Engineering topics

Im, Myungshin

Publications and source records attributed to Im, Myungshin.

5 records

DOE OSTI2024

A lanthanide-rich kilonova in the aftermath of a long gamma-ray burst

Observationally, kilonovae are astrophysical transients powered by the radioactive decay of nuclei heavier than iron, thought to be synthesized in the merger of two compact objects. Over the first few days, the kilonova evolution is dominated by a large number of radioactive isotopes contributing to the heating rate. On timescales of weeks to months, its behaviour is predicted to differ depending on the ejecta composition and the merger remnant. Previous work has shown that the kilonova associated with gamma-ray burst 230307A is similar to kilonova AT2017gfo, and mid-infrared spectra revealed an emission line at 2.15 micrometres that was attributed to tellurium. Here, in this study, we report a multi-wavelength analysis, including publicly available James Webb Space Telescope data and our own Hubble Space Telescope data, for the same gamma-ray burst. We model its evolution up to two months after the burst and show that, at these late times, the recession of the photospheric radius and the rapidly decaying bolometric luminosity (L bol ∝ t −2.7±0.4 , where t is time) support the recombination of lanthanide-rich ejecta as they cool.

79 ASTRONOMY AND ASTROPHYSICS↗

DOE OSTI2023

The Early Light Curve of the Type Ia Supernova 2021hpr in NGC 3147: Progenitor Constraints with the Companion Interaction Model

The progenitor system of Type Ia supernovae (SNe Ia) is expected to be a close binary system consisting of a carbon/oxygen white dwarf (WD) and a nondegenerate star or another WD. Here, we present results from high-cadence monitoring observations of SN 2021hpr in a spiral galaxy, NGC 3147, and constraints on the progenitor system based on its early multicolor light-curve data. First, we classify SN 2021hpr as a normal SN Ia from its long-term photometric and spectroscopic data. More interestingly, we found a significant "early excess" in the light curve over a simple power-law ~t 2 evolution. The early light curve evolves from blue to red to blue during the first week. To explain this, we fitted the early part of the BVRI-band light curves with a two-component model consisting of ejecta–companion interaction and a simple power-law model. The early excess and its color can be explained by shock-cooling emission due to a companion star having a radius of 8.84 ± 0.58 R ⊙ . We also examined Hubble Space Telescope preexplosion images, finding no detection of a progenitor candidate, consistent with the above result. However, we could not detect signs of a significant amount of stripped mass from a nondegenerate companion star (≲0.003 M ⊙ for Hα emission). The early excess light in the multiband light curve supports a nondegenerate companion in the progenitor system of SN 2021hpr. At the same time, the nondetection of emission lines opens the door for other methods to explain this event.

79 ASTRONOMY AND ASTROPHYSICS↗

NASA NTRS1995

The axis ratio distribution of faint galaxies: Evidence for a populatin of dwarfgalaxies at I approximately 20.5

The axis ratio distribution of faint galaxies observed by the Hubble Space Telescope (HST) as part of the Medium Deep Survey (MDS) key project suggests that a very large fraction of the total population at magnitudes I greater than or equal to 20 mag consists of a class of galaxies with luminosity profile, axis ratio distribution, angular size, and color that resemble local dwarfs. We find that galaxies with exponential light profiles and small angular sizes (half-light radius less than 0.6 sec) have an axis ratio distribution that is incompatible with their being intrinsically flattened objects and is instead consistent with local elliptical galaxies. We call these objects 'Small Exponential Ellipticals.' They are most likely dwarf galaxies, and they are numerous enough that, together with irregular galaxies, they can account for most if not all of the excess in the number counts at I approximately 20-21 mag with respect to the standard no-evolution models. This may suggest that the excess number counts are best explained by dwarf-rich models with strong luminosity evolution of the dwarf galaxies. Our data also supports a very mild luminosity evolution of the giant galaxy populations, which contributes little to the excess number counts.

Im, Myungshin↗

NASA NTRS1995

A test of galaxy evolutionary models via angular sizes

The relationship between angular size, magnitude, and redshift of faint galaxies is explored as a potential tool to distinguish between galaxy evolutionary models. Different models, based on merging, mild luminosity evolution, and no evolution, lead to different predictions of the angular size distribution, redshift- size relation, and magnitude-size relation. The merging model predicts significantly smaller sizes for faint galaxies than the standard model, because of the requirement for more intrinsically small faint objects at high redshift. A dwarf-rich no-evolution model also predicts small sizes for faint galaxies. The mild luminosity evolution model predicts more luminous galaxies of large angular size at high redshift, as does a standard no-evolution model. Prefurbishment Hubble Space Telescope (HST) Medium Deep Survey observations of magnitudes and sizes of faint galaxies indicate an excess of small versus large faint galaxies, favoring the dwarf rich, no evolution model with respect to the merging model; the other two models are more discrepant with the data. While these results cannot yet rule out with certainty any of the proposed models, they demonstrate the potential of angular size to discriminate between models of galaxy evolution, especially with the high-resolution HST wide field/planetary camera-2 (WFPC2) data.

Im, Myungshin↗

NASA NTRS1994

The Theta-z relation for HST bulges and disks out to z approximately equal 0.8

We present Hubble Space Telescope (HST) scale lengths and ground-based redshifts for 63 faint field galaxies down to I less than or approximately 21.5 mag from the Medium-Deep Survey. These have measured redshifts z less than or approximately 0.8 and half-light radii 0.1 sec less than or approximately r(sub e), r(sub s) less than or approximately 5 sec. We present the Theta-z relation for r(exp 1/4)-bulges and exponential disks separately for world models with q(sub 0) = 0.0-1.0. We show that selection against low surface brightness galaxies in the HST images and ground-based spectra is comparable to that in local surveys. We compare our HST disk-dominated galaxies to a magnitude-limited subsample of the European Southern Observatory (ESO)-Uppsala local spirals. Extrapolating the best-fit local exponential disk scale length (r(sub s) = 3.5 kpc for H(sub 0) = 75) out to z = 0.8, we find a rather symmetrical distribution of HST disks around this value. This implies that galaxy disks have been stable since z is approximately equal to 0.8. We compare our HST bulge-dominated galaxies with r(exp 1/4)-profiles to a magnitude-limited subsample of local Seven Samurai ellipticals. The latter have a local best fit r(sub e) = 5.7 kpc. Our HST bulge sample shows a similar distribution for z less than or approximately 0.8. Elliptical galaxy scale lengths have thus also remained rather constant since z less than or approximately 0.8. We set limits to their possible evolution.

Mutz, Steven B.↗