arXiv subjects

Jeong-Eun Lee

Publications and source records attributed to Jeong-Eun Lee.

230 records · Page 13Linked to original sources

arXiv2008

Massive-Star Forming Infrared Loop around the Crab-like Supernova Remnant G54.1+0.3: Post Main-Sequence Triggered Star Formation?

We report the discovery of a star-forming loop around the young, Crab-like supernova remnant (SNR) G54.1+0.3 using the AKARI infrared satellite. The loop consists of at least eleven young stellar objects (YSOs) embedded in a ring-like diffuse emission of radius ~1'. The YSOs are bright in the mid-infrared and are also visible in the Spitzer Space Telescope Galactic plane survey images. Their Spitzer colors are similar to those of class II YSOs in [3.6]-[5.8] but significantly redder in [8]-[24], i.e., 0<[3.6]-[5.8]<1.2 and 5<[8]-[24]<9. Most of them have near-infrared counterparts in the 2MASS JHKs images, and some of them have an optical counterpart too. Their JHKs colors and magnitudes indicate that the YSOs are massive (<= 10 Msun) pre-main-sequence stars at the same distance to the SNR, i.e., 8 kpc, which supports the association of the star-forming loop with the SNR. The dereddened spectral energy distributions are similar to eraly Herbig Be stars, which are early B-type pre-main-sequence stars with inner disks that have been destroyed. The confinement to a loop structure indicates that the YSOs are young, i.e., <= 2 Myr. We propose that their formation is triggered by the progenitor star of G54.1+0.3, which has a mass of <= 15 Msun. The triggering must have occurred near the end of the progenitor's life, possibly after it had evolved off the main sequence.

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arXiv2007

Chemical Evolution in VeLLOs

A new type of object called "Very Low Luminosity Objects (VeLLOs)" has been discovered by the Spitzer Space Telescope. VeLLOs might be substellar objects forming by accretion. However, some VeLLOs are associated with strong outflows, indicating the previous existence of massive accretion. The thermal history, which significantly affects the chemistry, between substellar objects with a continuous low accretion rate and objects in a quiescent phase after massive accretion (outburst) must be greatly different. In this study, the chemical evolution has been calculated in an episodic accretion model to show that CO and N2H+ have a relation different from starless cores or Class 0/I objects. Furthermore, the CO2 ice feature at 15.2 micron will be a good tracer of the thermal process in VeLLOs.

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arXiv2007

The Rich Mid-Infrared Environments of Two Highly-Obscured X-ray Binaries: Spitzer Observations of IGR J16318-4848 and GX 301-2

We present the results of Spitzer mid-infrared spectroscopic observations of two highly-obscured massive X-ray binaries: IGR J16318-4848 and GX301-2. Our observations reveal for the first time the extremely rich mid-infrared environments of this type of source, including multiple continuum emission components (a hot component with T > 700 K and a warm component with T ~ 180 K) with apparent silicate absorption features, numerous HI recombination lines, many forbidden ionic lines of low ionization potentials, and pure rotational H2 lines. This indicates that both sources have hot and warm circumstellar dust, ionized stellar winds, extended low-density ionized regions, and photo-dissociated regions. It appears difficult to attribute the total optical extinction of both sources to the hot and warm dust components, which suggests that there could be an otherwise observable colder dust component responsible for the most of the optical extinction and silicate absorption features. The observed mid-infrared spectra are similar to those from Luminous Blue Variables, indicating that the highly-obscured massive X-ray binaries may represent a previously unknown evolutionary phase of X-ray binaries with early-type optical companions. Our results highlight the importance and utility of mid-infrared spectroscopy to investigate highly-obscured X-ray binaries.

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arXiv2007

Molecular Line Observations of the Small Protostellar Group L1251B

We present molecular line observations of L1251B, a small group of pre- and protostellar objects, and its immediate environment in the dense C18O core L1251E. These data are complementary to near-infrared, submillimeter and millimeter continuum observations reported by Lee et al. (2006, ApJ, 648, 491; Paper I). The single-dish data of L1251B described here show very complex kinematics including infall, rotation and outflow motions, and the interferometer data reveal these in greater detail. Interferometer data of N2H+ 1-0 suggest a very rapidly rotating flattened envelope between two young stellar objects, IRS1 and IRS2. Also, interferometer data of CO 2-1 resolve the outflow associated with L1251B seen in single-dish maps into a few narrow and compact components. Furthermore, the high resolution data support recent theoretical studies of molecular depletions and enhancements that accompany the formation of protostars within dense cores. Beyond L1251B, single-dish data are also presented of a dense core located ~150" to the east that, in Paper I, was detected at 850 micron but has no associated point sources at near- and mid-infrared wavelengths. The relative brightness between molecules, which have different chemical timescales, suggests it is less chemically evolved than L1251B. This core may be a site for future star formation, however, since line profiles of HCO+, CS, and HCN show asymmetry with a stronger blue peak, which is interpreted as an infall signature.

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arXiv2006

The Spitzer c2d Survey of Nearby Dense Cores: III: Low Mass Star Formation in a Small Group, L1251B

We present a comprehensive study of a low-mass star-forming region,L1251B, at wavelengths from the near-infrared to the millimeter. L1251B, where only one protostar, IRAS 22376+7455, was known previously, is confirmed to be a small group of protostars based on observations with the Spitzer Space Telescope. The most luminous source of L1251B is located 5" north of the IRAS position. A near-infrared bipolar nebula, which is not associated with the brightest object and is located at the southeast corner of L1251B, has been detected in the IRAC bands. OVRO and SMA interferometric observations indicate that the brightest source and the bipolar nebula source in the IRAC bands are deeply embedded disk sources.Submillimeter continuum observations with single-dish telescopes and the SMA interferometric observations suggest two possible prestellar objects with very high column densities. Outside of the small group, many young stellar object candidates have been detected over a larger region of 12' x 12'. Extended emission to the east of L1251B has been detected at 850 micron; this "east core" may be a site for future star formation since no point source has been detected with IRAC or MIPS. This region is therefore a possible example of low-mass cluster formation, where a small group of pre- and protostellar objects (L1251B) is currently forming, alongside a large starless core (the east core).

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arXiv2005

Comparisons of an Evolutionary Chemical Model with Other Models

We compare an evolutionary chemical model with simple empirical models of the abundance and with static chemical models. We focus on the prediction of molecular line profiles that are commonly observed in low mass star forming cores. We show that empirical models can be used to constrain evaporation radii and infall radii using lines of some species. Species with more complex abundance profiles are not well represented by the empirical models. Static chemical models produce abundance profiles different from those obtained from an evolutionary calculation because static models do not account for the flow of matter inward from the outer regions. The resulting profiles of lines used to probe infall may differ substantially.

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arXiv2005

B335: A Laboratory for Astrochemistry in a Collapsing Cloud

We present observations of 25 transitions of 17 isotopologues of 9 molecules toward B335. With a goal of constraining chemical models of collapsing clouds, we compare our observations, along with data from the literature, to models of chemical abundances. The observed lines are simulated with a Monte Carlo code, which uses various physical models of density and velocity as a function of radius. The dust temperature as a function of radius is calculated self-consistently by a radiative transfer code. The gas temperature is then calculated at each radius, including gas-dust collisions, cosmic rays, photoelectric heating, and molecular cooling. The results provide the input to the Monte Carlo code. We consider both {\it ad hoc} step function models for chemical abundances and abundances taken from a self-consistent modeling of the evolution of a star-forming core. The step function models can match the observed lines reasonably well, but they require very unlikely combinations of radial variations in chemical abundances. Among the self-consistent chemical models, the observed lines are matched best by models with somewhat enhanced cosmic-ray ionization rates and sulfur abundances. We discuss briefly the steps needed to close the loop on the modeling of dust and gas, including off-center spectra of molecular lines.

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arXiv2004

Evolution of Chemistry and Molecular Line Profile during Protostellar Collapse

Understanding the chemical evolution in star-forming cores is a necessary pre-condition to correctly assess physical conditions when using molecular emission. We follow the evolution of chemistry and molecular line profiles through the entire star formation process, including a self-consistent treatment of dynamics, dust continuum radiative transfer, gas energetics, chemistry, molecular excitation, and line radiative transfer. In particular, the chemical code follows a gas parcel as it falls toward the center, passing through regimes of density, dust temperature, and gas temperature that are changing both because of the motion of the parcel and the evolving luminosity of the central source. We combine a sequence of Bonnor-Ebert spheres and the inside-out collapse model to describe dynamics from the pre-protostellar stage to later stages. The overall structures of abundance profiles show complex behavior that can be understood as interactions between freeze-out and evaporation of molecules. We find that the presence or absence of gas-phase CO has a tremendous effect on the less abundant species. In addition, the ambient radiation field and the grain properties have important effects on the chemical evolution, and the variations in abundance have strong effects on the predicted emission line profiles. Multi-transition and multi-position ob servations are necessary to constrain the parameters and interpret observations correctly in terms of physical conditions. Good spatial and spectral resolution is also important in distinguishing evolutionary stages.

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arXiv2004

Probing Pre-Protosellar Cores with Formaldehyde

We present maps of the 6 cm and 1.3 mm transitions of formaldehyde toward three cold, dense pre-protostellar cores: L1498, L1512, and L1544. The 6 cm transition is a unique probe of high density gas at low temperature. However, our models unequivocally indicate that H2CO is depleted in the interiors of PPCs, and depletion significantly affects how H2CO probes the earliest stages of star formation. Multi-stage, self-consistent models, including gas--dust energetics, of both H2CO transitions are presented, and the implications of the results are discussed.

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arXiv2002

Chemistry and Dynamics in Pre-Protostellar Cores

We have compared the intensity distribution of molecular line emission with that of dust continuum emission, and modeled molecular line profiles in three different preprotostellar cores in order to test how dynamical evolution is related to chemical evolution, and whether we can use different chemical tracers to identify specific dynamical evolutionary stages. We used dust continuum emission to obtain the input density and temperature structures by calculating radiative transfer of dust emission. Our results show that chemical evolution is dependent on dynamical processes, which can give different evolutionary timescales, as well as the density structure of the core.

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arXiv2002

Chemistry and Dynamics in Pre-Protostellar Cores

We have compared molecular line emission to dust continuum emission and modeled molecular lines using Monte Carlo simulations in order to study the depletion of molecules and the ionization fraction in three preprotostellar cores, L1512, L1544, and L1689B. L1512 is much less dense than L1544 and L1689B, which have similar density structures. L1689B has a different environment from those of L1512 and L1544. We used density and temperature profiles, calculated by modeling dust continuum emission in the submillimeter, for modeling molecular line profiles. In addition, we have used molecular line profiles and maps observed in several different molecules toward the three cores. We find a considerable diversity in chemical state among the three cores. The molecules include those sensitive to different timescales of chemical evolution such as CCS, the isotopes of CO and HCO+, DCO+, and N2H+. The CO molecule is significantly depleted in L1512 and L1544, but not in L1689B. CCS may be in the second enhancement of its abundance in L1512 and L1544 because of the significant depletion of CO molecules. N2H+ might already start to be depleted in L1512, but it traces very well the distribution of dust emission in L1544. On the other hand, L1689B may be so young that N2H+ has not reached its maximum yet. The ionization fraction has been calculated using H13CO+ and DCO+. This study suggests that chemical evolution depends on the absolute timescale during which a core stays in a given environment as well as its density structure.

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arXiv2001

Dust and Molecular Emission from the S235 Star Forming Region

We present the results of a multi-faceted study of the high-mass star-forming region S235. With the CalTech Submillimeter Observatory (CSO), we have observed and/or mapped over fifty molecular transitions and 350 micron dust emission in an attempt to learn more about the chemistry, kinematics, and mass distribution within this rich region. We have found two dust cores that are not resolved in our molecular line observations and suggest that one of these is a less evolved star-forming core.

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arXiv1999

Infrared Excess and Molecular Gas in Galactic Supershells

We have carried out high-resolution observations along one-dimensional cuts through the three Galactic supershells GS 064-01-97, GS 090-28-17, and GS 174+02-64 in the HI 21 cm and CO J=1-0 lines. By comparing the HI data with IRAS data, we have derived the distributions of the I_100 and tau_100 excesses, which are, respectively, the 100 mum intensity and 100 mum optical depth in excess of what would be expected from HI emission. We have found that both the I_100 and tau_100 excesses have good correlations with the CO integrated intensity W_CO in all three supershells. But the I_100 excess appears to underestimate H_2 column density N(H_2) by factors of 1.5-3.8. This factor is the ratio of atomic to molecular infrared emissivities, and we show that it can be roughly determined from the HI and IRAS data. By comparing the tau_100 excess with W_CO, we derive the conversion factor X = N(H_2)/W_CO = 0.26-0.66 in the three supershells. In GS 090-28-17, which is a very diffuse shell, our result suggests that the region with N(H_2) < 3*10^20 cm^-2 does not have observable CO emission, which appears to be consistent with previous results indicating that diffuse molecular gas is not observable in CO. Our results show that the molecular gas has a 60/100 mum color temperature T_d lower than the atomic gas. The low value of T_d might be due either to the low equilibrium temperature or to the lower abundance of small grains, or a combination of both.

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arXiv1999

Infrared Excess and Molecular Gas in the Galactic Worm GW46.4+5.5

We have carried out high-resolution (~3') HI and CO line observations along one-dimensional cuts through the Galactic worm GW46.4+5.5. By comparing the HI data with IRAS data, we have derived the distributions of I_100 excess and tau_100 excess, which are respectively the 100 mum intensity and 100 mum optical depth in excess of what would be expected from HI emission. In two observed regions, we were able to make a detailed comparison of the infrared excess and the CO emission. We have found that tau_100 excess has a very good correlation with the integrated intensity of CO emission, W_CO, but I_100 excess does not. There are two reasons for the poor correlation between I_100 excess and W_CO: firstly, there are regions with enhanced infrared emissivity without CO, and secondly, dust grains associated with molecular gas have a low infrared emissivity. In one region, these two factors completely hide the presence of molecular gas in the infrared. In the second region, we could identify the area with molecular gas, but I_100 excess significantly underestimates the column density of molecular hydrogen because of the second factor mentioned above. We therefore conclude that tau_100 excess, rather than I_100 excess, is an accurate indicator of molecular content along the line of sight. We derive tau_100/N(H)=(1.00+-0.02)*10^-5~(10^20 cm^-2)^-1, and X=N(H_2)/W_CO=~0.7*10^20 cm^-2 (K km s^-1)^-1. Our results suggest that I_100 excess could still be used to estimate the molecular content if the result is multiplied by a correction factor xi_c= _HI/ _H_2 (~2 in the second region), which accounts for the different infrared emissivities of atomic and molecular gas. We also discuss some limitations of this work.

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