adsabs/SciX_UAT_keywords huggingface dataset:
Magnetars from Neutron Star-White Dwarf Mergers: Application to Fast Radio Bursts

It is widely believed that magnetars could be born in core-collapse supernovae (SNe), binary neutron star (BNS) or binary white dwarf (BWD) mergers, or accretion-induced collapse (AIC) of white dwarfs. In this paper, we investigate whether magnetars could also be produced from neutron star-white dwarf (NSWD) mergers, motivated by FRB 180924-like fast radio bursts (FRBs) possibly from magnetars born in BNS/BWD/AIC channels suggested by Margalit et al. (2019). By a preliminary calculation, we find that NSWD mergers with unstable mass transfer could result in the NS acquiring an ultra-strong magnetic field via the dynamo mechanism due to differential rotation and convection or possibly via the magnetic flux conservation scenario of a fossil field. If NSWD mergers can indeed create magnetars, then such objects could produce at least a subset of FRB 180924-like FRBs within the framework of flaring magnetars, since the ejecta, local environments, and host galaxies of the final remnants from NSWD mergers resemble those of BNS/BWD/AIC channels. This NSWD channel is also able to well explain both the observational properties of FRB 180924-like and FRB 180916.J0158+65-like FRBs within a large range in local environments and host galaxies.

Keywords: radio bursts; gravitational waves; magnetars
An Increase in Small-planet Occurrence with Metallicity for Late-type Dwarf Stars in the Kepler Field and Its Implications for Planet Formation

While it is well-established that giant-planet occurrence rises rapidly with host star metallicity, it is not yet clear if small-planet occurrence around late-type dwarf stars depends on host star metallicity. Using the Kepler Data Release 25 planet candidate list and its completeness data products, we explore planet occurrence as a function of metallicity in the Kepler field's late-type dwarf stellar population. We find that planet occurrence increases with metallicity for all planet radii Rp down to at least Rp ≈ 2 R⊕, and that in the range 2 R⊕ ≲ Rp ≲ 5 R⊕, planet occurrence scales linearly with metallicity Z. Extrapolating our results, we predict that short-period planets with Rp ≲ 2 R⊕ should be rare around early-M dwarf stars with [M/H] ≲ -0.5 or late-M dwarf stars with [M/H] ≲ +0.0. This dependence of planet occurrence on metallicity observed in the Kepler field emphasizes the need to control for metallicity in estimates of planet occurrence for late-type dwarf stars like those targeted by Kepler's K2 extension and the Transiting Exoplanet Survey Satellite. We confirm the theoretical expectation that the small-planet occurrence-host star metallicity relation is stronger for low-mass stars than for solar-type stars. We establish that the expected solid mass in planets around late-type dwarfs in the Kepler field is comparable to the total amount of planet-making solids in their protoplanetary disks. We argue that this high efficiency of planet formation favors planetesimal accretion over pebble accretion as the origin of the small planets observed by Kepler around late-type dwarf stars.

Keywords: exoplanet formation; extrasolar rocky planets; extrasolar ice giants; late-type dwarf stars; planet hosting stars; exoplanets
Estimating Molecular Gas Content in Galaxies from Polycyclic Aromatic Hydrocarbon Emission

Emission from polycyclic aromatic hydrocarbons (PAHs), a commonly used indicator of star formation activity in galaxies, also has the potential to serve as an effective empirical tracer of molecular gas. We use a sample of 19 nearby galaxies with spatially resolved mid-infrared Spitzer spectroscopy, multiwavelength optical and mid-infrared imaging, and millimeter interferometric CO(1-0) maps to investigate the feasibility of using PAH emission as an empirical proxy to estimate molecular gas mass. PAH emission correlates strongly with CO emission on subkiloparsec scales over the diverse environments probed by our sample of star-forming galaxies and low-luminosity active galactic nuclei. The tight observed correlation, likely a consequence of photoelectronic heating of the diffuse interstellar gas by the PAHs, permits us to derive an empirical calibration to estimate molecular gas mass from the luminosity of PAH emission that has a total scatter of only ~0.2-0.25 dex. Mid-infrared bands sensitive to PAH emission (e.g., the Spitzer/IRAC4 and WISE/W3 filters) can also be used as a highly effective substitute for this purpose.

Keywords: polycyclic aromatic hydrocarbons; interstellar medium; interstellar molecules; interstellar dust; star formation; interstellar emissions; molecular gas
A JWST Survey for Planetary Mass Brown Dwarfs in IC 348

We have obtained images of the center of the star-forming cluster IC 348 with the James Webb Space Telescope and have identified brown dwarf candidates based on their photometry and point-like flux profiles. Low-resolution spectroscopy has been performed on four promising candidates, three of which have molecular absorption bands that indicate late spectral types. Among those late-type objects, the brightest is similar to known young L dwarfs while the other two show the so-called 3.4 μm feature that has been previously observed in the diffuse interstellar medium and in the atmospheres of Saturn and Titan, which has been attributed to an unidentified aliphatic hydrocarbon. Those two objects also exhibit features between 1.1 and 2.6 μm that we identify as the overtone and combination bands for that hydrocarbon. After accounting for the hydrocarbon bands, the remaining spectral features are consistent with youth and inconsistent with field dwarfs. Based on the low extinctions of those objects and the strengths of the overtone and combination bands, we conclude that the hydrocarbon resides in their atmospheres rather than in foreground material. Thus, our detections of the 3.4 μm feature are the first in atmospheres outside of the solar system. The presence of this hydrocarbon is not predicted by any atmospheric models of young brown dwarfs. Based on its luminosity and evolutionary models, the faintest new member of IC 348 has an estimated mass of 3-4 M Jup, making it a strong contender for the least massive free-floating brown dwarf that has been directly imaged to date. *Based on observations made with the NASA/ESA/CSA James Webb Space Telescope and the Gaia mission.

Keywords: brown dwarfs; l dwarfs; star forming regions; initial mass function; exoplanet atmospheres; t dwarfs; james webb space telescope
A Multiwavelength Study of the Sgr B Region: Contiguous Cloud-Cloud Collisions Triggering Widespread Star Formation Events?

The Sgr B region, including Sgr B1 and Sgr B2, is one of the most active star-forming regions in the Galaxy. Hasegawa et al. originally proposed that Sgr B2 was formed by a cloud-cloud collision (CCC) between two clouds with velocities of ~45 km s-1 and ~75 km s-1. However, some recent observational studies conflict with this scenario. We have reanalyzed this region, by using recent, fully sampled, dense-gas data and by employing a recently developed CCC identification methodology, with which we have successfully identified more than 50 CCCs and compared them at various wavelengths. We found two velocity components that are widely spread across this region and that show clear signatures of a CCC, each with a mass of ~106 M ⊙. Based on these observational results, we suggest an alternative scenario, in which contiguous collisions between two velocity features with a relative velocity of ~20 km s-1 created both Sgr B1 and Sgr B2. The physical parameters, such as the column density and the relative velocity of the colliding clouds, satisfy a relation that has been found to apply to the most massive Galactic CCCs, meaning that the triggering of high-mass star formation in the Galaxy and starbursts in external galaxies can be understood as being due to the same physical CCC process.

Keywords: interstellar medium; molecular clouds; co line emission; galactic center; star formation
The Impact of Stellar Clustering on the Observed Multiplicity and Orbital Periods of Planetary Systems

It has recently been shown that stellar clustering plays an important role in shaping the properties of planetary systems. We investigate how the multiplicity distributions and orbital periods of planetary systems depend on the 6D phase space density of stars surrounding planet host systems. We find that stars in high stellar phase space density environments (overdensities) have a factor of 1.6-2.0 excess in the number of single-planet systems compared to stars in low stellar phase space density environments (the field). The multiplicity distribution of planets around field stars is much flatter (i.e., there is a greater fraction of multiplanet systems) than in overdensities. This result is primarily driven by the combined facts that (i) "hot Jupiters" (HJs) are almost exclusively found in overdensities and (ii) HJs are predominantly observed to be single-planet systems. Nevertheless, we find that the difference in multiplicity is even more pronounced when only considering planets in the Kepler sample, which contains few HJs. This suggests that the Kepler dichotomy—an apparent excess of systems with a single transiting planet—plausibly arises from environmental perturbations. In overdensities, the orbital periods of single-planet systems are smaller than orbital periods of multiple-planet systems. As this difference is more pronounced in overdensities, the mechanism responsible for this effect may be enhanced by stellar clustering. Taken together, the pronounced dependence of planetary multiplicity and orbital period distributions on stellar clustering provides a potentially powerful tool to diagnose the impact of environment on the formation and evolution of planetary systems.

Keywords: exoplanet systems; solar-planetary interactions; exoplanet formation; planet formation; star formation; stellar dynamics
The Masses of Supernova Remnant Progenitors in M33

Using resolved optical stellar photometry from the Panchromatic Hubble Andromeda Treasury Triangulum Extended Region survey, we measured the star formation history near the position of 85 supernova remnants (SNRs) in M33. We constrained the progenitor masses for 60 of these SNRs, finding that the remaining 25 remnants had no local star formation in the last 56 Myr, consistent with core-collapse supernovae, making them potential Type Ia candidates. We then infer a progenitor mass distribution from the age distribution, assuming single star evolution. We find that the progenitor mass distribution is consistent with being drawn from a power law with an index of $-{2.9}_{-1.0}^{+1.2}$ . Additionally, we infer a minimum progenitor mass of ${7.1}_{-0.2}^{+0.1}$ M ⊙ from this sample, consistent with several previous studies, providing further evidence that stars with ages older than the lifetimes of single 8 M ⊙ stars are producing supernovae.

Keywords: supernovae; stellar evolution; massive stars; stellar populations
TOI-1696: A Nearby M4 Dwarf with a 3 R ⊕ Planet in the Neptunian Desert

We present the discovery and validation of a temperate sub-Neptune around the nearby mid-M dwarf TIC 470381900 (TOI-1696), with a radius of 3.09 ± 0.11 R ⊕ and an orbital period of 2.5 days, using a combination of Transiting Exoplanets Survey Satellite (TESS) and follow-up observations using ground-based telescopes. Joint analysis of multiband photometry from TESS, Multicolor Simultaneous Camera for studying Atmospheres of Transiting exoplanets (MuSCAT), MuSCAT3, Sinistro, and KeplerCam confirmed the transit signal to be achromatic as well as refined the orbital ephemeris. High-resolution imaging with Gemini/'Alopeke and high-resolution spectroscopy with the Subaru InfraRed Doppler (IRD) confirmed that there are no stellar companions or background sources to the star. The spectroscopic observations with IRD and Infrared Telescope Facility SpeX were used to determine the stellar parameters, and it was found that the host star is an M4 dwarf with an effective temperature of T eff = 3185 ± 76 K and a metallicity of [Fe/H] = 0.336 ± 0.060 dex. The radial velocities measured from IRD set a 2σ upper limit on the planetary mass to be 48.8 M ⊕. The large radius ratio (R p/R ⋆ ~ 0.1) and the relatively bright near-infrared magnitude (J = 12.2 mag) make this planet an attractive target for further follow-up observations. TOI-1696 b is one of the planets belonging to the Neptunian desert with the highest transmission spectroscopy metric discovered to date, making it an interesting candidate for atmospheric characterizations with JWST.

Keywords: exoplanet astronomy; exoplanet detection methods; exoplanet systems; transit photometry; photometry; multi-color photometry; high resolution spectroscopy; spectroscopy; observational astronomy; time series analysis; transits
H.E.S.S. Follow-up Observations of GRB 221009A

GRB 221009A is the brightest gamma-ray burst (GRB) ever detected. To probe the very-high-energy (VHE; >100 GeV) emission, the High Energy Stereoscopic System (H.E.S.S.) began observations 53 hr after the triggering event, when the brightness of the moonlight no longer precluded observations. We derive differential and integral upper limits using H.E.S.S. data from the third, fourth, and ninth nights after the initial GRB detection, after applying atmospheric corrections. The combined observations yield an integral energy flux upper limit of ${{\rm{\Phi }}}_{\mathrm{UL}}^{95 \% }=9.7\times {10}^{-12}\,\mathrm{erg}\,{\mathrm{cm}}^{-2}\,{{\rm{s}}}^{-1}$ above E thr = 650 GeV. The constraints derived from the H.E.S.S. observations complement the available multiwavelength data. The radio to X-ray data are consistent with synchrotron emission from a single electron population, with the peak in the spectral energy distribution occurring above the X-ray band. Compared to the VHE-bright GRB 190829A, the upper limits for GRB 221009A imply a smaller gamma-ray to X-ray flux ratio in the afterglow. Even in the absence of a detection, the H.E.S.S. upper limits thus contribute to the multiwavelength picture of GRB 221009A, effectively ruling out an IC-dominated scenario.

Keywords: gamma-rays; gamma-ray bursts; non-thermal radiation sources; high-energy cosmic radiation
Evaluation of a Magnetic Field Inversion Method Using Only Stokes I

We compare a method for inferring the photospheric vector magnetic field using only spectroscopy to a conventional method based on polarimetry. The magnetic field strength B and inclination angle can be inferred from the Zeeman splitting using only Stokes I. We applied this method to a sunspot observed with the Vacuum Tower Telescope and compared the results to vector magnetograms from the Helioseismic and Magnetic Imager on the Solar Dynamics Observatory, which used a polarimetric inversion. The spectroscopic inversion tends to show higher values in B compared to the polarimetric data. In quiet regions the discrepancy in B was typically a factor of two. In the strong sunspot fields, the differences averaged ≈22%. These discrepancies are significant, but comparable to those typically found among magnetograms from different instruments. Our results support the use of the spectroscopic inversion technique to provide a fast and reasonable estimate of B.

Keywords: solar magnetic fields; solar physics; sunspots; atomic spectroscopy; spectroscopy; spectropolarimetry; magnetogram
New Aspects of Energy Conversion in Magnetic Island Dynamics: Particle-in-cell Simulation of Multiple Island Coalescence and MMS Observations

Coalescence of multiple magnetic islands is recognized as an effective process to energize particles during magnetic reconnection, while its energy conversion process still remains unclear. Here, a two-dimensional fully kinetic simulation of multiple island coalescence with a small reconnection guide field is studied. In the analysis of energy conversion within a magnetic island, the dot product of ${{\boldsymbol{V}}}_{e}\cdot \left({\boldsymbol{j}}\times {\boldsymbol{B}}\right)={w}_{1}$ is a useful quantity to compare with j · E = w 2, since the average work done by the Lorentz force on the circulating particles is negligible in the island and ${w}_{2}-{w}_{1}={\boldsymbol{j}}\cdot \left({\boldsymbol{E}}+{{\boldsymbol{V}}}_{e}\times {\boldsymbol{B}}\right)={\boldsymbol{j}}\cdot {{\boldsymbol{E}}}^{{\prime} }={w}_{3}$ . A bipolar pattern of w 1 is found at a secondary island when the electrons are in circular motion inside the island. Significant energy dynamo (w 3 < 0) resulting from j ∥ E ∥ is found at the secondary island, which has not been reported before, where the parallel electric field E ∥ is highly correlated with w 3. Moreover, significant energy dissipation (w 3 > 0) due to ${{\boldsymbol{j}}}_{\perp }\cdot {{\boldsymbol{E}}}_{\perp }^{{\prime} }$ is seen in the merging region between two coalescing islands. Both types of energy conversions are accompanied by enhancements in j ∥ and the parallel electron temperature T e∥. Three ion-scale magnetic islands (FR1, FR2, and FR3) observed by the Magnetospheric Multiscale spacecraft are compared favorably with the simulated signatures of energy dynamo and dissipation of an evolving secondary island. In particular, FR1 displayed a similar energy dynamo signature as that simulated in an early stage of the secondary island. FR2 and FR3 showed a dominant ${{\boldsymbol{j}}}_{\perp }\cdot {{\boldsymbol{E}}}_{\perp }^{{\prime} }$ energy conversion similar to that obtained in a later stage of the secondary island.

Keywords: interplanetary physics
Two Quasi-periodic Fast-propagating Magnetosonic Wave Events Observed in Active Region NOAA 11167

We report a detailed observational study of two quasi-periodic fast-propagating (QFP) magnetosonic wave events that occurred on 2011 March 9 and 10, respectively. Interestingly, both the events have two wave trains (WTs): a strong main one (WT-1) and a small and weak secondary one (WT-2). Peculiar and common characteristics of the two events are observed, namely, (1) the two QFP waves are accompanied with brightenings during the whole stage of the eruptions; (2) both the two main WTs are nearly propagating along the same direction; (3) extreme-ultraviolet (EUV) waves are found to be associated with the two events. Investigating various aspects of the target events, we argue that (1) the second event is accompanied with a flux rope eruption during the whole stage; (2) the second event eruption produces a new filament-like dark feature; (3) the ripples of the two WT-2 QFP waves seem to result from different triggering mechanisms. Based on the obtained observational results, we propose that the funnel-like coronal loop system is indeed playing an important role in the two WT-1 QFP waves. The development of the second WT-2 QFP wave can be explained as due to the dispersion of the main EUV front. The coexistence of the two events offers thereby a significant opportunity to reveal what driving mechanisms and structures are tightly related to the waves.

Keywords: solar corona; magnetohydrodynamics; solar flares; solar extreme ultraviolet emission; solar active regions; solar coronal waves; space plasmas
Probing Microporous ASW with Near-infrared Spectroscopy: Implications for JWST's NIRSpec

The presence of porous amorphous solid water (ASW) in interstellar ice mantles has long been suspected but, to date, has not been identified. With the upcoming launch of the James Webb Space Telescope (JWST), future detections of porous ASW seem promising. To this end, we investigated the near-infrared spectra of ASW to confirm that two features that have been seen sporadically in literature and have either not been identified or only tentatively identified were indeed due to dangling bond (DB) absorption bands. These bands may provide an additional way to identify porous ASW with remote sensing spectroscopy. Our systematic study tested the thermal stability of these features and their spectral shifts during gas adsorption. We correlated these changes to what we observed in the well-studied fundamental DB absorptions at 3720 and 3696 cm-1. We find excellent agreement between the behavior of the fundamental DB absorption bands with those centered at 5326 and 7235 cm-1, as well as a shoulder located at 7285 cm-1, confirming that all these features are due to DBs present in porous ASW. We estimate that these weaker DB absorptions should be identifiable after 4-5 hr of observation time with JWST's NIRSpec instrument. These new features may not only prove useful in future observational campaigns in search of porous ASW but also may serve as a powerful way to indirectly detect weakly absorbing species that challenge the ability of remote sensing spectroscopy.

Keywords: laboratory astrophysics; solid matter physics; surface ices
Fast Blue Optical Transients Due to Circumstellar Interaction and the Mysterious Supernova SN 2018gep

The discovery of SN 2018gep (ZTF 18abukavn) challenged our understanding of the late-phase evolution of massive stars and their supernovae (SNe). The fast rise in luminosity of this SN (spectroscopically classified as a broad-lined Type Ic SN) indicates that the ejecta interacts with a dense circumstellar medium (CSM), while an additional energy source such as 56Ni decay is required to explain the late-time light curve. These features hint at the explosion of a massive star with pre-SN mass loss. In this work, we examine the physical origins of rapidly evolving astrophysical transients like SN 2018gep. We investigate the wave-driven mass-loss mechanism and how it depends on model parameters such as progenitor mass and deposition energy, searching for stellar progenitor models that can reproduce the observational data. A model with an ejecta mass ~2 M⊙, explosion energy ~1052 erg, a CSM of mass ~0.3 M⊙ and radius ~1000 R⊙, and a 56Ni mass ~0.3 M⊙ provides a good fit to the bolometric light curve. We also examine how interaction-powered light curves depend more generally on these parameters and how ejecta velocities can help break degeneracies. We find both wave-driven mass loss and mass ejection via pulsational pair instability can plausibly create the dense CSM in SN 2018gep, but we favor the latter possibility.

Keywords: supernovae; supernova dynamics; stellar pulsations; radiative transfer; light curves
Optical Spectral Variations of a Large Sample of Fermi Blazars

We have investigated the optical spectral behavior of a large sample of Fermi blazars (40 flat-spectrum radio quasars (FSRQs) and 13 BL Lacertae objects (BL Lacs)), and found two new universal optical spectral behaviors. In the low state the optical spectrum gradually becomes softer (steeper) or harder (flatter), but more and more slowly as the brightness increases, and then tends to stabilize in the high state. These are briefly named the redder-stable-when-brighter (RSWB) and bluer-stable-when-brighter (BSWB) behaviors, respectively. Thirty-four FSRQs and 7 BL Lacs exhibit clear RSWB behavior, and 2 FSRQs and 5 BL Lacs show distinct BSWB behavior, which means that FSRQs favor more RSWB than BSWB behavior, while BL Lacs have no clear preference between both behaviors. We have put forward a unified nonlinear formula to quantitatively characterize the optical spectral behaviors of FSRQs and BL Lacs, which can fit both kinds of behaviors very well. We argue that the RSWB and BSWB behaviors originate from the same mechanism, and that they are the universal optical spectral behaviors for blazars. The frequently observed redder-when-brighter and bluer-when-brighter trends can be considered to be the approximations of the RSWB and BSWB behaviors, respectively. The rarely observed stable-when-brighter trend can also be viewed as an approximation or a special case of the RSWB or BSWB behaviors. We have developed a model with two constant-spectral-index components, which cannot only explain the two kinds of optical spectral behaviors well, but also successfully interpret the differential behaviors between FSRQs and BL Lacs.

Keywords: active galactic nuclei; flat-spectrum radio quasars; bl lacertae objects; blazars