18487685
Triggered Star Formation in the Orion Bright-rimmed Clouds
We have developed an empirical and effective set of criteria, based on the Two Micron All Sky Survey (2MASS) colors, to select candidate classical T Tauri stars (CTTSs). This provides a useful tool to study the young stellar population in star-forming regions. Here we present our analysis of the bright-rimmed clouds (BRCs) B35, B30, IC 2118, LDN 1616, LDN 1634, and Ori East to show how massive stars interact with molecular clouds to trigger star formation. Our results support the radiation-driven implosion model, in which the ionization fronts from OB stars compress a nearby cloud until the local density exceeds the critical value, thereby inducing the cloud to collapse to form stars. We find that only BRCs associated with strong IRAS 100 μm emission (a tracer of high density) and Hα emission (a tracer of ionization fronts) show signs of ongoing star formation. Relevant timescales, including the ages of O stars, expanding H II regions, and the ages of CTTSs, are consistent with sequential star formation. We also find that CTTSs are only seen between the OB stars and the BRCs, with those closer to the BRCs being progressively younger. There are no CTTSs leading the ionization fronts, i.e., within the molecular clouds. All of these findings provide strong evidence of triggered star formation and show the major roles massive stars play in sustaining the star-forming activities in the region.
2927391
A large sample of low surface brightness disc galaxies from the SDSS – I. The sample and the stellar populations
We present the properties of a large sample (12 282) of nearly face-on low surface brightness (LSB) disc galaxies selected from the main galaxy sample of SDSS-DR4. These properties include B-band central surface brightness μ 0 (B), scalelengths h, integrated magnitudes, colours and distances D. This sample has μ 0 (B) values from 22 to 24.5 mag arcsec -2 with a median value of 22.42 mag arcsec -2 , and disc scalelengths ranging from 2 to 19 kpc. They are quite bright with M B taking values from -18 to -23 mag with a median value of -20.08 mag. There exist clear correlations between log h and M B , log h and log D, log D and M B . However, no obvious correlations are found between μ 0 (B) and log h, colours, etc. The correlation between colours and log h is weak even though it exists. Both the optical-optical and optical-NIR colour-colour diagrams indicate that most of them have a mixture of young and old stellar populations. They also satisfy colour-magnitude relations, which indicate that brighter galaxies tend generally to be redder. The comparison between the LSBGs and a control sample of nearly face-on disc galaxies with higher surface brightness (HSB) with μ 0 (B) from 18.5 to 22 mag arcsec -2 show that, at a given luminosity or distance, the observed LSB galaxies tend to have larger scalelengths. These trends could be seen gradually by dividing both the LSBGs and HSBGs into two subgroups according to surface brightness. A volume-limited subsample was extracted to check the incompleteness of surface brightness. The only one of the property relations having an obvious change is the relation of log h versus μ 0 (B), which shows a correlation in this subsample.
118411365
The photometric system of the Tsinghua-NAOC 80-cm telescope at NAOC Xinglong Observatory
The Tsinghua-NAOC (National Astronomical Observatories of China) Telescope (hereafter, TNT) is an 80-cm Cassegrain reflecting telescope located at the Xinglong Observatory of NAOC, with the main scientific goals of monitoring various transients in the universe such as supernovae, gamma-ray bursts, novae, variable stars, and active galactic nuclei. We present a systematic test and analysis of the photometric performance of this telescope. Based on the calibration observations taken on 12 photometric nights, spanning the period from 2004 to 2012, we derived an accurate transformation equation between the instrumental ubvri magnitudes and standard Johnson UBV and Cousins RI magnitudes. In particular, the color terms and the extinction coefficients of different passbands are well determined. With these data, we also obtained the limiting magnitudes and the photometric precision of TNT. It is worthwhile to point out that the sky background at the Xinglong Observatory became gradually worse over the period from 2005 to 2012 (e.g., ~21.4 mag vs. ~20.1 mag in the V band).
14830508
SDSS J121811.0+465501.2: A New Low Surface Brightness Galaxy with Low Metallicity
We have serendipitously found a new nearby low surface brightness (LSB) galaxy from the SDSS database. We estimate the oxygen abundance of its H II region SDSS J121811.0+465501.2 from its electron temperature, as well as that of another H II region, SDSS J135440.5+535309.6, located in the irregular LSB galaxy UGC 8837. These two extragalactic H II regions were classified as stars in the SDSS DR4 database and were found occasionally by us in the automatic recognition and classification of stellar spectra. Their optical spectra show obvious emission lines, i.e., strong [O III] λλ4959, 5007 and Balmer emission lines, but very weak [N II] λλ6548, 6583 and [S II] λλ6317, 6731, which could indicate that they are metal-poor star-forming regions. The derived oxygen abundances of the two objects are 12 + log(O/H) ~ 7.88 ± 0.30 and 7.70 ± 0.30, respectively. The host of the H II region SDSS J121811.0+465501.2 is identified as a new nearly edge-on LSB disk galaxy (almost without a bulge), with a B-band central surface brightness μ0(B) of 23.68 mag arcsec-2 and an inclination angle of ~75° by using the GIM2D software to analyze its g- and r-band images independently. It is a nearby dwarf galaxy with redshift z ~ 0.00157, disk scale length ~0.40 kpc, and B-band absolute magnitude MB ~ -13.51 mag. The very low oxygen abundances of these two objects confirm the low metallicities of LSB galaxies.
119237976
A large sample of low surface brightness disc galaxies from the SDSS – II. Metallicities in surface brightness bins
We study the spectroscopic properties of a large sample of low surface brightness galaxies (LSBGs) [with B-band central surface brightness μ0(B) > 22 mag arcsec −2 ] selected from the Sloan Digital Sky Survey Data Release 4 (SDSS-DR4) main galaxy sample. A large sample of disc-dominated high surface brightness galaxies [HSBGs, with μ0(B) < 22 mag arcsec −2 ] are also selected for comparison simultaneously. To study them in more details, these sample galaxies are further divided into four subgroups according to μ0(B) (in units of mag arcsec −2 ): very low surface brightness galaxies (vLSBGs) (24.5–22.75), intermediate low surface brightness galaxies (iLSBGs) (22.75–22.0), intermediate high surface brightness galaxies (iHSBGs) (22.0–21.25) and very high surface brightness galaxies (vHSBGs) (<21.25). The diagnostic diagram from spectral emission-line ratios shows that the active galactic nucleus fractions of all the four subgroups are small (<9 per cent). The 21 032 star-forming galaxies with good-quality spectroscopic observations are further selected for studying their dust extinction, strong-line ratios, metallicities and stellar mass–metallicity relations. The vLSBGs have lower extinction values and have less metal-rich and massive galaxies than the other subgroups. The oxygen abundances of our LSBGs are not as low as those of the H II regions in LSBGs studied in literature, which could be because our samples are more luminous, and because of the different metallicity calibrations used. We find a correlation between 12 + log(O/H) and μ0(B) for vLSBGs, iLSBGs and iHSBGs but show that this could be a result of correlation between μ0(B) and stellar mass and the well-known mass–metallicity relation. This large sample shows that LSBGs span a wide range in metallicity and stellar mass, and they lie nearly on the stellar mass versus metallicity and N/O versus O/H relations of normal galaxies. This suggests that LSBGs and HSBGs have not had dramatically different star formation and chemical enrichment histories.
119256368
The properties of a large sample of low surface brightness galaxies from SDSS
A large sample of low surface brightness (LSB) disk galaxies is selected from SDSS with B-band central surface brightness mu_0(B) from 22 to 24.5 mag arcsec^(-2). Some of their properties are studied, such as magnitudes, surface brightness, scalelengths, colors, metallicities, stellar populations, stellar masses and multiwavelength SEDs from UV to IR etc. These properties of LSB galaxies have been compared with those of the galaxies with higher surface brightnesses. Then we check the variations of these properties following surface brightness.
120532642
Configuration dependence of moments of inertia in the odd-A nucleus from BCS theory
The moments of inertia (Mol) of odd-A and even-even rare-earth nuclei are calculated using the Nilsson + BCS method with and without the blocking effect taken into account. The calculated MoIs with blocking considered is smaller than that without blocking considered. The Mor of an odd-A nucleus depends strongly on which orbital is occupied by the odd nucleon. Although the calculated MoI's are systematically smaller than the data, the large fluctuations in the experimental odd-even differences in MoI's can be reproduced qualitatively. This is contrary to the common assertion that the MoI of an odd-A nucleus is some 15% larger than those of its neighboring even-even nuclei with the BCS theory and is helpful to understand the appearance of normally deformed identical bands in an odd-A nucleus and its even-even neighbors under the mean-field (BCS) approximation.
13913450
Ten Overdensities and an Arc Structure in the Galactic Halo
Aims. We use Sloan Digital Sky Survey (SDSS) DR5 photometric data to explore the structure of the Galaxy halo and hope to discover new companions or substructures which may reveal the forming history of the halo. Methods. We study overdensities in a sky area where � = 120 ◦ � 270 ◦ and � = 25 ◦ � 70 ◦ and pick out ten overdensity areas by comparing color-magnitude diagrams with known globular clusters or dwarf spheroidal galaxies. An automatic template matching method is developed to fit the color-magnitude diagram for ea ch overdensities with a suitable stellar evolution isochro nic line so that distance module and metalicity are roughly estimated. The performance test of the method indicates that our estimates on distance is acceptable. Results. The 10 overdensities and three known objects, UMa I, UMa II and Willman 1 are found to be connected with a long arc substructure starts from � 8kpc above the disk and spread to � 150kpc into north galactic cap. Evidence also shows a relationship between metalicities and distance modules of the overdensities. Conclusions. C7, one of the 10 overdensities is most likely a dwarf spheroidal galaxy according to radial density profile study and its absolute magnitude. Other overdensties are not classifi ed for poor information. The arc structure may either be a long tidal stream or be an intrinsic substructure of the halo. Follow-up study need to be done to clarify this problem.
11077788
A GRB Follow-up System at the Xinglong Observatory and Detection of the High-Redshift GRB 060927 ⋆
A gamma-ray burst (GRB) optical photometric follow-up system at the Xinglong Observatory of National Astronomical Observatories of China (NAOC) has been constructed. It uses the 0.8-m Tsinghua-NAOC Telescope (TNT) and the 1-m EST telescope, and can automatically respond to GRB Coordinates Network (GCN) alerts. Both telescopes slew relatively fast, being able to point to a new target field within ~1 min upon a request. Whenever available, the 2.16-m NAOC telescope is also used. In 2006 the system responded to 15 GRBs and detected seven early afterglows. In 2007 six GRBs have been detected among 18 follow-up observations. TNT observations of the second most distant GRB 060927 (z = 5.5) are shown, which started as early as 91 s after the GRB trigger. The afterglow was detected in the combined image of the first 19×20 s unfiltered exposures. This GRB follow-up system has joined the East-Asia GRB Follow-up Observation Network (EAFON).
119402342
ROSAC: Studying the clustering properties of X-ray selected AGNs
We present the first ROSAC results of an AGN clustering analysis. This study comprises a sample of 200 AGNs, 75% of which being at low redshifts z<0.5, in the Ursa Major constellation. The spatial 2-point-correlation function (SCF) as well as the minimal spanning tree (MST) technique were applied. Some evidence for clustering is found in the SCF, although with low significance. Using the MST technique, we could find two AGN groups. This result is preliminary and the exact significance will be tested with careful simulations.
67821379
Skyrme-Hartree-Fock plus Tensor Correction for Nuclear Matter
We study the equation of states of symmetric and pure neutron matter in the SkyrmeHartree-Fock (SHF) model with tensor corrections. We are aware now that the pion exchange interaction has a significant contribution to nuclear structure in light nuclei. The pion generates a strong tensor interaction between two nucleons, which cannot be treated within the Hartree-Fock framework for the spin-saturated system such as homogeneous nuclear matter. Therefore, we study the role of the tensor interaction based on the SHF model, in which we extend it by explicitly introducing two-particle-two-hole (2p-2h) excitations for the treatment of the tensor interaction in symmetric nuclear matter and pure neutron matter. We are able to describe infinite matter very well using the SHF model with tensor corrections. We also discuss the connection between the symmetry energy and the tensor interaction in this framework. Subject Index: 201