28 research outputs found
Investigation of Kepler Objects of Interest Stellar Parameters from Observed Transit Durations
The Kepler mission discovery of candidate transiting exoplanets (KOIs)
enables a plethora of ensemble analysis of the architecture and properties of
exoplanetary systems. We compare the observed transit durations of KOIs to a
synthetic distribution generated from the known eccentricities of radial
velocity (RV) discovered exoplanets. We find that the Kepler and RV
distributions differ at a statistically significant level. We identify three
related systematic trends that are likely due to errors in stellar radii, which
in turn affect the inferred exoplanet radii and the distribution thereof, and
prevent a valid analysis of the underlying ensemble eccentricity distribution.
First, 15% of KOIs have transit durations >20% longer than the transit duration
expected for an edge-on circular orbit, including 92 KOIs with transit
durations >50% longer, when only a handful of such systems are expected.
Second, the median transit duration is too long by up to ~25%. Random errors of
<50% in the stellar radius are not adequate to account for these two trends,
and they are present for all spectral types in the Kepler sample. We identify
that incorrect estimates of stellar metallicity and extinction could account
for these anomalies, rather than astrophysical effects such as eccentric
exoplanets improbably transiting near apastron. Third, we find that the median
transit duration is correlated with stellar radius, when no such trend is
expected. All three effects are still present, although less pronounced, when
considering only multiple transiting KOI systems which are thought to have a
low false positive rate. Improved stellar parameters for KOIs are necessary for
the validity of future ensemble tests of exoplanetary systems found by Kepler.Comment: PASP, in pres
A new and unusual LBV-like outburst from a Wolf–Rayet star in the outskirts of M33
MCA-1B (also called UIT003) is a luminous hot star in the western outskirts of M33, classified over 20 yr ago with a spectral type of Ofpe/WN9 and identified then as a candidate luminous blue variable (LBV). Palomar Transient Factory data reveal that this star brightened in 2010, with a light curve resembling that of the classic LBV star AF And in M31. Other Ofpe/WN9 stars have erupted as LBVs, but MCA-1B was unusual because it remained hot. It showed a WN-type spectrum throughout its eruption, whereas LBVs usually get much cooler. MCA-1B showed an almost four-fold increase in bolometric luminosity and a doubling of its radius, but its temperature stayed ≳29 kK. As it faded, it shifted to even hotter temperatures, exhibiting a WN7/WN8-type spectrum, and doubling its wind speed. MCA-1B is reminiscent of some supernova impostors, and its location resembles the isolated environment of SN 2009ip. It is most similar to HD 5980 (in the Small Magellanic Cloud) and GR 290 (also in M33). Whereas these two LBVs exhibited B-type spectra in eruption, MCA-1B is the first clear case where a Wolf–Rayet (WR) spectrum persisted at all times. Together, MCA-1B, HD 5980, and GR 290 constitute a class of WN-type LBVs, distinct from S Doradus LBVs. They are most interesting in the context of LBVs at low metallicity, a possible post-LBV/WR transition in binaries, and as likely Type Ibn supernova progenitors
SN2013fs and SN2013fr: Exploring the circumstellar-material diversity in Type II supernovae
We present photometry and spectroscopy of SN2013fs and SN2013fr in the first
100 days post-explosion. Both objects showed transient, relatively narrow
H emission lines characteristic of SNeIIn, but later resembled normal
SNeII-P or SNeII-L, indicative of fleeting interaction with circumstellar
material (CSM). SN2013fs was discovered within 8hr of explosion. Its light
curve exhibits a plateau, with spectra revealing strong CSM interaction at
early times. It is a less luminous version of the transitional SNIIn PTF11iqb,
further demonstrating a continuum of CSM interaction intensity between SNeII-P
and IIn. It requires dense CSM within 6.510~cm of the
progenitor, from a phase of advanced pre-SN mass loss shortly before explosion.
Spectropolarimetry of SN2013fs shows little continuum polarization, but
noticeable line polarization during the plateau phase. SN2013fr morphed from a
SNIIn at early times to a SNII-L. After the first epoch its narrow lines
probably arose from host-galaxy emission, but the bright, narrow H
emission at early times may be intrinsic. As for SN2013fs, this would point to
a short-lived phase of strong CSM interaction if proven to be intrinsic,
suggesting a continuum between SNeIIn and II-L. It is a low-velocity SNII-L,
like SN2009kr but more luminous. SN2013fr also developed an IR excess at later
times, due to warm CSM dust that require a more sustained phase of strong
pre-SN mass loss.Comment: MNRAS accepted. 28 pages, 23 figures, 8 table
Multi-Epoch Spectropolarimetry for a Sample of Type IIn Supernovae: Persistent Asymmetry in Dusty Circumstellar Material
We present multi-epoch spectropolarimetry and spectra for a sample of 14 Type
IIn supernovae (SNe IIn). We find that after correcting for likely interstellar
polarization, SNe IIn commonly show intrinsic continuum polarization of 1--3%
at the time of peak optical luminosity, although a few show weaker or
negligible polarization. While some SNe IIn have even stronger polarization at
early times, their polarization tends to drop smoothly over several hundred
days after peak. We find a tendency for the intrinsic polarization to be
stronger at bluer wavelengths, especially at early times. While polarization
from an electron scattering region is expected to be grey, scattering of SN
light by dusty circumstellar material (CSM) may induce such a
wavelength-dependent polarization. For most SNe IIn, changes in polarization
degree and wavelength dependence are not accompanied by changes in the position
angle, requiring that asymmetric pre-SN mass loss had a persistent geometry.
While 2--3% polarization is typical, about 30% of SNe IIn have very low or
undetected polarization. Under the simplifying assumption that all SN IIn
progenitors have axisymmetric CSM (i.e. disk/torus/bipolar), then the
distribution of polarization values we observe is consistent with similarly
asymmetric CSM seen from a distribution of random viewing angles. This
asymmetry has very important implications for understanding the origin of
pre-SN mass loss in SNe IIn, suggesting that it was shaped by binary
interaction.Comment: 76 pages, 54 figures (13 in main text, 41 in appendix A
SN2012ab: A Peculiar Type IIn Supernova with Aspherical Circumstellar Material
We present photometry, spectra, and spectropolarimetry of supernova (SN)
2012ab, mostly obtained over the course of days after discovery. SN
2012ab was a Type IIn (SN IIn) event discovered near the nucleus of spiral
galaxy 2MASXJ12224762+0536247. While its light curve resembles that of SN
1998S, its spectral evolution does not. We see indications of CSM interaction
in the strong intermediate-width emission features, the high luminosity (peak
at absolute magnitude ), and the lack of broad absorption features in
the spectrum. The H emission undergoes a peculiar transition. At early
times it shows a broad blue emission wing out to km
and a truncated red wing. Then at late times (
100days) it shows a truncated blue wing and a very broad red emission wing
out to roughly km . This late-time broad red wing
probably arises in the reverse shock. Spectra also show an asymmetric
intermediate-width H component with stronger emission on the red side
at late times. The evolution of the asymmetric profiles requires a density
structure in the distant CSM that is highly aspherical. Our spectropolarimetric
data also suggest asphericity with a strong continuum polarization of % and depolarization in the H line, indicating asphericity in the
CSM at a level comparable to that in other SNe IIn. We estimate a mass-loss
rate of for km extending back at least 75yr prior to the
SN. The strong departure from axisymmetry in the CSM of SN 2012ab may suggest
that the progenitor was an eccentric binary system undergoing eruptive mass
loss.Comment: 18 pages, 12 figure
A new and unusual LBV-like outburst from a Wolf–Rayet star in the outskirts of M33
MCA-1B (also called UIT003) is a luminous hot star in the western outskirts of M33, classified over 20 yr ago with a spectral type of Ofpe/WN9 and identified then as a candidate luminous blue variable (LBV). Palomar Transient Factory data reveal that this star brightened in 2010, with a light curve resembling that of the classic LBV star AF And in M31. Other Ofpe/WN9 stars have erupted as LBVs, but MCA-1B was unusual because it remained hot. It showed a WN-type spectrum throughout its eruption, whereas LBVs usually get much cooler. MCA-1B showed an almost four-fold increase in bolometric luminosity and a doubling of its radius, but its temperature stayed ≳29 kK. As it faded, it shifted to even hotter temperatures, exhibiting a WN7/WN8-type spectrum, and doubling its wind speed. MCA-1B is reminiscent of some supernova impostors, and its location resembles the isolated environment of SN 2009ip. It is most similar to HD 5980 (in the Small Magellanic Cloud) and GR 290 (also in M33). Whereas these two LBVs exhibited B-type spectra in eruption, MCA-1B is the first clear case where a Wolf–Rayet (WR) spectrum persisted at all times. Together, MCA-1B, HD 5980, and GR 290 constitute a class of WN-type LBVs, distinct from S Doradus LBVs. They are most interesting in the context of LBVs at low metallicity, a possible post-LBV/WR transition in binaries, and as likely Type Ibn supernova progenitors
A new and unusual LBV-like outburst from a Wolf-Rayet star in the outskirts of M33
MCA-1B (also called UIT003) is a luminous hot star in the western outskirts
of M33, classified over 20yr ago with a spectral type of Ofpe/WN9 and
identified then as a candidate luminous blue variable (LBV). Palomar Transient
Factory data reveal that this star brightened in 2010, with a light curve
resembling that of the classic LBV star AFAnd in M31. Other Ofpe/WN9 stars have
erupted as LBVs, but MCA-1B was unusual because it remained hot. It showed a
WN-type spectrum throughout its eruption, whereas LBVs usually get much cooler.
MCA-1B showed an almost four-fold increase in bolometric luminosity and a
doubling of its radius, but its temperature stayed around 29kK. As it faded, it
shifted to even hotter temperatures, exhibiting a WN7/WN8-type spectrum, and
doubling its wind speed. MCA-1B is reminiscent of some supernova impostors, and
its location resembles the isolated environment of SN 2009ip. It is most
similar to HD5980 (in the SMC) and GR 290 (also in M33). Whereas these two LBVs
exhibited B-type spectra in eruption, MCA-1B is the first clear case where a
Wolf-Rayet (WR) spectrum persisted at all times. Together, MCA-1B, HD 5980, and
GR 290 constitute a class of WN-type LBVs, distinct from S Doradus LBVs. They
are most interesting in the context of LBVs at low metallicity, a possible
post-LBV/WR transition in binaries, and as likely Type~Ibn supernova
progenitors.Comment: accepted in MNRAS. 20 pages, 15 figure
Stars Don't Eat Their Young Migrating Planets - Empirical Constraints On Planet Migration Halting Mechanisms
Abridged: The discovery of "hot Jupiters" very close to their parent stars
confirmed that Jovian planets migrate inward via several potential mechanisms.
We present empirical constraints on planet migration halting mechanisms. We
compute model density functions of close-in exoplanets in the orbital
semi-major axis - stellar mass plane to represent planet migration that is
halted via several mechanisms, including the interior 1:2 resonance with the
magnetospheric disk truncation radius, the interior 1:2 resonance with the dust
sublimation radius, and several scenarios for tidal halting. The models differ
in the predicted power law dependence of the exoplanet orbital semi-major axis
as a function stellar mass, and thus we also include a power law model with the
exponent as a free parameter. We use a Bayesian analysis to assess the model
success in reproducing empirical distributions of confirmed exoplanets and
Kepler candidates that orbit interior to 0.1 AU. Our results confirm a
correlation of the halting distance with stellar mass. Tidal halting provides
the best fit to the empirical distribution of confirmed Jovian exoplanets at a
statistically robust level, consistent with the Kozai mechanism and the
spin-orbit misalignment of a substantial fraction of hot Jupiters. For Kepler
candidates, which have a more restricted range in stellar mass compared to
confirmed planets, we are unable to discern between the tidal dissipation and
magnetospheric disk truncation braking mechanisms at a statistically
significant level. The power law model favors exponents in the range of
0.38-0.9. This is larger than that predicted for tidal halting (0.23-0.33),
which suggests that additional physics may be missing in the tidal halting
theory.Comment: 22 pages, 9 figures, accepted for publication in Ap
SN 2014ab: An Aspherical Type IIn Supernova with Low Polarization
We present photometry, spectra, and spectropolarimetry of supernova (SN)
2014ab, obtained through days after peak brightness. SN 2014ab was a
luminous Type IIn SN ( mag) discovered after peak brightness near
the nucleus of its host galaxy, VV 306c. Prediscovery upper limits constrain
the time of explosion to within 200 days prior to discovery. While SN 2014ab
declined by mag over the course of our observations, the observed
spectrum remained remarkably unchanged. Spectra exhibit an asymmetric
emission-line profile with a consistently stronger blueshifted component,
suggesting the presence of dust or a lack of symmetry between the far side and
near side of the SN. The Pa emission line shows a profile very similar
to that of H, implying that this stronger blueshifted component is
caused either through obscuration by large dust grains, occultation by
optically thick material, or a lack of symmetry between the far side and near
side of the interaction region. Despite these asymmetric line profiles, our
spectropolarimetric data show that SN 2014ab has little detected polarization
after accounting for the interstellar polarization. This suggests that we are
seeing emission from a photosphere that has only small deviation from circular
symmetry face-on. We are likely seeing a SN IIn with nearly circular symmetry
in the plane normal to our line of sight, but with either large-grain dust or
significant asymmetry in the density of circumstellar material or SN ejecta
along our line of sight. We suggest that SN 2014ab and SN 2010jl (as well as
other SNe IIn) may be similar events viewed from different directions.Comment: 20 pages, 19 figure
The electron-capture origin of supernova 2018zd
In the transitional mass range ( 8-10 solar masses) between white dwarf
formation and iron core-collapse supernovae, stars are expected to produce an
electron-capture supernova. Theoretically, these progenitors are thought to be
super-asymptotic giant branch stars with a degenerate O+Ne+Mg core, and
electron capture onto Ne and Mg nuclei should initiate core collapse. However,
no supernovae have unequivocally been identified from an electron-capture
origin, partly because of uncertainty in theoretical predictions. Here we
present six indicators of electron-capture supernovae and show that supernova
2018zd is the only known supernova having strong evidence for or consistent
with all six: progenitor identification, circumstellar material, chemical
composition, explosion energy, light curve, and nucleosynthesis. For supernova
2018zd, we infer a super-asymptotic giant branch progenitor based on the faint
candidate in the pre-explosion images and the chemically-enriched circumstellar
material revealed by the early ultraviolet colours and flash spectroscopy. The
light-curve morphology and nebular emission lines can be explained with the low
explosion energy and neutron-rich nucleosynthesis produced in an
electron-capture supernova. This identification provides insights into the
complex stellar evolution, supernova physics, cosmic nucleosynthesis, and
remnant populations in the transitional mass range.Comment: Author version of the published letter in Nature Astronomy, 28 June
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