Color excesses of type Ia supernovae from the single-degenerate channel model

The single degenerate model is the most widely accepted progenitor model of type Ia supernovae （SNe Ia）, in which a carbon-oxygen white dwarf （CO WD） accretes hydrogen-rich material from a main sequence or a slightly evolved star （WD＋MS） to increase its mass, and explodes when its mass approaches the Chandrasekhar mass limit. During the mass transfer phase between the two components, an optically thick wind may occur and the material lost as wind may exist as circumstellar material （CSM）. Searching for the CSM around a progenitor star is helpful for discriminating different progenitor models of SNe Ia. In addition, the CSM is a source of color excess. The purpose of this paper is to study the color excess produced from the single-degenerate progenitor model with an optically thick wind, and reproduce the distribution of color excesses of SNe Ia. Meng et al. systemically carded out binary evolution calculations of the WD ＋MS systems for various metallicities and showed the parameters of the systems before Roche lobe overflow and at the moment of supernova explosion in Meng ＆ Yang. With the results of Meng et al., we calculate the color excesses of SNe Ia at maximum light via a simple analytic method. We reproduce the distribution of color excesses of SNe Ia by our binary population synthesis approach if the velocity of the optically thick wind is taken to be an order of magnitude of 10km s^-1. However, if the wind velocity is larger than 100km s^-1, the reproduction is bad.

Gnomon shadow lengths recorded in the Zhoubi Suanjing:the earliest meridian observations in China?

The Zhoubi Suanjing, one of the most important ancient Chinese books on mathematical astronomy, was compiled about 100 BC in the Western Han dynasty （BC 206 - AD 23）. We study the gnomon shadow lengths for the 24 solar terms as recorded in the book. Special attention is paid to the so-called law of ‘cun qian li’, which says the shadow length of a gnomon of 8 chi （about 1.96 m） high will increase （or decrease） 1 cun （1/10chi） for every 10001i （roughly 400kin） the gnomon moves northward （or south- ward）. From these data, one can derive the time and location of the observations. The resuits, however, do not fit historical facts. We suggest that compilers of the Zhoubi Suanjing must have modified the original data according to the law of ‘cun qian li’. Through reversing the situation, we recovered the original data, our analysis of which reveals the best possible observation time as 564 BC and the location of observation as 35.78° N latitude. We conclude that this must be the earliest records of solar meridian observations in China. In the meantime, we give the errors of solar altitudes for the 24 solar terms. The average deviation is 5.22°, and the mean absolute deviation is 5.52°, signifying the accuracy of astronomical calculations from that time.

Constraining Dark Energy and Cosmological Transition Redshift with Type Ia Supernovae

The property of dark energy and the physical reason for the acceleration of the present universe are two of the most difficult problems in modern cosmology. The dark energy contributes about two-thirds of the critical density of the present universe from the observations of type-Ia supernovae （SNe Ia） and anisotropy of cosmic microwave background （CMB）. The SN Ia observations also suggest that the universe expanded from a deceleration to an acceleration phase at some redshift, implying the existence of a nearly uniform component of dark energy with negative pressure. We use the ＂Gold＂ sample containing 157 SNe Ia and two recent well-measured additions, SNe Ia 1994ae and 1998aq to explore the properties of dark energy and the transition redshift. For a fiat universe with the cosmological constant, we measure ΩM=0.28-0.05^＋0.04 which is consistent with Riess et al. The transition redshift is zT=0.60-0.08^＋0.04. We also discuss several dark energy models that define w（z） of the parameterized equation of state of dark energy including one parameter and two parameters （w（z） being the ratio of the pressure to energy density）. Our calculations show that the accurately calculated transition redshift varies from zT =0.29-0.06^＋0.07 to zT =0.60-0.08^＋0.06 across these models. We also calculate the minimum redshift zc at which the current observations need the universe to accelerate.

Constraints on Deceleration Parameter of a 5D Bounce Cosmological Model from Recent Cosmic Observations

We study the constraint on deceleration parameter q from the recent SNela Gold dataset and observational Hubble data by using a model-independent deceleration parameter q（z） = 1/2 - a/（1 ＋ z）^b under the flve-dimensional bounce cosmological model. For the cases of SNeIa Gold dataset, Hubble data, and their combination, the present results show that the constraints on transition redshift ZT are 0.35-0.07^＋0.14,0.68-0.58^＋1.47,and 0.55-0.09^＋0.18 with 1σ errors,respectively.

Are Type Ia Supernovae Reliable Distance Indicators?

Recent applications of type Ia supernovae （SNe Ia） in cosmology have successfully revealed the accelerating expansion of the universe. However, as distance indicators used in measuring the expansion history of the universe and probing the nature of dark energy, these objects must pass more strict tests. We propose a K-S test to investigate if there exists any systematic bias when deriving the luminosity distances under the standard candle assumption. Two samples, one comprising 71 high-redshift SNe Ia and the other, 44 nearby ones, are used in our investigation. We find that it is likely there exists a bias in the adopted samples, which is probably caused by a systematic error, e.g. in the color parameter used in the luminosity calibration and a bias may be caused by the SN evolution or by varying properties of the dust surrounding the SNe Ia.

Testing the distance-duality relation with a combination of cosmological distance observations

We propose an accurate test of the distance-duality （DD） relation, η = DL（z）（1 ＋ z）^-2/DA（z） = 1 （where DL and DA are the luminosity distances and angular diameter distances, respectively）, with a combination of cosmological observational data of Type Ia Supernovae （SNe Ia） from the Union2 set and the galaxy cluster sample under an assumption of the spherical model. In order to avoid bias brought on by redshift non-coincidence between observational data and to consider redshift error bars of both clusters and SNe Ia in the analysis, we carefully choose the SNe Ia points which have the minimum acceptable redshift difference of the galaxy cluster sample （│△Z│min = σz,SN ＋σz,cluster）. By assuming η to be a constant and defined as functions of the redshift parameterized by six different expressions, we find that there exists no observable evidence for variations in the DD relation based on the collected data, since related statistical tests are well satisfied within the 1σ confidence level for most cases. Further, considering different values of △z as constraints, we also find that the choice of △z may play an important role in this model-independent test of the DD relation for the spherical sample of galaxy clusters.

The cooling time of white dwarfs produced from type Iasupernovae

Type Ia supernovae （SNe Ia） play a key role in measuring cosmological parameters, in which the Phillips relation is adopted. However, the origin of the relation is still unclear. Several parameters are suggested, e.g. the relative content of carbon to oxygen （C/O） and the central density of the white dwarf （WD） at ignition. These parameters are mainly determined by the WD＇s initial mass and its cooling time, respectively. Using the progenitor model developed by Meng ＆ Yang, we present the distributions of the initial WD mass and the cooling time. We do not find any correlation between these parameters. However, we notice that as the range of the WD＇s mass decreases, its average value increases with the cooling time. These results could provide a constraint when simulating the SN Ia explosion, i.e. the WDs with a high C/O ratio usually have a lower central density at ignition, while those having the highest central density at ignition generally have a lower C/O ratio. The cooling time is mainly determined by the evolutionary age of secondaries, and the scatter of the cooling time decreases with the evolutionary age. Our results may indicate that WDs with a long cooling time have more uniform properties than those with a short cooling time, which may be helpful to explain why SNe Ia in elliptical galaxies have a more uniform maximum luminosity than those in spiral galaxies.

Galaxy formation in the reionization epoch as hinted by WideField Camera 3 observations of the Hubble Ultra Deep Field

We present a large sample of candidate galaxies at z ≈ 7 - 10, selected in the Hubble Ultra Deep Field using the new observations of the Wide Field Camera 3 that was recently installed on the Hubble Space Telescope. Our sample is composed of 20 z850-dropouts （four new discoveries）, 15 Y105-dropouts （nine new discoveries） and 20 J125-dropouts （all new discoveries）. The surface densities of the Z850-dropouts are close to what was predicted by earlier studies, however, those of the Y105- and J125-dropouts are quite unexpected. While no Y105- or J125-dropouts have been found at AB ≤ 28.0 mag, their surface densities seem to increase sharply at fainter levels. While some of these candidates seem to be close to foreground galaxies and thus could possibly be gravitationally lensed, the overall surface densities after excluding such cases are still much higher than what would be expected if the luminosity function does not evolve from z ～ 7 to 10. Motivated by such steep increases, we tentatively propose a set of Schechter function parameters to describe the luminosity functions at z ≈8 and 10. As compared to their counterpart at z ≈ 7, here L^＊ decreases by a factor of ～ 6.5 and Ф^＊ increases by a factor of 17-90. Although such parameters are not yet demanded by the existing observations, they are allowed and seem to agree with the data better than other alternatives. If these luminosity functions are still valid beyond our current detection limit, this would imply a sudden emergence of a large number of low-luminosity galaxies when looking back in time to z ≈ 10, which, while seemingly exotic, would naturally fit in the picture of the cosmic hydrogen reionization. These early galaxies could easily account for the ionizing photon budget required by the reionization, and they would imply that the global star formation rate density might start from a very high value at z ≈ 10, rapidly reach the minimum at z≈ 7, and start to rise again towards z ≈ 6. In this scenario, the majority of the stellar mass that the universe assembled through the reionization epoch seems still undetected by current observations at z ≈ 6.

Full-disk synoptic observations of the chromosphere using Hα telescope at the Kodaikanal Observatory

This paper reports on the installation and observations of a new solar telescope installed on 2014 October 7 at the Kodaikanal Observatory. The telescope is a refractive type equipped with a tunable Lyot Hα filter. A CCD camera with size 2k ×2k acquires images of the Sun and has a pixel size of 1.21″ pixel^-1 and a full field-of-view of 41＇. The telescope is equipped with a guiding system which keeps the image of the Sun within a few pixels throughout the observations. The FWHM of the Lyot filter is 0.4A and the filter is motorized, capable of scanning the Hα line profile at a smaller step size of 0.01 A. Partial-disk imaging covering about 10＇ is also possible with the help of a relay lens kept in front of the CCD camera. In this paper, we report the detailed specifications of the telescope, filter unit, the installation, observations and the procedures we have followed to calibrate and align the data. We also present preliminary results with this new full-disk telescope.

The Peak Luminosity of Type Ia Supernovae and its Implications for the Cosmic Expansion Rate

Supernovae of type Ia (SNe Ia) are confirmed to be the best distance indicators to derive the cosmic expansion rate. The dispersion of their peak luminosity at optical bands (BVI) is approximate to 0.13 mag, after taking into account the effects of the second parameters (i.e., the Antial decline rate m15(B) and (B - V) color at maximum light). The local calibrations from HST indicate an absolute magnitude of 19.48 ± 0.08mag (in V band) for SNe Ia in spiral galaxies. The current expansion rate, Ho, is found to be 63.6 ± 1.8 (random) ±5.7 (systematic) kms-1 Mpc-1. This Value will decrease by 3% when the metallicity effect on the cepheid distances is considered. In addition, a marginal local outward flow of 4.0 ± 4.5% within the velocity-distance of 7000 km s-1 can be inferred from SNe Ia for the Einstein-de Sitter universe; however, this outward flow is only 2.2 ± 4.4% for an accelerating expansion universe (which is supported by high-z SNe Ia).

Redshift drift reconstruction for some cosmological models from observations

Redshift drift is a tool to directly probe the expansion history of the uni- verse. Based on the Friedmann-Robertson-Walker framework, we reconstruct the ve- locity drift and deceleration factor for several cosmological models using observa- tional H（z） data from the differential ages of galaxies and baryon acoustic oscillation peaks, luminosity distance of Type Ia supernovae, cosmic microwave background shift parameter, and baryon acoustic oscillation distance parameter. They can, for the first time, provide an objective and quantifiable measure of the redshift drift. We find that reconstructed velocity drift with different peak values and corresponding redshifts can potentially provide a method to distinguish the quality of competing dark energy mod- els at low redshifts. Better fitting between models and observational data indicate that current data are insufficient to distinguish the quality of these models. However, by comparing with the simulated velocity drift from Liske et al, we find that the Dvali- Gabadadze-Porrati model is inconsistent with the data at high redshift, which origi- nally piqued the interest of researchers in the topic of redshift drift. Considering the deceleration factor, we are able to give a stable instantaneous estimation of a transition redshift of zt ~ 0.7 from joint constraints, which incorporates a more complete set of values than the previous study that used a single data set.

Gamma-ray bursts and their links with supernovae and cosmology

Gamma-ray bursts are the most luminous explosions in the Universe, whose origin and mechanism are the focus of intense interest. They appear connected to su- pernova remnants from massive stars or the merger of their remnants, and their bright- ness makes them temporarily detectable out to the largest distances yet explored in the universe. After pioneering breakthroughs from space and ground experiments, their study is entering a new phase with observations from the recently launched Fermi satellite, as well as the prospect of detections or limits from large neutrino and gravitational wave detectors. The interplay between such observations and theoretical models of gamma-ray bursts is reviewed, and cosmology. as well as their connections to supernovae

The two promising scenarios to explode core collapse supernovae

I compare to each other what I consider to be the two most promising scenarios to explode core-collapse supernovae （CCSNe）. Both are based on the negative jet feedback mechanism （JFM）. In the jittering jets scenario a collapsing core of a single slowly-rotating star can launch jets. The accretion disk or belt （a sub-Keplerian accretion flow concentrated toward the equatorial plane） that launches the jets is intermittent with varying directions of the axis. Instabilities, such as the standing accretion shock instability （SASI）, lead to stochastic angular momentum variations that allow the formation of the inter- mittent accretion disks/belts. According to this scenario no failed CCSNe exist. According to the fixed axis scenario, the core of the progenitor star must be spun up during its late evolutionary phases, and hence all CCSNe are descendants of strongly interacting binary systems, most likely through a common envelope evolution （whether the companion survives or not）. Due to the strong binary interaction, the axis of the accretion disk that is formed around the newly born neutron star has a more or less fixed direction. According to the fixed axis scenario, accretion disks/belts are not formed around the newly born neutron star of single stars; they rather end in failed CCSNe. I also raise the possibility that the jittering jets scenario operates for progenitors with initial mass of 8 Me⊙ MZAMS≤18 M⊙, while the fixed axis scenario operates for MZAMS≤ 18 M⊙. For the first time these two scenarios are compared to each other, as well as to some aspects of the neutrino-driven explosion mechanism. These new comparisons further suggest that the JFM plays a major role in exploding massive stars.

Dependence of low redshift Type Ia Supernovae luminosities on host galaxies

We study the relation between Type Ia Supernovae （SNe Ia） and properties of their host galaxies using a large sample with low redshift. By examining the Hubble residuals of the entire sample from the best-fit cosmology, we show that SNe Ia in passive hosts are brighter than those in star-forming hosts after light curve correction at the 2. 1σ confidence level. We find that SNe Ia in high luminosity hosts are brighter after light-curve correction at the 〉 3σ confidence level. We also find that SNe Ia in large galaxies are brighter after light-curve correction at the ≥2σ confidence level. We demonstrate that the residuals depend linearly on host luminosity at a confidence of 4or or host size at a confidence of 3.3σ.

WD+RG systems as the progenitors of type Ia supernovae

Type Ia supernovae （SNe Ia） play an important role in the study of cosmic evolution, especially in cosmology. There are several progenitor models for SNe Ia proposed in the past years. By considering the effect of accretion disk instability on the evolution of white dwarf （WD） binaries, we performed detailed binary evolution calculations for the WD ＋ red-giant （RG） channel of SNe Ia, in which a carbon--oxygen WD accretes material from a RG star to increase its mass to the Chandrasekhar mass limit. According to these calculations, we mapped out the initial and final parameters for SNe Ia in the orbital period-secondary mass （log Pi_ M2^i） plane for various WD masses for this channel. We discussed the influence of the variation of the duty cycle value on the regions for producing SNe Ia. Similar to previous studies, this work also indicates that the long-period dwarf novae offer possible ways for producing SNe Ia. Meanwhile, we find that the surviving companion stars from this channel have a low mass after the SN explosion, which may provide a means for the formation of the population of single low-mass WDs （〈0.45 M⊙）.

Double-detonation model of type Ia supernovae with a variable helium layer ignition mass

Although Type Ia supernovae （SNe Ia） play an important role in the study of cosmology, their progenitors are still poorly understood. Thermonuclear explosions from the helium double-detonation sub-Chandrasekhar mass model have been considered as an alternative method for producing SNe Ia. By adopting the assumption that a double detonation occurs when a He layer with a critical ignition mass accumulates on the surface of a carbon-oxygen white dwarf （CO WD）, we perform detailed binary evolution calculations for the He double-detonation model, in which a He layer from a He star accumulates on a CO WD. According to these calculations, we obtain the initial parameter spaces for SNe Ia in the orbital period and secondary mass plane for various initial WD masses. We implement these results into a detailed binary population synthesis approach to calculate SN Ia birthrates and delay times. From this model, the SN Ia birthrate in our Galaxy is ～0.4 - 1.6 × 10^-3 yr^-1. This indicates that the double-detonation model only produces part of the SNe la. The delay times from this model are ～ 70 - 710 Myr, which contribute to the young population of SNe Ia in the observations. We found that the CO WD ＋ sdB star system CD-30 11223 could produce an SN Ia via the double-detonation model in its future evolution.

How to co-add images? I. A new iterative method for image reconstruction of dithered observations

By employing the previous Voronoi approach and replacing its nearest neighbor approx- imation with Drizzle in iterative signal extraction, we develop a fast iterative Drizzle algorithm, namedfiDrizzle, to reconstruct the underlying band-limited image from undersampled dithered frames. Compared with the existing iDrizzle, the new algorithm improves rate of convergence and accelerates the computational speed. Moreover, under the same conditions （e.g. the same number of dithers and iterations）, fiDrizzle can make a better quality reconstruction than iDrizzle, due to the newly discov- ered High Sampling caused Decelerating Convergence （HSDC） effect in the iterative signal extraction process.fiDrizzle demonstrates its powerful ability to perform image deconvolution from undersampled dithers.

Preliminary results of CCD observations targeting Himalia acquired at Yunnan Observatories in 2015

In order to study the potential associated with high precision CCD astrometry of irregular satel- lites, we have acquired experimental observations of Himalia, the sixth and irregular satellite of Jupiter. A total of 185 CCD observations were obtained by using the 2.4m and 1 m telescopes administered by Yunnan Observatories over ten nights. Preliminary analysis of the observations were made, including geo- metric distortion, atmospheric refraction, and also the phase effect. All positions of Himalia are measured relative to the reference stars from the catalog UCAC4 in each CCD field of view. The theoretical positions of Himalia were retrieved from the Institute de M6chanique C61este et de Calcul des t^ph6m6rides, while the positions of Jupiter were obtained based on the planetary ephemeris INPOP13c. The results show that the means of observed minus computed （O - C） residuals are -0.004＂ and -0.002＂ in right ascension and declination, respectively. The standard deviations of （O - C） residuals are estimated to be about 0.04＂ in each direction.

Swift XRT Analysis of Type II-P Supernova SN 2008ij

This study investigates the X-ray properties and evolution of Type II supernovae (SNe II) observed by Swift, examining variations among supernovae and exploring their X-ray characteristics over time. We present the first X-ray study of the Type IIp supernova SN 2008ij using data from the Swift X-ray mission. This investigation focuses on its spectral properties, identifying an X-ray flux of 1.20 (+0.11, −0.10) × 10−13 erg/cm2/s and a plasma temperature of 4.76 (+1.22, −0.83) keV. Our study marks an advancement in understanding SN 2008ij, providing crucial results into its X-ray emission characteristics. These results lay the groundwork for future studies of Type IIp supernovae, offering a foundation for exploring their evolutionary and physical processes.

A critique of supernova data analysis in cosmology

Observational astronomy has shown significant growth over the last decade and has made important contributions to cosmology. A major paradigm shift in cosmology was brought about by observations of Type Ia supernovae. The notion that the universe is accelerating has led to several theoretical challenges. Unfortunately, although high-quality supernovae data-sets are being produced, their statistical anal- ysis leaves much to be desired. Instead of using the data to directly test the model, several studies seem to concentrate on assuming the model to be correct and limiting themselves to estimating model parameters and internal errors. As shown here, the important purpose of testing a cosmological theory is thereby vitiated.