Browsing by Author "Zhang, Hong Xin"

Now showing 1 - 4 of 4
  • Thumbnail Image
    Item
    Evolutionary stages and disk properties of young stellar objects in the Perseus cloud
    (NATL ASTRONOMICAL OBSERVATORIES, CHIN ACAD SCIENCES, 2015) Zhang, Hong Xin; Gao, Yu; Fang, Min; Yuan, Hai Bo; Zhao, Ying He; Chang, Rui Xiang; Jiang, Xue Jian; Liu, Xiao Wei; Luo, A Li; Ma, Hong Jun; Shao, Zheng Yi; Wang, Xiao Long
    We investigated the evolutionary stages and disk properties of 211 young stellar objects (YSOs) across the Perseus cloud by modeling their broadband optical to mid-infrared (IR) spectral energy distribution (SED). Our optical gri photometry data were obtained from the recently finished Purple Mountain Observatory Xuyi Schmidt Telescope Photometric Survey of the Galactic Anti-center (XSTPS-GAC). About 81% of our sample fall into the Stage II phase which is characterized by having optically thick disks, while 14% into the Stage I phase characterized by having significant infalling envelopes, and the remaining 5% into the Stage III phase characterized by having optically thin disks. The median stellar age and mass of the Perseus YSOs are 3.1 Myr and 0.3 M-circle dot respectively. By exploring the relationships among the turnoff wave bands lambda(turnoff) (longward of which significant IR excesses above the stellar photosphere are observed), the excess spectral index alpha(excess) as determined for lambda > lambda(turnoff), and the disk inner radius R-in (determined from SED modeling) for YSOs at different evolutionary stages, we found that the median and standard deviation of alpha(excess) for YSOs with optically thick disks tend to increase with lambda(turnoff), especially at lambda(turnoff) >= 5.8 mu m, whereas the median fractional dust luminosities L-dust/L star tend to decrease with increasing lambda(turnoff). This points to an inside-out process of disk clearing for small dust grains. Moreover, a positive correlation between alpha(excess) and R-in was found at alpha(excess) greater than or similar to 0 and R-in greater than or similar to 10 x the dust sublimation radius R-sub, irrespective of lambda(turnoff), L-dust/L star and disk flaring. This suggests that the outer disk flaring either does not evolve synchronously with the inside-out disk clearing of small dust grains or has little appreciable influence on the spectral slopes at lambda less than or similar to 24 mu m. About 23% of our YSO disks are classified as transitional disks, which have lambda(turnoff) = 5.8 mu m and L-dust/L star > 10(-3). The transitional disks and full disks occupy distinctly different regions on the L-dust/L star vs. alpha(excess) diagram. Taking L-dust/L star as an approximate discriminator of disks with (>0.1) and without (<0.1) considerable accretion activity, we found that 65% and 35% of the transitional disks may be consistent with being dominantly cleared by photoevaporation and dynamical interaction with giant planets respectively. None of our transitional disks have alpha(excess) (<0.0) or L-dust/L star (>0.1) values that would otherwise be suggestive of disk clearing dominanted by grain growth.
  • Thumbnail Image
    Item
    Intrinsic Morphology of Ultra-diffuse Galaxies
    (IOP PUBLISHING LTD, 2020) Rong, Yu; Dong, Xiao Yu; Puzia, Thomas H.; Galaz, Gaspar; Sanchez Janssen, Ruben; Cao, Tianwen; van der Burg, Remco F. J.; Sifon, Cristobal; Pina, Pavel E. Mancera; Marcelo, Mora; D'Ago, Giuseppe; Zhang, Hong Xin; Johnston, Evelyn J.; Eigenthaler, Paul
    With the published data of apparent axis ratios for 1109 ultra-diffuse galaxies (UDGs) located in 17 low-redshift (z similar to 0.020-0.063) galaxy clusters and 84 UDGs in two intermediate-redshift (z similar to 0.308-0.348) clusters, we take advantage of a Markov Chain Monte Carlo approach and assume a triaxial model to investigate the intrinsic morphologies of UDGs. In contrast to the conclusion of Burkert, i.e., the underlying shapes of UDGs are purely prolate (C = B < A), we find that the data favor the oblate-triaxial models (i.e., thick disks with C < B less than or similar to A) over the nearly prolate ones. We also find that the intrinsic morphologies of UDGs are related to their stellar masses/luminosities, environments, and redshifts. First, the more luminous UDGs have puffier morphologies compared with the less luminous counterparts; the UDG morphologic dependence on luminosity is distinct from that of the typical quiescent dwarf ellipticals (dEs) and dwarf spheroidals (dSphs); in this sense, UDGs may not be simply treated as an extension of the dE/dSph class with similar evolutionary histories; they may differ not only in size. Second, the UDGs with smaller clustercentric distances are more puffed up, compared with the counterparts with larger clustercentric distances; in combination with the UDG thickness dependence on luminosity, the puffier morphologies of UDGs with high luminosities or located in the denser environments are very likely to be attributed to tidal interactions with massive galaxies. Third, we find that the intermediate-redshift UDGs are more flattened, compared with the low-redshift counterparts, which plausibly suggests a "disky" origin for the high-redshift, newly born UDGs.
  • Thumbnail Image
    Item
    MASS-TO-LIGHT VERSUS COLOR RELATIONS FOR DWARF IRREGULAR GALAXIES
    (IOP PUBLISHING LTD, 2016) Herrmann, Kimberly A.; Hunter, Deidre A.; Zhang, Hong Xin; Elmegreen, Bruce G.
    We have determined new relations between UBV colors and mass-to-light ratios (M/L) for dwarf irregular (dIrr) galaxies, as well as for transformed g' - r'. These M/L to color relations (MLCRs) are based on stellar mass density profiles determined for 34 LITTLE THINGS dwarfs from spectral energy distribution fitting to multi-wavelength surface photometry in passbands from the FUV to the NIR. These relations can be used to determine stellar masses in dIrr galaxies for situations where other determinations of stellar mass are not possible. Our MLCRs are shallower than comparable MLCRs in the literature determined for spiral galaxies. We divided our dwarf data into four metallicity bins and found indications of a steepening of the MLCR with increased oxygen abundance, perhaps due to more line blanketing occurring at higher metallicity.
  • Thumbnail Image
    Item
    Stellar population properties of ultracompact dwarfs in M87 : a mass-metallicity correlation connecting low-metallicity globular clusters and compact ellipticals
    (2018) Zhang, Hong Xin; Puzia, Thomas H.; Peng, Eric W.; Liu, Chengze; Coté, Patrick; Ferrarese, Laura; Duc, Pierre Alain; Eigenthaler, Paul; Lim, Sungsoon; Muñoz, Roberto P.