, and SNe II of 30.4(-11.5)(+3.7) %, 16.3(-7.4)(+3.7) %, and 53.3(-18.7)(+9.5) %, respectively. In the SN Ia sample, the fraction of the 91T-like subtype becomes relatively low (similar to 5.4%), while that of the 02cx-like subtype shows a moderate increase (similar to 6.8%). In the SN Ibc sample, we find significant fractions of broadlined SNe Ic (similar to 18.0%) and SNe Ibn (similar to 8.8%). The fraction of the 87A-like subtype was determined to be similar to 2.3%, 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 straightforward evidence for the presence of "prompt" and "delayed" progenitor components that give rise to SN Ia explosions. Several subtypes of SNe such as 02cx-like SNe Ia, broadlined SNe Ic, and SNe IIn (and perhaps SNe Ibn) are found to occur preferentially in less massive spiral galaxies (i.e., with stellar mass <0.5x10(10) M-circle dot), thus favoring their associations with young stellar progenitors. Moreover, the 02cx-like subtype shows a trend of exploding in the outer skirt of their hosts, which is suggestive of metal-poor progenitors.

Context. We present a comprehensive photometric and spectroscopic study of the nearby Type II supernova (SN) 2023ixf; our extensive observations span the phases from similar to 3 to over 600 days after the first light. Aims. The aim of this study is to obtain key information on the explosion properties of SN 2023ixf and the nature of its progenitor. Methods. The observational properties of SN 2023ixf were compared with those of a representative sample of Type IIP and IIL SNe to investigate commonalities and diversities. We conducted a detailed analysis of the temporal evolution of major spectral features observed throughout differ-ent phases of the SN 2023ixf explosion. Several key interpretations are addressed through a comparison between the data and the model spectra predicted by nonlocal thermodynamic equilibrium (non-LTE) radiative-transfer calculations for progenitor stars within a range of zero-age main sequence (ZAMS) masses. Results. Our observations indicate that SN 2023ixf is a transitional SN that bridges the gap between the Type IIP and IIL subclasses of H-rich SNe; it is characterized by a relatively short plateau (70 d) in the light curve. It shows a rather prompt spectroscopic evolution toward the nebular phase; emission lines of Na, O, H, and Ca in nebular spectra all exhibit multi-peak profiles, which might be due to a bipolar distribution of the ejecta. In particular, the H alpha profile can be separated into two central peaked components (with velocities of about 1500 km s(-1)) that are likely due to nickel-powered ejecta and two outer box components (with velocities of up to similar to 8000 km s-1) that can arise from interactions of the outermost ejecta with a circumstellar shell at a distance of similar to 6.2 x 10(15) cm. The nebular-phase spectra of SN 2023ixf show good agreement with those predicted by a non-LTE radiative-transfer code for progenitor stars with ZAMS masses ranging from 15 to 19 M-circle dot A distance of 6.35(+0.31-)0.39 Mpc is estimated for M101 based on the expanding photosphere method.

Binary evolution theory predicts that the second common envelope ejection can produce low-mass (0.32–0.36 M⊙) subdwarf B (sdB) stars inside ultrashort-orbital-period binary systems, as their helium cores are ignited under nondegenerate conditions. With the orbital decay driven by gravitational-wave (GW) radiation, the minimum orbital periods of detached sdB binaries could be as short as ∼20 min. However, only four sdB binaries with orbital periods below an hour have been reported so far, and none of them has an orbital period approaching the above theoretical limit. Here we report the discovery of a 20.5-min-orbital-period ellipsoidal binary, TMTS J052610.43+593445.1, in which the visible star is being tidally deformed by an invisible carbon–oxygen white dwarf companion. The visible component is inferred to be an sdB star with a mass ∼0.33 M⊙ approaching the helium-ignition limit, although a He-core white dwarf cannot be completely ruled out. In particular, the radius of this low-mass sdB star is only 0.066 R⊙, about seven Earth radii. Such a system provides a key clue in mapping the binary evolution scheme from the second common envelope ejection to the formation of AM CVn stars having a helium-star donor. It may also serve as a crucial verification binary of space-borne GW observatories such as LISA and TianQin in the future. A very uncommon detached binary system with a 20.5-min orbital period has been discovered to harbour a carbon–oxygen white dwarf star and a low-mass subdwarf B star with a seven-Earth radius that traces the theoretical limit of binary evolution predicted 20 years ago.

We present a detailed analysis of the progenitor and its local environment for the recently discovered Type II supernova (SN) 2024ggi at a distance of about 6.7 Mpc, by utilizing the pre-explosion images from the Hubble Space Telescope and Spitzer Space Telescope. The progenitor is identified as a red bright variable star, with absolute F814W-band magnitudes being -6.2 mag in 1995 to -7.2 mag in 2003, respectively, consistent with that of a normal red supergiant star. Combining with the historical mid-infrared light curves, a pulsational period of about 379 days can be inferred for the progenitor star. Fitting its spectral energy distribution with stellar spectral models yields the stellar parameters of temperature, radius, and bolometric luminosity as T*=3290(-27)(+19) K, R*=887(-51)(+60) R-circle dot, and log(L/L-circle dot) =4.92(-0.04)(+0.05 ), respectively. The above parameters indicate that the progenitor of SN 2024ggi is consistent with the stellar evolutionary track of a solar-metallicity massive star with an initial mass of 13(-1)(+1) M-circle dot. Moreover, our analysis indicates a relatively low mass-loss rate (i.e., <3 x 10-6 M-circle dot yr(-1)) for the progenitor compared to that inferred from flash spectroscopy and X-ray detection (i.e., 10(-2)-10(-5) M-circle dot yr(-1)), implying a significant enhancement in mass loss within a few years prior to the explosion.

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.

ABSTRACT The Tsinghua University-Ma Huateng Telescope for Survey (TMTS) has been constantly monitoring the northern sky since 2020 in search of rapidly variable stars. To find variable white dwarfs (WDs), the TMTS catalogue is cross-matched with the WD catalogue of Gaia EDR3, resulting in over 3000 light curves of WD candidates. The WD TMTS J17184064+2524314 (hereafter J1718) is the second ZZ Ceti star discovered among these common sources. Based on the light curves from TMTS, follow-up photometric observations, and Transiting Exoplanet Survey Satellite, 10 periods and three combination periods are detected. A rotation period of 25.12 ± 0.18 h is derived, according to the identified rotational splitting. Our spectroscopic observation indicates that this WD belongs to DA type with Teff = 11 670 ± 604 K, log g = 8.16 ± 0.36, M = 0.70 ± 0.23 M⊙, and age = 0.51 ± 0.34 Gyr. Based on core-parametrized asteroseismological model grids (≥14 million), we derive the best-fitting solution of Teff = 11 640 ± 20 K, log g = 8.267 ± 0.008, and M = 0.750 ± 0.005 M⊙ for J1718, consistent with the spectral fitting results. For this WD, the corresponding carbon and oxygen abundances in the core are 0.43 and 0.57, respectively. The distance derived from the intrinsic luminosity given by asteroseismology is 64 ± 15 pc, in accord with the distance of 70.1 ± 0.2 pc from Gaia DR3 within the uncertainties.

The Tsinghua University–Ma Huateng Telescopes for Survey (TMTS) started to monitor the LAMOST plates in 2020, leading to the discovery of numerous short-period eclipsing binaries, peculiar pulsators, flare stars, and other variable objects. Here, we present the uninterrupted light curves for a sample of 64 cataclysmic variables (CVs) observed/discovered using the TMTS during its first three-year observations, and we introduce new CVs and new light-variation periods (from known CVs) revealed through the TMTS observations. Thanks to the high-cadence observations of TMTS, diverse light variations, including superhumps, quasi-periodic oscillations, large-amplitude orbital modulations, and rotational modulations, are able to be detected in our CV samples, providing key observational clues for understanding the fast-developing physical processes in various CVs. All of these short-timescale light-curve features help further classify the subtypes of CV systems. We highlight the light-curve features observed in our CV sample and discuss further implications of minute-cadence light curves for CV identifications and classifications. Moreover, we examine the Hα emission lines in the spectra from our nonmagnetic CV samples (i.e., dwarf novae and nova-like subclasses) and find that the distribution of Hα emission strength shows significant differences between the sources with orbital periods above and below the period gap, which agrees with the trend seen from the SDSS nonmagnetic CV sample.

SN 2019neq was a very fast evolving superluminous supernova. At a redshift z=0.1059, its peak absolute magnitude was -21.5+/-0.2 mag in g band. In this work, we present data and analysis from an extensive spectrophotometric follow-up campaign using multiple observational facilities. Thanks to a nebular spectrum of SN 2019neq, we investigated some of the properties of the host galaxy at the location of SN 2019neq and found that its metallicity and specific star formation rate are in a good agreement with those usually measured for SLSNe-I hosts. We then discuss the plausibility of the magnetar and the circumstellar interaction scenarios to explain the observed light curves, and interpret a nebular spectrum of SN 2019neq using published SUMO radiative-transfer models. The results of our analysis suggest that the spindown radiation of a millisecond magnetar with a magnetic field B 6e14 G could boost the luminosity of SN 2019neq.

Periodic variables are always of great scientific interest in astrophysics. Thanks to the rapid advancement of modern large-scale time-domain surveys, the number of reported variable stars has experienced substantial growth for several decades, which significantly deepened our comprehension of stellar structure and binary evolution. The Tsinghua University-Ma Huateng Telescopes for Survey (TMTS) has started to monitor the LAMOST sky areas since 2020, with a cadence of 1 min. During the period from 2020 to 2022, this survey has resulted in densely sampled light curves for similar to 30 000 variables of the maximum powers in the Lomb-Scargle periodogram above the 5 sigma threshold. In this paper, we classified 11 638 variable stars into six main types using xgboost and Random Forest classifiers with accuracies of 98.83 per cent and 98.73 per cent, respectively. Among them, 5301 (45.55 per cent) variables are newly discovered, primarily consisting of delta Scuti stars, demonstrating the capability of TMTS in searching for short-period variables. We cross-matched the catalogue with Gaia's second Data Release and LAMOST's seventh Data Release to obtain important physical parameters of the variables. We identified 5504 delta Scuti stars (including 4876 typical delta Scuti stars and 628 high-amplitude delta Scuti stars), 5899 eclipsing binaries (including EA-, EB-, and EW-type), and 226 candidates of RS Canum Venaticorum. Leveraging the metal abundance data provided by LAMOST and the Galactic latitude, we discovered eight candidates of SX Phe stars within the class of 'delta Scuti stars'. Moreover, with the help of Gaia colour-magnitude diagram, we identified nine ZZ Ceti stars.

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$\beta$ 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$\alpha$ and H$\beta$) have moderate correlations with the ratio of absorption to emission for H$\alpha$ (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.

We present extensive photometric and spectroscopic observations of the nearby Type Ia supernova (SN) 2023wrk at a distance of about 40 Mpc. The earliest detection of this SN can be traced back to a few hours after the explosion. Within the first few days, the light curve shows a bump feature, while the B − V color is blue and remains nearly constant. The overall spectral evolution is similar to that of an SN 1991T/SN 1999aa-like SN Ia, while the C ii λ 6580 absorption line appears to be unusually strong in the first spectrum taken at t ≈ −15.4 days after the maximum light. This carbon feature disappears quickly in subsequent evolution but it reappears at around the time of peak brightness. The complex evolution of the carbon line and the possible detection of Ni iii absorption around 4700 Å and 5300 Å in the earliest spectra indicate macroscopic mixing of fuel and ash. The strong carbon lines are likely related to the collision of SN ejecta with unbound carbon, consistent with the predictions of pulsational delayed-detonation or carbon-rich circumstellar-matter interaction models. Among those carbon-rich SNe Ia with strong C ii λ 6580 absorption at very early times, the line-strength ratio of C ii to Si ii and the B − V color evolution are found to exhibit large diversity, which may be attributed to different properties of unbound carbon and outward-mixing 56 Ni.

We present optical observations and analysis of a bright type Iax SN~2020udy hosted by NGC 0812. The light curve evolution of SN~2020udy is similar to other bright Iax SNe. Analytical modeling of the quasi bolometric light curves of SN 2020udy suggests that 0.08$\pm$0.01 M$_{\odot}$ of $^{56}$Ni would have been synthesized during the explosion. Spectral features of SN 2020udy are similar to the bright members of type Iax class showing weak Si {\sc II} line. The late-time spectral sequence is mostly dominated by Iron Group Elements (IGEs) with broad emission lines. Abundance tomography modeling of the spectral time series of SN~2020udy using TARDIS indicates stratification in the outer ejecta, however, to confirm this, spectral modeling at a very early phase is required. After maximum light, uniform mixing of chemical elements is sufficient to explain the spectral evolution. Unlike the case of normal type Ia SNe, the photospheric approximation remains robust until +100 days, requiring an additional continuum source. Overall, the observational features of SN 2020udy are consistent with the deflagration of a Carbon-Oxygen white dwarf.