219 research outputs found
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 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
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
Discovery and Rapid Follow-up Observations of the Unusual Type II SN 2018ivc in NGC 1068
We present the discovery and high-cadence follow-up observations of SN 2018ivc, an unusual SNe II that exploded in NGC 1068 (D = 10.1 Mpc). The light curve of SN 2018ivc declines piecewise-linearly, changing slope frequently, with four clear slope changes in the first 30 days of evolution. This rapidly changing light curve indicates that interaction between the circumstellar material and ejecta plays a significant role in the evolution. Circumstellar interaction is further supported by a strong X-ray detection. The spectra are rapidly evolving and dominated by hydrogen, helium, and calcium emission lines. We identify a rare high-velocity emission-line feature blueshifted at ∼7800 (in Hα, Hβ, Pβ, Pγ, He I, and Ca II), which is visible from day 18 until at least day 78 and could be evidence of an asymmetric progenitor or explosion. From the overall similarity between SN 2018ivc and SN 1996al, the Hα equivalent width of its parent H II region, and constraints from pre-explosion archival Hubble Space Telescope images, we find that the progenitor of SN 2018ivc could be as massive as 52 but is more likely <12 . SN 2018ivc demonstrates the importance of the early discovery and rapid follow-up observations of nearby supernovae to study the physics and progenitors of these cosmic explosions
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
Early Lightcurves of Type Ia Supernovae are Consistent with Nondegenerate Progenitor Companions
If Type Ia supernovae (SNe~Ia) result from a white dwarf being ignited by
Roche lobe overflow from a nondegenerate companion, then as the supernova
explosion runs into the companion star its ejecta will be shocked, causing an
early blue excess in the lightcurve. A handful of these excesses have been
found in single-object studies, but inferences about the population of SNe~Ia
as a whole have been limited because of the rarity of multiwavelength followup
within days of explosion. Here we present a three-year investigation yielding
an unbiased sample of nine nearby () SNe~Ia with exemplary early data.
The data are truly multiwavelength, covering and Swift bandpasses, and
also early, with an average first epoch 16.0 days before maximum light. Of the
nine objects, three show early blue excesses. We do not find enough statistical
evidence to reject the null hypothesis that SNe~Ia predominantly arise from
Roche-lobe-overflowing single-degenerate systems (). When looking at
the objects' colors, we find the objects are almost uniformly near-UV-blue, in
contrast to earlier literature samples which found that only a third of SNe~Ia
are near-UV-blue, and we find a seemingly continuous range of colors in
the days after explosion, again in contrast with earlier claims in the
literature. This study highlights the importance of early, truly
multiwavelength, high-cadence data in determining the progenitor systems of
SNe~Ia and in revealing their diverse early behavior.Comment: 28 pages, 10 figure
From Discovery to the First Month of the Type II Supernova 2023ixf: High and Variable Mass Loss in the Final Year Before Explosion
We present the discovery of Type II supernova (SN) 2023ixf in M101, among the
closest core-collapse SNe in the last several decades, and follow-up
photometric and spectroscopic observations in the first month of its evolution.
The light curve is characterized by a rapid rise ( days) to a
luminous peak ( mag) and plateau ( mag)
extending to days with a smooth decline rate of mag
day. During the rising phase, color shows blueward evolution,
followed by redward evolution in the plateau phase. Prominent flash features of
hydrogen, helium, carbon, and nitrogen dominate the spectra up to
days after first light, with a transition to a higher ionization state in the
first days. Both the color and flash ionization states suggest
a rise in the temperature, indicative of a delayed shock-breakout inside dense
circumstellar material (CSM). From the timescales of CSM interaction, we
estimate its compact radial extent of cm. We then
construct numerical light-curve models based on both continuous and eruptive
mass-loss scenarios shortly before explosion. For the continuous mass-loss
scenario, we infer a range of mass-loss history with in the final years before explosion, with a potentially
decreasing mass loss of in
years towards the explosion. For the eruptive mass-loss scenario, we favor
eruptions releasing of the envelope at about a year before
explosion, which result in CSM with mass and extent similar to the continuous
scenario. We discuss the implications of the available multi-wavelength
constraints obtained thus far on the progenitor candidate and SN 2023ixf to our
variable CSM models.Comment: 15 pages, 5 figures, submitted to ApJ
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