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Kenji Hamaguchi

Publications and source records attributed to Kenji Hamaguchi.

13 records
All sourcesNASA NTRSDOE OSTI
NASA NTRS2023

Delayed Development of Cool Plasmas in X-Ray Flares from the Young Sun-like Star κ1 Ceti

The Neutron star Interior Composition Explorer (NICER) X-ray observatory observed two powerful X-ray flares equivalent to superflares from the nearby young solar-like star κ 1 Ceti in 2019. NICER follows each flare from the onset through the early decay, collecting over 30 counts s −1 near the peak, enabling a detailed spectral variation study of the flare rise. The flare in September varies quickly in ∼800 s, while the flare in December has a few times longer timescale. In both flares, the hard-band (2–4 keV) light curves show typical stellar X-ray flare variations with a rapid rise and slow decay, while the soft X-ray light curves, especially of the September flare, have prolonged flat peaks. The time-resolved spectra require two temperature plasma components at kT ∼0.3–1 and ∼2–4 keV. Both components vary similarly, but the cool component lags by ∼200 s with a four to six times smaller emission measure (EM) compared to the hot component. A comparison with hydrodynamic flare loop simulations indicates that the cool component originates from X-ray plasma near the magnetic loop footpoints that mainly cools via thermal conduction. The time lag represents the travel time of the evaporated gas through the entire flare loop. The cool component has a several times smaller EM than its simulated counterpart, suggesting a suppression of conductive cooling, possibly by the expansion of the loop cross-sectional area or turbulent fluctuations. The cool component's time lag and EM ratio provide important constraints on the flare loop geometry.

main sequence stars↗
NASA NTRS2022

Nested Dust Shells Around the Wolf–Rayet Binary WR 140 Observed With JWST

Massive colliding-wind binaries that host a Wolf–Rayet (WR) star present a potentially important source of dust and chemical enrichment in the interstellar medium. However, the chemical composition and survival of dust formed from such systems is not well understood. The carbon-rich Wolf–Rayet binary WR 140 presents an ideal astrophysical laboratory for investigating these questions, given its well-defined orbital period and predictable dust-formation episodes every 7.93 years around periastron passage. We present observations from our Early Release Science programme (ERS 1349) with the James Webb Space Telescope Mid-Infrared Instrument (MIRI) Medium-Resolution Spectrometer and Imager that reveal the spectral and spatial signatures of nested circumstellar dust shells around WR 140. MIRI medium-resolution spectroscopy of the second dust shell and Imager detections of over 17 shells formed throughout approximately the past 130 years confirm the survival of carbonaceous dust grains from WR 140 that are probably carriers of ‘unidentified infrared’-band features at 6.4 and 7.7 μm. The observations indicate that dust-forming carbon-rich Wolf–Rayet binaries can enrich the interstellar medium with organic compounds and carbonaceous dust.

massive stars↗
NASA NTRS2022

Eta Carinae: An Evolving View of the Central Binary, Its Interacting Winds and Its Foreground Ejecta

FUV spectra of η Car, recorded across two decades with HST/STIS, document multiple changes in resonant lines caused by dissipating extinction in our line of sight. The FUV flux has increased nearly tenfold, which has led to increased ionization of the multiple shells within the Homunculus and photodestruction of H 2 . Comparison of observed resonant line profiles with CMFGEN model profiles allows separation of wind–wind collision and shell absorptions from the primary wind P Cygni profiles. The dissipating occulter preferentially obscured the central binary and interacting winds relative to the very extended primary wind. We are now able to monitor changes in the colliding winds with orbital phase. High-velocity transient absorptions occurred across the most recent periastron passage, indicating acceleration of the primary wind by the secondary wind, which leads to a downstream, high-velocity bow shock that is newly generated every orbital period. There is no evidence of changes in the properties of the binary winds.

Massive stars↗
NASA NTRS2021

Eta Carinae: A Tale of Two Periastron Passages

Since 2002, the far-ultraviolet(FUV)flux(1150–1680Å)of Eta Carinae, monitored by the Hubble SpaceTelescope/Space Telescope Imaging Spectrograph, has increased by an order of magnitude. This increase isattributed to partial dissipation of a line-of-sight(LOS)occulter that blocks the central core of the system. Acrossthe 2020 February periastron passage, changes in the FUV emission show a stronger wavelength dependence thanoccurred across the 2003 July periastron passage. Across both periastron passages, most of the FUV spectrumdropped influx then recovered a few months later. The 2020 periastron passage included enhancements of FUVflux in narrow spectral intervals near periastron followed by a transient absorption and recovery to pre-periastronflux levels. The drop influx is due to increased absorption by singly ionized species as the secondary star plungesdeep into the wind of the primary star, which blocks the companion’s ionizing radiation. The enhanced FUVemission is caused by the companion’s wind-blown cavity briefly opening a window to deeper layers of theprimary star. This is thefirst time transient brightening has been seen in the FUV comparable to transientspreviously seen at longer wavelengths. Changes in resonance line-velocity profiles hint that the dissipating occulteris associated with material in LOS moving at−100 to−300 km s−1, similar in velocity of structures previouslyassociated with the 1890s lesser eruption.

Theodore R Gull↗
NASA NTRS2021

Simultaneous Multiwavelength Flare Observations of EV Lacertae

We present the first results of our ongoing project conducting simultaneous multiwavelength observations of flares on nearby active M dwarfs. We acquired data of the nearby dM3.5e star EV Lac using five different observatories: NASA's Transiting Exoplanet Survey Satellite (TESS), NASA's Neil Gehrels Swift Observatory (Swift), NASA's Neutron Interior Composition Explorer (NICER), the University of Hawaii 2.2-meter telescope (UH88), and the Las Cumbres Observatory Global Telescope (LCOGT) Network. During the ∼25 days of TESS observations, we acquired three simultaneous UV/X-ray observations using Swift that total ∼18 ks, 21 simultaneous epochs totaling ∼98 ks of X-ray data using NICER, one observation (∼3 hr) with UH88, and one observation (∼3 hr) with LCOGT. We identified 56 flares in the TESS light curve with estimated energies in the range log E(T) (erg) = (30.5–33.2), nine flares in the Swift UVM2 light curve with estimated energies in the range log E(UV) (erg) = (29.3–31.1), 14 flares in the NICER light curve with estimated minimum energies in the range log E(N) (erg) = (30.5–32.3), and 1 flare in the LCOGT light curve with log E(L) (erg) = 31.6. We find that the flare frequency distributions (FFDs) of TESS and NICER flares have comparable slopes, β(T) = −0.67 ± 0.09 and β(N) = − 0.65 ± 0.19, and the FFD of UVOT flares has a shallower slope (β(U) = −0.38 ± 0.13). Furthermore, we do not find conclusive evidence for either the first ionization potential (FIP) or the inverse FIP effect during coronal flares on EV Lac.

Rishi R. Paudel↗
NASA NTRS2021

The RS CVn Type star GT Mus Shows Most Energetic X-ray Flares Throughout the 2010s

We report that the RS CVn-type star GT Mus (HR4492, HD101379+HD101380) was the most active star in the X-ray sky in the last decade in terms of the scale of recurrent energetic flares. We detected eleven flares from GT Mus in eight years of observations with MAXI (Monitor of All-sky X-ray Image)from 2009 August to 2017 August. The detected flare peak luminosities were 1–4×1033erg s−1in the2.0–20.0 keV band for its distance of 109.6 pc. The durations of the flares ranged from 1–4 days. The flare energies are calculated to be 1–8×1038erg, which are at the upper end of the observed stellar flare. We carried out X-ray follow-up observations for one of the eleven flares with NICER (Neutron star Interior Composition Explorer) on 2017 July 18 and found that the flare cooled quasi-statically. On the basis of a quasi-static cooling model, the flare loop length is derived to be 4×1012cm (or60R). The electron density is derived to be1×1010cm−3, which is consistent with the typical value of solar and stellar flares (1010−11cm−3).

Ryo Sasaki↗
NASA NTRS2021

Conditions in the WR 140 Wind-collision Region Revealed by the 1.083-um He I Line Profile

We present spectroscopy of the P Cygni profile of the 1.083-μm He i line in the WC7 + O5 colliding-wind binary (CWB) WR140 (HD 193793), observed in 2008, before its periastron passage in 2009, and in 2016–17, spanning the subsequent periastron passage. Both absorption and emission components showed strong variations. The variation of the absorption component as the O5 star was occulted by the wind-collision region (WCR) sets a tight constraint on its geometry. While the sightline to the O5 star traversed the WCR, the strength and breadth of the absorption component varied significantly on time-scales of days. An emission sub-peak was observed on all our profiles. The variation of its radial velocity with orbital phase was shown to be consistent with formation in the WCR as it swung round the stars in their orbit. Modelling the profile gave a measure of the extent of the sub-peak forming region. In the phase range 0.93–0.99, the flux in the sub-peak increased steadily, approximately inversely proportionally to the stellar separation, indicating that the shocked gas in the WCR where the line was formed was adiabatic. After periastron, the sub-peak flux was anomalously strong and varied rapidly, suggesting formation in clumps down-stream in the WCR. For most of the time, its flux exceeded the 2–10-keV X-ray emission, showing it to be a significant coolant of the shocked wind.

Peredur M Williams↗
NASA NTRS2021

Conditions in the WR 140 Wind-Collision Region Revealed by the 1.083-mu m Hei Line Profile

We present spectroscopy of the P Cygni profile of the 1.083-μm He I line in the WC7+O5 colliding-wind binary (CWB) WR 140 (HD 193793), observed in 2008, before its periastron passage in 2009, and in 2016–2017, spanning the subsequent periastron passage. Both absorption and emission components showed strong variations. The variation of the absorption component as the O5 star was occulted by the wind-collision region (WCR) sets a tight constraint on its geometry. While the sightline to the O5 star traversed the WCR, the strength and breadth of the absorption component varied significantly on time-scales of days. An emission subpeak was observed on all our profiles. The variation of its radial velocity with orbital phase was shown to be consistent with formation in the WCR as it swung round the stars in their orbit. Modelling the profile gives a measure of the extent of the subpeak-forming region. In the phase range 0.93–0.99, the flux in the subpeak increased steadily, approximately inversely proportionally to the stellar separation, indicating that the shocked gas in the WCR where the line was formed was adiabatic. After periastron, the subpeak flux was anomalously strong and varied rapidly, suggesting formation in clumps downstream in the WCR. For most of the time, its flux exceeded the 2–10-keV X-ray emission, showing it to be a significant coolant of the shocked wind.

Peredur M Williams↗
NASA NTRS2020

Evidence for Magnetic Activity at Starbirth: a Powerful X-ray Flare From the Class 0 Protostar HOPS 383

Context. Class 0 protostars represent the earliest evolutionary stage of solar-type stars, during which the majority of the system mass resides in an infalling envelope of gas and dust and not yet in the central, nascent star. Although X-rays are a key signature of magnetic activity in more evolved protostars and young stars, whether such magnetic activity is present at the Class 0 stage is still debated. Aims. We aim to detect a bona fide Class 0 protostar in X-rays. Methods. We observed HOPS 383 in 2017 December in X-rays with the Chandra X-ray Observatory (~84 ks) and in near-infrared imaging with the Southern Astrophysical Research Telescope (SOAR). Results. HOPS 383 was detected in X-rays during a powerful flare. This hard (E > 2 keV) X-ray counterpart was spatially coincident with the northwest 4-cm component of HOPS 383, which would be the base of the radio thermal jet launched by HOPS 383. The flare duration was ~3.3 h; at the peak, the X-ray luminosity reached ~4 x 10(exp 31) erg/s in the 2–8 keV energy band, a level at least an order of magnitude larger than that of the undetected quiescent emission from HOPS 383. The X-ray flare spectrum is highly absorbed (N(H) ~ 7 x 10(exp 23) per sq.cm), and displays a 6.4 keV emission line with an equivalent width of ~1:1 keV, arising from neutral or low-ionization iron. Conclusions. The detection of a powerful X-ray flare from HOPS 383 constitutes direct proof that magnetic activity can be present at the earliest formative stages of solar-type stars.

Nicolas Grosso↗