12 research outputs found
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 to
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 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.Comment: 20 pages, 10 figures, accepted for publication in MNRA
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 > 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., , 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 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.,
mag) and moderate post-peak decline rate (i.e.,
mag). The nickel mass synthesized in the explosion is estimated as
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 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 - and -band light curves of ATLAS and in the
-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
Properties and Asteroseismological analysis of a new ZZ ceti discovered by TMTS
Tsinghua university-Ma Huateng Telescope for Survey (TMTS) aims to discover
rapidly evolving transients by monitoring the northern sky. The TMTS catalog is
cross-matched with the white dwarf (WD) catalog of Gaia EDR3, and light curves
of more than a thousand WD candidates are obtained so far. Among them, the WD
TMTS J23450729+5813146 (hereafter J2345) is one interesting common source.
Based on the light curves from the TMTS and follow-up photometric observations,
periods of 967.113 s, 973.734 s, 881.525 s, 843.458 s, 806.916 s and 678.273 s
are identified. In addition, the TESS observations suggest a 3.39 h period but
this can be attributed to the rotation of a comoving M dwarf located within 3".
The spectroscopic observation indicates that this WD is DA type with Teff =
11778+/-617K,log g = 8.38+/-0.31,mass=0.84+/-0.20Msun and age=0.704+/-0.377
Gyrs. Asteroseismological analysis reveals a global best-fit solution of Teff
=12110+/-10K and mass=0.760+/-0.005Msun,consistent with the spectral fitting
results, and Oxygen and Carbon abundances in the core center are 0.73 and 0.27,
respectively. The distance derived from the intrinsic luminosity given by
asteroseismology is 93 parsec, which is in agreement with the distance of 98
parsec from Gaia DR3. Additionally, kinematic study shows that this WD is
likely a thick disk star. The mass of its zero-age main-sequence mass is
estimated to be 3.08 Msun and has a main-sequence plus cooling age of roughly
900 Myrs.Comment: 10 pages, 10 figures, accepted for publication in 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
min (classical BLAPs, hereafter) and the other group with pulsation period
below 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-
Forecast of Cosmological Constraints with Type Ia Supernovae from the Chinese Space Station Telescope
The 2-m aperture Chinese Space Station Telescope (CSST), which observes at
wavelengths ranging from 255 to 1000 nm, is expected to start science
operations in 2024. An ultra-deep field observation program covering
approximately 10 square degrees is proposed with supernovae (SNe) and other
transients as one of its primary science drivers. This paper presents the
simulated detection results of type Ia supernovae (SNe Ia) and explores the
impact of new datasets on the determinations of cosmological parameters. The
simulated observations are conducted with an exposure time of 150 s and
cadences of 10, 20, and 30 days. The survey mode covering a total of 80
observations but with a random cadence in the range of 4 to 14 days is also
explored. Our simulation results indicate that the CSST can detect up to SNe Ia at z 1.3. The simulated SNe Ia are then used to constrain the
cosmological parameters. The constraint on can be improved by 37.5%
using the 10-day cadence sample in comparison with the Pantheon sample. A
deeper measurement simulation with a 300 s exposure time together with the
Pantheon sample improves the current constraints on by 58.3% and
by 47.7%. Taking future ground-based SNe Ia surveys into
consideration, the constraints on can be improved by 59.1%. The CSST
ultra-deep field observation program is expected to discover large amounts of
SNe Ia over a broad redshift span and enhance our understanding of the nature
of dark energy.Comment: 10 pages, 6 figures, 2 tables, accepted for publication in SCIENCE
CHINA Physics, Mechanics & Astronom
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., = 14.5 mag). Our
photometric analysis reveals that SN 2016ije is subluminous ( =
17.650.06 mag) but exhibits relatively broad light curves
( = 1.350.14 mag), similar to the behavior of SN
2002es. Our analysis of the bolometric light curve indicates that only
0.140.04 of 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
( 4500 km s) is much lower than that from silicon lines (
8300 km s) 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.Comment: 25 pages, 13 figure