5 research outputs found
Inferring the parallax of Westerlund 1 from Gaia DR2
Westerlund 1 (Wd1) is potentially the largest star cluster in the Galaxy.
That designation critically depends upon the distance to the cluster, yet the
cluster is highly obscured, making luminosity-based distance estimates
difficult. Using {\it Gaia} Data Release 2 (DR2) parallaxes and Bayesian
inference, we infer a parallax of mas corresponding to a
distance of kpc. To leverage the combined statistics of all
stars in the direction of Wd1, we derive the Bayesian model for a cluster of
stars hidden among Galactic field stars; this model includes the parallax
zero-point. Previous estimates for the distance to Wd1 ranged from 1.0 to 5.5
kpc, although values around 5 kpc have usually been adopted. The {\it Gaia} DR2
parallaxes reduce the uncertainty from a factor of 3 to 18\% and rules out the
most often quoted value of 5 kpc with 99\% confidence. This new distance allows
for more accurate mass and age determinations for the stars in Wd1. For
example, the previously inferred initial mass at the main-sequence turn-off was
around 40 M; the new {\it Gaia} DR2 distance shifts this down to
about 22 M. This has important implications for our understanding of
the late stages of stellar evolution, including the initial mass of the
magnetar and the LBV in Wd1. Similarly, the new distance suggests that the
total cluster mass is about four times lower than previously calculated.Comment: 14 pages, 10 figure
On the Gaia DR2 distances for Galactic Luminous Blue Variables
We examine parallaxes and distances for Galactic luminous blue variables
(LBVs) in Gaia DR2. The sample includes 11 LBVs and 14 LBV candidates. For
about half of the sample, DR2 distances are either similar to commonly adopted
literature values, or the DR2 values have large uncertainties. For the rest,
reliable DR2 distances differ significantly from values in the literature, and
in most cases the Gaia DR2 distance is smaller. Two key results are that the S
Doradus instability strip may not be as clearly defined as previously thought,
and that there exists a population of LBVs at relatively low luminosities. LBVs
seem to occupy a wide swath from the end of the main sequence at the blue edge
to 8000 K at the red side, with a spread in luminosity reaching as low as
log(L/Lsun)=4.5. The lower-luminosity group corresponds to effective
single-star initial masses of 10-25 Msun, and includes objects that have been
considered as confirmed LBVs. We discuss implications for LBVs including (1)
their instability and origin in binary evolution, (2) connections to some
supernova (SN) impostors such as the class of SN 2008S-like objects, and (3)
LBVs that may be progenitors of SNe with dense circumstellar material across a
wide initial mass range. Although some of the Gaia DR2 distances for LBVs have
large uncertainty, this represents the most direct and consistent set of
Galactic LBV distance estimates available in the literature.Comment: 20 pages, 5 figures, MNRAS accepted. Updated inclusion of excess
astrometric noise and discussion of literature distance
Recurring outbursts of the supernova impostor AT 2016blu in NGC 4559
We present the first photometric analysis of the supernova (SN) impostor AT
2016blu in NGC 4559. This transient was discovered by the Lick Observatory
Supernova Search in 2012 and has continued its outbursts since then. Optical
and infrared photometry of AT 2016blu reveals at least 19 outbursts in
2012-2022. Similar photometry from 1999-2009 shows no outbursts, indicating
that the star was relatively stable in the decade before discovery. Archival
{\it Hubble Space Telescope} observations suggest that the progenitor had a
minimum initial mass of M and a luminosity of L. AT 2016blu's outbursts show irregular variability with
multiple closely spaced peaks having typical amplitudes of 1-2 mag and
durations of 1-4 weeks. While individual outbursts have irregular light curves,
concentrations of these peaks recur with a period of d. Based
on this period, we predict times for upcoming outbursts in 2023 and 2024. AT
2016blu shares similarities with SN 2000ch in NGC 3432, where outbursts may
arise from periastron encounters in an eccentric binary containing a luminous
blue variable (LBV). We propose that AT 2016blu's outbursts are also driven by
interactions that intensify around periastron in an eccentric system. Intrinsic
variability of the LBV-like primary star may cause different intensity and
duration of binary interaction at each periastron passage. AT 2016blu also
resembles the periastron encounters of Carinae prior to its Great
Eruption and the erratic pre-SN eruptions of SN 2009ip. This similarity and the
onset of eruptions in the past decade hint that AT 2016blu may also be headed
for a catastrophe, making it a target of great interest.Comment: 18 pages, 14 figures, 6 tables, MNRAS Accepte
Hubble Space Telescope Imaging Reveals that SN 2015bh is Much Fainter than its Progenitor
We present Hubble Space Telescope (HST) imaging of the site of SN 2015bh in
the nearby spiral galaxy NGC 2770 taken between 2017 and 2019, nearly four
years after the peak of the explosion. In 2017-2018, the transient fades
steadily in optical filters before declining more slowly to mag
in 2019, nearly 4 mag below the level of its eruptive luminous blue variable
(LBV) progenitor observed with HST in 2008-2009. The source fades at a constant
color of mag until 2018, similar to SN 2009ip and
consistent with a spectrum dominated by continued interaction of the ejecta
with circumstellar material (CSM). A deep optical spectrum obtained in 2021
lacks signatures of ongoing interaction ( erg s for broadened emission km s), but
but indicates the presence of a nearby H II region ( pc). The
color evolution of the fading source makes it unlikely that emission from a
scattered light echo or binary OB companion of the progenitor contributes
significantly to the flattening of the late-time light curve. The remaining
emission in 2019 may plausibly be attributed to an unresolved (
pc), young stellar cluster. Importantly, the color evolution of SN 2015bh also
rules out scenarios in which the surviving progenitor evolves back to a hot,
quiescent, optically faint state or is obscured by nascent dust. The simplest
explanation is that the massive progenitor did not survive. SN 2015bh,
therefore, likely represents a remarkable example of the terminal explosion of
a massive star preceded by decades of end-stage eruptive variability.Comment: 15 pages, 5 figures, submitted to ApJ
Limit on Supernova Emission in the Brightest Gamma-Ray Burst, GRB 221009A
We present photometric and spectroscopic observations of the extraordinary gamma-ray burst (GRB) 221009A in search of an associated supernova. Some past GRBs have shown bumps in the optical light curve that coincide with the emergence of supernova spectral features, but we do not detect any significant light-curve features in GRB 221009A, nor do we detect any clear sign of supernova spectral features. Using two well-studied GRB-associated supernovae (SN 2013dx, SN 2016jca, ) at a similar redshift as GRB 221009A ( z = 0.151), we modeled how the emergence of a supernova would affect the light curve. If we assume the GRB afterglow to decay at the same rate as the X-ray data, the combination of afterglow and a supernova component is fainter than the observed GRB brightness. For the case where we assume the best-fit power law to the optical data as the GRB afterglow component, a supernova contribution should have created a clear bump in the light curve, assuming only extinction from the Milky Way. If we assume a higher extinction of E ( B − V ) = 1.74 mag (as has been suggested elsewhere), the supernova contribution would have been hard to detect, with a limit on the associated supernova of 19.54. We do not observe any clear supernova features in our spectra, which were taken around the time of expected maximum light. The lack of a bright supernova associated with GRB 221009A may indicate that the energy from the explosion is mostly concentrated in the jet, leaving a lower energy budget available for the supernova