arXiv subjects

Subo Dong

Publications and source records attributed to Subo Dong.

At least 145 records · Page 8Linked to original sources

arXiv2015

Type Ia Supernovae with Bi-Modal Explosions Are Common -- Possible Smoking Gun for Direct Collisions of White-Dwarfs

We discover clear doubly-peaked line profiles in 3 out of ~20 type Ia supernovae (SNe Ia) with high-quality nebular-phase spectra. The profiles are consistently present in three well-separated Co/Fe emission features. The two peaks are respectively blue-shifted and red-shifted relative to the host galaxies and are separated by ~5000 km/s. The doubly-peaked profiles directly reflect a bi-modal velocity distribution of the radioactive Ni56 in the ejecta that powers the emission of these SNe. Due to their random orientations, only a fraction of SNe with intrinsically bi-modal velocity distributions will appear as doubly-peaked spectra. Therefore SNe with intrinsic bi-modality are likely common, especially among the SNe in the low-luminosity part on the Philips relation (Δm15(B) >~ 1.3; ~40% of all SNe Ia). Such bi-modality is naturally expected from direct collisions of white dwarfs (WDs) due to the detonation of both WDs and is demonstrated in a 3D 0.64 M_Sun-0.64 M_Sun WD collision simulation. In the future, with a large sample of nebular spectra and a comprehensive set of numerical simulations, the collision model can be unambiguously tested as the primary channel for type Ia SNe, and the distribution of nebular line profiles will either be a smoking gun or rule it out.

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arXiv2015

Spitzer Parallax of OGLE-2015-BLG-0966: A Cold Neptune in the Galactic Disk

We report the detection of a Cold Neptune m_planet=21+/-2MEarth orbiting a 0.38MSol M dwarf lying 2.5-3.3 kpc toward the Galactic center as part of a campaign combining ground-based and Spitzer observations to measure the Galactic distribution of planets. This is the first time that the complex real-time protocols described by Yee et al. (2015), which aim to maximize planet sensitivity while maintaining sample integrity, have been carried out in practice. Multiple survey and follow-up teams successfully combined their efforts within the framework of these protocols to detect this planet. This is the second planet in the Spitzer Galactic distribution sample. Both are in the near-to-mid disk and clearly not in the Galactic bulge.

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arXiv2015

LAMOST observations in the Kepler field. Database of low-resolution spectra

The nearly continuous light curves with micromagnitude precision provided by the space mission Kepler are revolutionising our view of pulsating stars. They have revealed a vast sea of low-amplitude pulsation modes that were undetectable from Earth. The long time base of Kepler light curves allows an accurate determination of frequencies and amplitudes of pulsation modes needed for in-depth asteroseismic modeling. However, for an asteroseismic study to be successful, the first estimates of stellar parameters need to be known and they can not be derived from the Kepler photometry itself. The Kepler Input Catalog (KIC) provides values for the effective temperature, the surface gravity and the metallicity, but not always with a sufficient accuracy. Moreover, information on the chemical composition and rotation rate is lacking. We are collecting low-resolution spectra for objects in the Kepler field of view with the Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST, Xinglong observatory, China). All of the requested fields have now been observed at least once. In this paper we describe those observations and provide a database of use to the whole astronomical community.

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arXiv2015

Confirmation of the Planetary Microlensing Signal and Star and Planet Mass Determinations for Event OGLE-2005-BLG-169

We present Hubble Space Telescope (HST) Wide Field Camera 3 (WFC3) observations of the source and lens stars for planetary microlensing event OGLE-2005-BLG-169, which confirm the relative proper motion prediction due to the planetary light curve signal observed for this event. This (and the companion Keck result) provide the first confirmation of a planetary microlensing signal, for which the deviation was only 2%. The follow-up observations determine the flux of the planetary host star in multiple passbands and remove light curve model ambiguity caused by sparse sampling of part of the light curve. This leads to a precise determination of the properties of the OGLE-2005-BLG-169Lb planetary system. Combining the constraints from the microlensing light curve with the photometry and astrometry of the HST/WFC3 data, we find star and planet masses of M_* = 0.69+- 0.02 M_solar and m_p = 14.1 +- 0.9 M_earth. The planetary microlens system is located toward the Galactic bulge at a distance of D_L = 4.1 +- 0.4 kpc, and the projected star-planet separation is a_perp = 3.5 +- 0.3 AU, corresponding to a semi-major axis of a = 4.0 (+2.2 -0.6) AU.

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arXiv2015

Triple Microlens OGLE-2008-BLG-092L: Binary Stellar System with a Circumprimary Uranus-type Planet

We present the gravitational microlensing discovery of a 4 M_Uranus planet that orbits a 0.7 M_Sun star at ~18 AU. This is the first known analog of Uranus. Similar planets, i.e., cold ice-giants, are inaccessible to either radial velocity or transit methods because of the long orbital periods, while low reflected light prevents direct imaging. We discuss how similar planets may contaminate the sample of the very short microlensing events that are interpreted as free-floating planets with an estimated rate of 1.8 per main sequence star. Moreover, the host star has a nearby stellar (or brown dwarf) companion. The projected separation of the planet is only ~3 times smaller than that of the companion star, suggesting significant dynamical interactions.

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arXiv2015

MOA-2007-BLG-197: Exploring the brown dwarf desert

We present the analysis of MOA-2007-BLG-197Lb, the first brown dwarf companion to a Sun-like star detected through gravitational microlensing. The event was alerted and followed-up photometrically by a network of telescopes from the PLANET, MOA, and uFUN collaborations, and observed at high angular resolution using the NaCo instrument at the VLT. From the modelling of the microlensing light curve, we derived the binary lens separation in Einstein radius units (s~1.13) and a mass ratio of (4.732+/-0.020)x10^{-2}. Annual parallax, lens orbital motion and finite source effects were included in the models. To recover the lens system's physical parameters, we combined the resulting light curve best-fit parameters with (J,H,Ks) magnitudes obtained with VLT NaCo and calibrated using IRSF and 2MASS data. We derived a lens total mass of 0.86+/-0.04 Msun and a lens distance of 4.2+/-0.3 kpc. We find that the companion of MOA-2007-BLG-197L is a brown dwarf of 41+/-2 Mjup observed at a projected separation of 4.3+/-0.1 AU, and orbits a 0.82+/-0.04 Msun G-K dwarf star. We study the statistical properties of this population of brown dwarfs detected by microlensing, transit, radial velocity, and direct imaging (most of these objects orbit solar-type stars), and we performed a two-dimensional, non-parametric probability density distribution fit to the data, which draws a structured brown dwarf landscape. We confirm the existence of a region that is strongly depleted in objects at short periods and intermediate masses (P<30 d, M~30-60 Mjup), but also find an accumulation of objects around P~500 d and M~20 Mjup, as well as another depletion region at long orbital periods (P>500 d) and high masses (M>50 Mjup). While these data provide important clues on mechanisms of brown dwarfs formation, more data are needed to establish their relative importance, in particular as a function of host star mass.

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arXiv2014

OGLE-LMC-ECL-11893: The discovery of a long-period eclipsing binary with a circumstellar disk

We report the serendipitous discovery of a disk-eclipse system OGLE-LMC-ECL-11893. The eclipse occurs with a period of 468 days, a duration of about 15 days and a deep (up to ΔI ~1.5), peculiar and asymmetric profile. A possible origin of such an eclipse profile involves a circumstellar disk. The presence of the disk is confirmed by the H-alpha line profile from the follow-up spectroscopic observations, and the star is identified as Be/Ae type. Unlike the previously known disk-eclipse candidates (Epsilon Aurigae, EE Cephei, OGLE-LMC-ECL-17782, KH 15D), the eclipses of OGLE-LMC-ECL-11893 retain the same shape throughout the span of ~17 years (13 orbital periods), indicating no measurable orbital precession of the disk.

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arXiv2014

On the Metallicities of Kepler Stars

We use 12000 stars from Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST) spectroscopic data to show that the metallicities of Kepler field stars as given in the Kepler Input Catalog (KIC) systematically underestimate both the true metallicity and the dynamic range of the Kepler sample. Specifically, to the first order approximation, we find [Fe/H]_KIC = -0.20 + 0.43 [Fe/H]_LAMOST, with a scatter of ~0.25 dex, due almost entirely to errors in KIC. This relation is most secure for -0.3<[Fe/H]_LAMOST<+0.4 where we have >200 comparison stars per 0.1 dex bin and good consistency is shown between metallicities determined by LAMOST and high-resolution spectra. It remains approximately valid in a slightly broader range. When the relation is inverted, the error in true metallicity as derived from KIC is (0.25 dex)/0.43~0.6 dex. We thereby quantitatively confirm the cautionary note by Brown et al. (2011) that KIC estimates of [Fe/H] should not be used by "anyone with a particular interest in stellar metallicities". Fortunately, many more LAMOST spectroscopic metallicities will be available in the near future.

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arXiv2014

OGLE-2012-BLG-0455/MOA-2012-BLG-206: Microlensing event with ambiguity in planetary interpretations caused by incomplete coverage of planetary signal

Characterizing a microlensing planet is done from modeling an observed lensing light curve. In this process, it is often confronted that solutions of different lensing parameters result in similar light curves, causing difficulties in uniquely interpreting the lens system, and thus understanding the causes of different types of degeneracy is important. In this work, we show that incomplete coverage of a planetary perturbation can result in degenerate solutions even for events where the planetary signal is detected with a high level of statistical significance. We demonstrate the degeneracy for an actually observed event OGLE-2012-BLG-0455/MOA-2012-BLG-206. The peak of this high-magnification event $(A_{\rm max}\sim400)$ exhibits very strong deviation from a point-lens model with $Δχ^{2}\gtrsim4000$ for data sets with a total number of measurement 6963. From detailed modeling of the light curve, we find that the deviation can be explained by four distinct solutions, i.e., two very different sets of solutions, each with a two-fold degeneracy. While the two-fold (so-called "close/wide") degeneracy is well-understood, the degeneracy between the radically different solutions is not previously known. The model light curves of this degeneracy differ substantially in the parts that were not covered by observation, indicating that the degeneracy is caused by the incomplete coverage of the perturbation. It is expected that the frequency of the degeneracy introduced in this work will be greatly reduced with the improvement of the current lensing survey and follow-up experiments and the advent of new surveys.

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arXiv2014

Candidate Gravitational Microlensing Events for Future Direct Lens Imaging

The mass of the lenses giving rise to Galactic microlensing events can be constrained by measuring the relative lens-source proper motion and lens flux. The flux of the lens can be separated from that of the source, companions to the source, and unrelated nearby stars with high-resolution images taken when the lens and source are spatially resolved. For typical ground-based adaptive optics (AO) or space-based observations, this requires either inordinately long time baselines or high relative proper motions. We provide a list of microlensing events toward the Galactic Bulge with high relative lens-source proper motion that are therefore good candidates for constraining the lens mass with future high-resolution imaging. We investigate all events from 2004 -- 2013 that display detectable finite-source effects, a feature that allows us to measure the proper motion. In total, we present 20 events with mu >~ 8 mas/yr. Of these, 14 were culled from previous analyses while 6 are new, including OGLE-2004-BLG-368, MOA-2005-BLG-36, OGLE-2012-BLG-0211, OGLE-2012-BLG-0456, MOA-2012-BLG-532, and MOA-2013-BLG-029. In <~12 years the lens and source of each event will be sufficiently separated for ground-based telescopes with AO systems or space telescopes to resolve each component and further characterize the lens system. Furthermore, for the most recent events, comparison of the lens flux estimates from images taken immediately to those estimated from images taken when the lens and source are resolved can be used to empirically check the robustness of the single-epoch method currently being used to estimate lens masses for many events.

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arXiv2014

Luminosity function suggests up to 100 white dwarfs within 20 pc may be hiding in multiple systems

We examine the luminosity function of white dwarfs (WDs) in the local ``complete'' WD sample ($d<20$ pc) of Holberg et. al. 2008. We find that the fraction of bright and young WDs is anomalously high among the WDs detected in multiple systems with main sequence (MS) companions compared to that of the single WDs and theoretical expectations. This indicates a significant observation bias against finding relatively faint WDs in multiple systems. At the bright end ($M_V<11.5$), the amount of WDs with MS companions is approximately equal to that of single white dwarfs, indicating that $\gtrsim 50\%$ of WDs have MS companions, consistent with the high multiplicity fraction of early-type MS stars. If true, a significant fraction of WDs in multiple systems within 20 pc may have not been detected yet, and the number density of WDs in the solar neighborhood and elsewhere may be up to twice as much as presently believed.

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arXiv2014

Super-massive planets around late-type stars - the case of OGLE-2012-BLG-0406Lb

The core accretion theory of planetary formation does not predict that super-Jupiters will form beyond the snow line of a low mass stars. We present a discovery of 3.9 +- 1.2 M_Jup mass planet orbiting the 0.59 +- 0.17 M_Sun star using the gravitational microlensing method. During the event, the projected separation of the planet and the star is 3.9 +- 1.0 AU i.e., the planet is significantly further from the host star than the snow line. This is a fourth such planet discovered using the microlensing technique and challenges the core accretion theory.

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arXiv2014

Directly Imaging Tidally Powered Migrating Jupiters

Upcoming direct-imaging experiments may detect a new class of long-period, highly luminous, tidally powered extrasolar gas giants. Even though they are hosted by ~ Gyr-"old" main-sequence stars, they can be as "hot" as young Jupiters at ~100 Myr, the prime targets of direct-imaging surveys. They are on years-long orbits and presently migrating to "feed" the "hot Jupiters." They are expected from "high-e" migration mechanisms, in which Jupiters are excited to highly eccentric orbits and then shrink semi-major axis by a factor of ~10-100 due to tidal dissipation at close periastron passages. The dissipated orbital energy is converted to heat, and if it is deposited deep enough into the atmosphere, the planet likely radiates steadily at luminosity L ~ 100-1000 L_Jup(2 x 10-7-2 x 10-6 L_Sun) during a typical ~ Gyr migration timescale. Their large orbital separations and expected high planet-to-star flux ratios in IR make them potentially accessible to high-contrast imaging instruments on 10 m class telescopes. ~10 such planets are expected to exist around FGK dwarfs within ~50 pc. Long-period radial velocity planets are viable candidates, and the highly eccentric planet HD 20782b at maximum angular separation ~0.''08 is a promising candidate. Directly imaging these tidally powered Jupiters would enable a direct test of high-e migration mechanisms. Once detected, the luminosity would provide a direct measurement of the migration rate, and together with mass (and possibly radius) estimate, they would serve as a laboratory to study planetary spectral formation and tidal physics.

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arXiv2013

A Sub-Earth-Mass Moon Orbiting a Gas Giant Primary or a High Velocity Planetary System in the Galactic Bulge

We present the first microlensing candidate for a free-floating exoplanet-exomoon system, MOA-2011-BLG-262, with a primary lens mass of M_host ~ 4 Jupiter masses hosting a sub-Earth mass moon. The data are well fit by this exomoon model, but an alternate star+planet model fits the data almost as well. Nevertheless, these results indicate the potential of microlensing to detect exomoons, albeit ones that are different from the giant planet moons in our solar system. The argument for an exomoon hinges on the system being relatively close to the Sun. The data constrain the product M pi_rel, where M is the lens system mass and pi_rel is the lens-source relative parallax. If the lens system is nearby (large pi_rel), then M is small (a few Jupiter masses) and the companion is a sub-Earth-mass exomoon. The best-fit solution has a large lens-source relative proper motion, mu_rel = 19.6 +- 1.6 mas/yr, which would rule out a distant lens system unless the source star has an unusually high proper motion. However, data from the OGLE collaboration nearly rule out a high source proper motion, so the exoplanet+exomoon model is the favored interpretation for the best fit model. However, the alternate solution has a lower proper motion, which is compatible with a distant (so stellar) host. A Bayesian analysis does not favor the exoplanet+exomoon interpretation, so Occam's razor favors a lens system in the bulge with host and companion masses of M_host = 0.12 (+0.19 -0.06) M_solar and m_comp = 18 (+28 -100 M_earth, at a projected separation of a_perp ~ 0.84 AU. The existence of this degeneracy is an unlucky accident, so current microlensing experiments are in principle sensitive to exomoons. In some circumstances, it will be possible to definitively establish the low mass of such lens systems through the microlensing parallax effect. Future experiments will be sensitive to less extreme exomoons.

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arXiv2013

Warm Jupiters Need Close "Friends" for High-Eccentricity Migration -- A Stringent Upper Limit on the Perturber's Separation

We propose a stringent observational test on the formation of warm Jupiters (gas-giant planets with 10 d <~ P <~ 100 d) by high-eccentricity (high-e) migration mechanisms. Unlike hot Jupiters, the majority of observed warm Jupiters have pericenter distances too large to allow efficient tidal dissipation to induce migration. To access the close pericenter required for migration during a Kozai-Lidov cycle, they must be accompanied by a strong enough perturber to overcome the precession caused by General Relativity (GR), placing a strong upper limit on the perturber's separation. For a warm Jupiter at a ~ 0.2 AU, a Jupiter-mass (solar-mass) perturber is required to be <~ 3 AU (<~ 30 AU) and can be identified observationally. Among warm Jupiters detected by Radial Velocities (RV), >~ 50% (5 out of 9) with large eccentricities (e >~ 0.4) have known Jovian companions satisfying this necessary condition for high-e migration. In contrast, <~ 20 % (3 out of 17) of the low-e (e <~ 0.2) warm Jupiters have detected additional Jovian companions, suggesting that high-e migration with planetary perturbers may not be the dominant formation channel. Complete, long-term RV follow-ups of the warm-Jupiter population will allow a firm upper limit to be put on the fraction of these planets formed by high-e migration. Transiting warm Jupiters showing spin-orbit misalignments will be interesting to apply our test. If the misalignments are solely due to high-e migration as commonly suggested, we expect that the majority of warm Jupiters with low-e (e <~0.2) are not misaligned, in contrast with low-e hot Jupiters.

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arXiv2013

Head-on collisions of white dwarfs in triple systems could explain type Ia supernova

Type Ia supernovae (SNe Ia), thermonuclear explosions of carbon-oxygen white dwarfs (CO-WDs), are currently the best cosmological "standard candles", but the triggering mechanism of the explosion is unknown. It was recently shown that the rate of head-on collisions of typical field CO-WDs in triple systems may be comparable to the SNe Ia rate. Here we provide evidence supporting a scenario in which the majority of SNe Ia are the result of such head-on collisions of CO-WDs. In this case, the nuclear detonation is due to a well understood shock ignition, devoid of commonly introduced free parameters such as the deflagration velocity or transition to detonation criteria. By using two-dimensional hydrodynamical simulations with a fully resolved ignition process, we show that zero-impact-parameter collisions of typical CO-WDs with masses $0.5-1\,M_{\odot}$ result in explosions that synthesize $^{56}$Ni masses in the range of $\sim0.1-1\,M_{\odot}$, spanning the wide distribution of yields observed for the majority of SNe Ia. All collision models yield the same late-time ($>60$ days since explosion) bolometric light curve when normalized by $^{56}$Ni masses (to better than $30\%$), in agreement with observations. The calculated widths of the $^{56}$Ni-mass-weighted-line-of-sight velocity distributions are correlated with the calculated $^{56}$Ni yield, agreeing with the observed correlation. The strong correlation, shown here for the first time, between $^{56}$Ni yield and total mass of the colliding CO-WDs (insensitive to their mass ratio), is suggestive as the source for the continuous distribution of observed SN Ia features, possibly including the Philips relation.

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arXiv2013

Fast Rise of "Neptune-Size" Planets ($4-8 R_{\rm Earth}$) from $P\sim10$ to $\sim250$ days -- Statistics of Kepler Planet Candidates Up to $\sim 0.75 {\rm AU}$

We infer the period ($P$) and size ($R_p$) distribution of Kepler transiting planet candidates with $R_p\ge 1 R_{\rm Earth}$ and $P < 250$ days hosted by solar-type stars. The planet detection efficiency is computed by using measured noise and the observed timespans of the light curves for $\sim 120,000$ Kepler target stars. We focus on deriving the shape of planet period and radius distribution functions. We find that for orbital period $P>10$ days, the planet frequency d$N_p$/d$\log$P for "Neptune-size" planets ($R_p = 4-8 R_{\rm Earth}$) increases with period as $\propto P^{0.7\pm0.1}$. In contrast, d$N_p$/d$\log$P for "super-Earth-size" ($2-4 R_{\rm Earth}$) as well as "Earth-size" ($1-2 R_{\rm Earth}$) planets are consistent with a nearly flat distribution as a function of period ($\propto P^{0.11\pm0.05}$ and $\propto P^{-0.10\pm0.12}$, respectively), and the normalizations are remarkably similar (within a factor of $\sim 1.5$ at $50 $ days). Planet size distribution evolves with period, and generally the relative fractions for big planets ($\sim 3-10 R_{\rm Earth}$) increase with period. The shape of the distribution function is not sensitive to changes in selection criteria of the sample. The implied nearly flat or rising planet frequency at long period appears to be in tension with the sharp decline at $\sim 100$ days in planet frequency for low mass planets (planet mass $m_p < 30 M_{\rm Earth}$) recently suggested by HARPS survey. Within $250$ days, the cumulative frequencies for Earth-size and super-Earth-size planets are remarkably similar ($\sim 28 %$ and $25%$), while Neptune-size and Jupiter-size planets are $\sim 7%$, and $\sim 3%$, respectively. A major potential uncertainty arises from the unphysical impact parameter distribution of the candidates.

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arXiv2013

MOA-2010-BLG-311: A planetary candidate below the threshold of reliable detection

We analyze MOA-2010-BLG-311, a high magnification (A_max>600) microlensing event with complete data coverage over the peak, making it very sensitive to planetary signals. We fit this event with both a point lens and a 2-body lens model and find that the 2-body lens model is a better fit but with only Delta chi^2~80. The preferred mass ratio between the lens star and its companion is $q=10^(-3.7+/-0.1), placing the candidate companion in the planetary regime. Despite the formal significance of the planet, we show that because of systematics in the data the evidence for a planetary companion to the lens is too tenuous to claim a secure detection. When combined with analyses of other high-magnification events, this event helps empirically define the threshold for reliable planet detection in high-magnification events, which remains an open question.

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