323 research outputs found
Trigger, an active release experiment that stimulated auroral particle precipitation and wave emissions
The experiment design, including a description of the diagnostic and chemical release payload, and the general results are given for an auroral process simulation experiment. A drastic increase of the field aligned charged particle flux was observed over the approximate energy range 10 eV to more than 300 keV, starting about 150 ms after the release and lasting about one second. The is evidence of a second particle burst, starting one second after the release and lasting for tens of seconds, and evidence for a periodic train of particle bursts occurring with a 7.7 second period from 40 to 130 seconds after the release. A transient electric field pulse of 200 mv/m appeared just before the particle flux increase started. Electrostatic wave emissions around 2 kHz, as well as a delayed perturbation of the E-region below the plasma cloud were also observed. Some of the particle observations are interpreted in terms of field aligned electrostatic acceleration a few hundred kilometers above the injected plasma cloud. It is suggested that the acceleration electric field was created by an instability driven by field aligned currents originating in the plasma cloud
Constraints on the multiplicity of the most massive stars known: R136 a1, a2, a3, and c
The most massive stars known to date are R 136 a1, a2, a3, and c within the
central cluster R 136a of the Tarantula nebula in the Large Magellanic Cloud
(LMC), with reported masses in excess of 150-200. However, the mass
estimation of these stars relies on the assumption that they are single. We
collected three epochs of spectroscopy for R 136 a1, a2, a3, and c with the
Space Telescope Imaging Spectrograph (STIS) of the Hubble Space Telescope (HST)
in the years 2020-2021 to probe potential radial-velocity (RV) variations. We
combine these epochs with an additional HST/STIS observation taken in 2012. We
use cross-correlation to quantify the RVs, and establish constraints on
possible companions to these stars up to periods of ~10 yr. Objects are
classified as binaries when the peak-to-peak RV shifts exceed 50 km/s, and when
the RV shift is significant with respect to errors.
R 136 a1, a2, and a3 do not satisfy the binary criteria and are thus
classified as putatively single, although formal peak-to-peak RV variability on
the level 40 km/s is noted for a3. Only R 136 c is classified as binary, in
agreement with literature. We can generally rule out massive companions (M2 >
~50 Msun) to R 136 a1, a2, and a3 out to orbital periods of < 1 yr (separations
< 5 au) at 95% confidence, or out to tens of years (separations < ~100 au) at
50% confidence. Highly eccentric binaries (e > ~0.9) or twin companions with
similar spectra could evade detection down to shorter periods (> ~10 d), though
their presence is not supported by the relative X-ray faintness of R 136 a1,
a2, and a3. We derive a preliminary orbital solution with a 17.2 d period for
the X-ray bright binary R 136 c, though more data are needed to conclusively
derive its orbit.
Our study supports a lower bound of 150-200 on the upper-mass limit
at LMC metallicityComment: Accepted to A&
The R136 star cluster dissected with Hubble Space Telescope/STIS. I. Far-ultraviolet spectroscopic census and the origin of HeII 1640 in young star clusters
We introduce a HST/STIS stellar census of R136a, the central ionizing star
cluster of 30 Doradus. We present low resolution far-ultraviolet STIS/MAMA
spectroscopy of R136 using 17 contiguous 52x0.2 arcsec slits which together
provide complete coverage of the central 0.85 parsec (3.4 arcsec). We provide
spectral types of 90% of the 57 sources brighter than m_F555W = 16.0 mag within
a radius of 0.5 parsec of R136a1, plus 8 additional nearby sources including
R136b (O4\,If/WN8). We measure wind velocities for 52 early-type stars from CIV
1548-51, including 16 O2-3 stars. For the first time we spectroscopically
classify all Weigelt & Baier members of R136a, which comprise three WN5 stars
(a1-a3), two O supergiants (a5-a6) and three early O dwarfs (a4, a7, a8). A
complete Hertzsprung-Russell diagram for the most massive O stars in R136 is
provided, from which we obtain a cluster age of 1.5+0.3_-0.7 Myr. In addition,
we discuss the integrated ultraviolet spectrum of R136, and highlight the
central role played by the most luminous stars in producing the prominent HeII
1640 emission line. This emission is totally dominated by very massive stars
with initial masses above ~100 Msun. The presence of strong HeII 1640 emission
in the integrated light of very young star clusters (e.g A1 in NGC 3125)
favours an initial mass function extending well beyond a conventional upper
limit of 100 Msun. We include montages of ultraviolet spectroscopy for LMC O
stars in the Appendix. Future studies in this series will focus on optical
STIS/CCD medium resolution observations.Comment: 20 pages plus four Appendices providing LMC UV O spectral templates,
UV spectral atlas in R136, wind velocities of LMC O stars and photometry of
additional R136 source
The Properties of Fast Yellow Pulsating Supergiants: FYPS Point the Way to Missing Red Supergiants
Fast yellow pulsating supergiants (FYPS) are a recently-discovered class of
evolved massive pulsator. As candidate post-red supergiant objects, and one of
the few classes of pulsating evolved massive stars, these objects have
incredible potential to change our understanding of the structure and evolution
of massive stars. Here we examine the lightcurves of a sample of 126 cool
supergiants in the Magellanic Clouds observed by the Transiting Exoplanet
Survey Satellite (\tess~) in order to identify pulsating stars. After making
quality cuts and filtering out contaminant objects, we examine the distribution
of pulsating stars in the Hertzprung-Russel (HR) diagram, and find that FYPS
occupy a region above . This luminosity boundary
corresponds to stars with initial masses of 18-20 , consistent
with the most massive red supergiant progenitors of supernovae (SNe) II-P, as
well as the observed properties of SNe IIb progenitors. This threshold is in
agreement with the picture that FYPS are post-RSG stars. Finally, we
characterize the behavior of FYPS pulsations as a function of their location in
the HR diagram. We find low frequency pulsations at higher effective
temperatures, higher frequency pulsations at lower temperatures, with a
transition between the two behaviors at intermediate temperatures. The observed
properties of FYPS make them fascinating objects for future theoretical study.Comment: Consistent with published version which contains significantly
improved detection and rejection of contaminant objects. Comments welcom
Three-dimensional distribution of ejecta in Supernova 1987A at 10 000 days
Due to its proximity, SN 1987A offers a unique opportunity to directly
observe the geometry of a stellar explosion as it unfolds. Here we present
spectral and imaging observations of SN 1987A obtained ~10,000 days after the
explosion with HST/STIS and VLT/SINFONI at optical and near-infrared
wavelengths. These observations allow us to produce the most detailed 3D map of
H-alpha to date, the first 3D maps for [Ca II] \lambda \lambda 7292, 7324, [O
I] \lambda \lambda 6300, 6364 and Mg II \lambda \lambda 9218, 9244, as well as
new maps for [Si I]+[Fe II] 1.644 \mu m and He I 2.058 \mu m. A comparison with
previous observations shows that the [Si I]+[Fe II] flux and morphology have
not changed significantly during the past ten years, providing evidence that it
is powered by 44Ti. The time-evolution of H-alpha shows that it is
predominantly powered by X-rays from the ring, in agreement with previous
findings. All lines that have sufficient signal show a similar large-scale 3D
structure, with a north-south asymmetry that resembles a broken dipole. This
structure correlates with early observations of asymmetries, showing that there
is a global asymmetry that extends from the inner core to the outer envelope.
On smaller scales, the two brightest lines, H-alpha and [Si I]+[Fe II] 1.644
\mu m, show substructures at the level of ~ 200 - 1000 km/s and clear
differences in their 3D geometries. We discuss these results in the context of
explosion models and the properties of dust in the ejecta.Comment: Accepted for publication in Ap
Predicting the Presence of Companions for Stripped-envelope Supernovae:The Case of the Broad-lined Type Ic SN 2002ap
The R136 star cluster dissected with Hubble Space Telescope/STIS. I. Far-ultraviolet spectroscopic census and the origin of He II lambda 1640 in young star clusters
We introduce a Hubble Space Telescope (HST)/Space Telescope Imaging Spectrograph (STIS) stellar census of R136a, the central ionizing star cluster of 30 Doradus. We present low resolution far-ultraviolet STIS spectroscopy of R136 using 17 contiguous 52 arcsec × 0.2 arcsec slits which together provide complete coverage of the central 0.85 parsec (3.4 arcsec). We provide spectral types of 90 per cent of the 57 sources brighter than mF555W = 16.0 mag within a radius of 0.5 parsec of R136a1, plus 8 additional nearby sources including R136b (O4 If/WN8). We measure wind velocities for 52 early-type stars from C IVλλ1548–51, including 16 O2–3 stars. For the first time, we spectroscopically classify all Weigelt and Baier members of R136a, which comprise three WN5 stars (a1–a3), two O supergiants (a5–a6) and three early O dwarfs (a4, a7, a8). A complete Hertzsprung–Russell diagram for the most massive O stars in R136 is provided, from which we obtain a cluster age of 1.5
+0.3−0.7
−0.7+0.3
Myr. In addition, we discuss the integrated ultraviolet spectrum of R136, and highlight the central role played by the most luminous stars in producing the prominent He II λ1640 emission line. This emission is totally dominated by very massive stars with initial masses above ∼100 M⊙. The presence of strong He II λ1640 emission in the integrated light of very young star clusters (e.g. A1 in NGC 3125) favours an initial mass function extending well beyond a conventional upper limit of 100 M⊙. We include montages of ultraviolet spectroscopy for Large Magellanic Cloud O stars in the appendix. Future studies in this series will focus on optical STIS medium resolution observations
AT 2021loi: A Bowen Fluorescence Flare with a Rebrightening Episode, Occurring in a Previously-Known AGN
AT 2021loi is an optical-ultraviolet transient located at the center of its
host galaxy. Its spectral features identify it as a member of the ``Bowen
Fluorescence Flare'' (BFF) class. The first member of this class was considered
to be related to a tidal disruption event, but enhanced accretion onto an
already active supermassive black hole was suggested as an alternative
explanation. AT 2021loi, having occurred in a previously-known unobscured AGN,
strengthens the latter interpretation. Its light curve is similar to those of
previous BFFs, showing a rebrightening approximately one year after the main
peak (which was not explicitly identified, but might be the case, in all
previous BFFs). An emission feature around 4680 A, seen in the pre-flare
spectrum, strengthens by a factor of 2 around the optical peak of the
flare, and is clearly seen as a double peaked feature then, suggesting a blend
of NIII with HeII as its origin. The appearance of
OIII 3133 and possible NIII (blended with
H) during the flare further support a Bowen Fluorescence
classification. Here, we present ZTF, ATLAS, Keck, Las Cumbres Observatory,
NEOWISE-R, , AMI and VLA observations of AT 2021loi, making it one of
the best observed BFFs to date. AT 2021loi thus provides some clarity on the
nature of BFFs but also further demonstrates the diversity of nuclear
transients.Comment: Submitted to ApJ. This version addresses comments from the refere
Type Ia Supernova Rate Measurements To Redshift 2.5 From CANDELS: Searching For Prompt Explosions In The Early Universe
dThe Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey (CANDELS) was a multi-cycle treasury program on the Hubble Space Telescope (HST) that surveyed a total area of -0.25 deg2 with -900 HST orbits spread across five fields over three years. Within these survey images we discovered 65 supernovae (SNe) of all types, out to z 2.5. We classify -24 of these as Type Ia SNe (SNe Ia) based on host galaxy redshifts and SN photometry (supplemented by grism spectroscopy of six SNe). Here we present a measurement of the volumetric SN Ia rate as a function of redshift, reaching for the first time beyond z =- 2 and putting new constraints on SN Ia progenitor models. Our highest redshift bin includes detections of SNe that exploded when the universe was only -3 Gyr old and near the peak of the cosmic star formation history. This gives the CANDELS high redshift sample unique leverage for evaluating the fraction of SNe Ia that explode promptly after formation ( 40 Myr. However, mild tension is apparent between ground-based low-z surveys and space-based high-z surveys. In both CANDELS and the sister HST program CLASH (Cluster Lensing And Supernova Survey with Hubble), we find a low rate of SNe Ia at z > 1. This could be a hint that prompt progenitors are in fact relatively rare, accounting for only 20% of all SN Ia explosions-though further analysis and larger samples will be needed to examine that suggestion. Key words: infrared: general - supernovae:Astronom
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