Xiaodian Chen

The helium star donor channel for the progenitors of Type Ia supernovae

Type Ia supernovae (SNe Ia) play an important role in astrophysics, especially in the study of cosmic evolution. Several progenitor models for SNe Ia have been proposed in the past. In this paper we carry out a detailed study of the He star donor channel, in which a carbon-oxygen white dwarf (CO WD) accretes material from a He main-sequence star or a He subgiant to increase its mass to the Chandrasekhar mass. Employing Eggletons stellar evolution code with an optically thick wind assumption, and adopting the prescription of Kato & Hachisu for the mass accumulation efficiency of the He-shell flashes on to the WDs, we performed binary evolution calculations for about 2600 close WD binary systems. According to these calculations, we mapped out the initial parameters for SNe Ia in the orbital period-secondary mass (log P(i)-M(2)(i)) plane for various WD masses from this channel. The study shows that the He star donor channel is noteworthy for producing SNe Ia (similar to 1.2 x 10(-3) yr(-1) in our Galaxy), and that the progenitors from this channel may appear as supersoft X-ray sources. Importantly, this channel can explain SNe Ia with short delay times (similar to 10(8) yr), which is consistent with the recent observational implications of young populations of SN Ia progenitors.

MEASUREMENT OF 21 cm BRIGHTNESS FLUCTUATIONS AT z ∼ 0.8 IN CROSS-CORRELATION

In this Letter, 21 cm intensity maps acquired at the Green Bank Telescope are cross-correlated with large-scale structure traced by galaxies in the WiggleZ Dark Energy Survey. The data span the redshift range 0.6 < z < 1 over two fields totaling {approx}41 deg. sq. and 190 hr of radio integration time. The cross-correlation constrains {Omega}{sub HI} b{sub HI} r = [0.43 {+-} 0.07(stat.) {+-} 0.04(sys.)] Multiplication-Sign 10{sup -3}, where {Omega}{sub HI} is the neutral hydrogen (H I) fraction, r is the galaxy-hydrogen correlation coefficient, and b{sub HI} is the H I bias parameter. This is the most precise constraint on neutral hydrogen density fluctuations in a challenging redshift range. Our measurement improves the previous 21 cm cross-correlation at z {approx} 0.8 both in its precision and in the range of scales probed.

Evolution of Supermassive Black Hole Binaries and Acceleration of Jet Precession in Galactic Nuclei

Supermassive black hole binaries (SMBHBs) are expected with the hierarchical galaxy formation model. Currently, physics processes dominating the evolution of a SMBHB are unclear. An interesting question is whether we could observationally determine the evolution of SMBHBs and give constraints on the physical processes. Jet precession has been observed in many active galactic nuclei (AGNs) and is generally attributed to disk precession. In this paper we calculate the time variation of jet precession and conclude that jet precession is accelerated in SMBHB systems but decelerated in others. The acceleration of jet precession, -->dPpr/dt, is related to the jet precession timescale, -->Ppr, and the SMBHB evolution timescale, -->?a, as -->dPpr/dt ? ? (Ppr/?a) . Our calculations based on the models for jet precession and SMBHB evolution show that -->dPpr/dt can be as high as about ?1.0, with a typical value of ?0.2, and can be easily detected. We discuss the differential jet precession for NGC 1275 that has been observed in the literature. If its observed rapid acceleration of jet precession is true, the jet precession is due to the orbital motion of an unbound SMBHB with a mass ratio of -->q ? 0.76. When jets precess from ancient bubbles to the currently active jets, the separation of the SMBHB decreases from about 1.46 kpc to 0.80 kpc, with an averaged decreasing velocity of -->da/dt ? 1.54 ? 106 cm s?1 and an evolution timescale of -->?a ? 7.5 ? 107 yr. However, if we assume steady jet precession for many cycles, the observations imply a hard SMBHB with a mass ratio of a -->q ? 0.21 and a separation of -->a ? 0.29 pc.

Corrosion behavior of Ti-6Al-4V alloy welded by scanning electron beam

The corrosion behavior of Ti-6Al-4V alloy welded by SEE (scanning electron beam) is described in this paper. Solidification macrostructure of weld was transformed from 650-mu m columnar crystals to 100-mu m equiaxed crystals by SEE. Anodic polarization behavior of the weld was different following the change in solidification macrostructure. Anodic polarization curve of 150-mu m equiaxed-grain weld coincided with that of base metal, which indicated a best state of equal corrosion tendency between the base and the weld metal. Alloy element homogeneity in the equiaxed solidification structures made the weld more corrosion resistant than the columnar solidification structures. Fine microstructure and a small quantity of dislocations have a good effect on uniform corrosion resistance of the weld, but coarse microstructure, a spinodal decomposition of the metastable beta-phase and a large quantity of dislocations were related to the inferior resistance either to uniform corrosion or to local corrosion

A search for open cluster Cepheids in the Galactic plane

We analyse all potential combinations of Galactic Cepheids and open clusters (OCs) in the most up-to-date catalogues available. Isochrone fitting and proper-motion calcula- tion are applied to all potential OC{Cepheid combinations. Five selection criteria are used to select possible OC Cepheids: (i) the Cepheid of interest must be located within 60 arcmin of the OCs centre; (ii) the Cepheids proper motion is located within the 1 sigma distribution of that of its host OC; (iii) the Cepheid is located in the instability strip of its postulated host OC; (iv) the Cepheid and OC distance moduli should differ by less than 1 mag; and (v) the Cepheid and OC ages (and, where available, their metal- licities) should be comparable: {\Delta}log(t yr^-1) < 0.3. Nineteen possible OC Cepheids are found based on our near-infrared (NIR) analysis; eight additional OC{Cepheid associations may be genuine pairs for which we lack NIR data. Six of the Cepheids analysed at NIR wavelengths are new, high-probability OC Cepheids, since they lie on the near-infrared (NIR) period (P){luminosity relation (PLR). These objects include TY Sct and CN Sct in Dolidze 34, XX Sgr in Dolidze 52, CK Sct in NGC 6683, VY Car in ASCC 61 and U Car in Feinstein 1. Two additional new OC Cepheids lack NIR data: V0520 Cyg in NGC 6991 and CS Mon in Juchert 18. The NIR PLR for our confirmed sample of OC Cepheids is M_J = (-3.12 +/- 0.29) log(P day^-1)-(2.17 +/- 0.29) mag, which is in good agreement with the best NIR PLR available for all Galactic Cepheids.

CONTACT BINARIES AS VIABLE DISTANCE INDICATORS: NEW, COMPETITIVE (V)JHK s PERIOD–LUMINOSITY RELATIONS

Based on the largest catalogs currently available, comprising 6090 contact binaries (CBs) and 2167 open clusters, we determine the near-infrared

Physical parameter study of eight W Ursae Majoris-type contact binaries in NGC 188

JHK_{\rm s}

The Near-infrared optimal distances method applied to Galactic Classical Cepheids tightly constrains mid-infrared period-luminosity relations

CB period--luminosity (PL) relations, for the first time achieving the low levels of intrinsic scatter that make these relations viable as competitive distance calibrators. To firmly establish our distance calibration on the basis of open cluster CBs, we require that (i) the CB of interest must be located inside the core radius of its host cluster; (ii) the CBs proper motion must be located within the

The Optical–Mid-infrared Extinction Law of the l = 165° Sightline in the Galactic Plane: Diversity of the Extinction Law in the Diffuse Interstellar Medium

2\sigma

Optical–Mid-infrared Period–Luminosity Relations for W UMa-type Contact Binaries Based on Gaia DR 1: 8% Distance Accuracy

distribution of that of its host open cluster; and (iii) the CBs age,