24 research outputs found
Distance and age of the massive stellar cluster Westerlund 1. I. Parallax method using Gaia-EDR3
Westerlund 1 (Wd 1) is one of the most massive young star clusters in the
Milky Way. Although relevant for star formation and evolution, its fundamental
parameters are not yet very well constrained. Our goal is to derive an accurate
distance and provide constraints on the cluster age. We used the photometric
and astrometric information available in the Gaia Early Data Release 3
(Gaia-EDR3) to infer its distance of 4.06 kpc. Modelling of
the eclipsing binary system W36 reported in Paper II led to the distance of
4.340.25 kpc, in agreement with the Gaia-EDR3 distance and, therefore,
validating the parallax zero-point correction approach appropriate for red
objects. By taking advantage of another two recent distance determinations
using the Gaia-EDR3, we obtained a weighted mean distance for the cluster as
d=4.23 kpc (=13.13 mag),
which has an unprecedented accuracy of 4\%. We adopted recent Geneva
evolutionary tracks for supra-solar metallicity objects to infer the age of the
faintest RSG source from Wd 1, leading to a cluster age of 11.00.5 Myr, in
excellent agreement with recent work by Beasor \& Davies (10.4
Myr) based on MIST evolutionary models. The age of W36 was reported to be
3.50.5 Myr in Paper II, supporting recent claims of a temporal spread of
several Myr for the star-forming process within Wd 1 instead of a monolithic
starburst scenario.Comment: 16 pages, 9 figures, revised version submitted to MNRAS on April
20th, 202
VVV-WIT-12 and Its Fashionable Nebula: A 4 yr Long-period Young Stellar Object with a Light Echo?
© 2023. The Author(s). Published by the American Astronomical Society. This is an open access article distributed under the terms of the Creative Commons Attribution License (CC BY), https://creativecommons.org/licenses/by/4.0/We report the serendipitous discovery of VVV-WIT-12, an unusual variable source that seems to induce variability in its surrounding nebula. The source belongs to the rare objects that we call WITs (short for What Is This?) discovered within the VISTA Variables in the Vía Láctea (VVV) survey. VVV-WIT-12 was discovered during a pilot search for light echoes from distant supernovae in the Milky Way using the near-IR images of the VVV survey. This source has an extremely red spectral energy distribution, consistent with a very reddened (A V ∼ 100 mag) long-period variable star (P ∼ 1525 days). Furthermore, it is enshrouded in a nebula that changes brightness and color with time, apparently in sync with the central source variations. The near-IR light curve and complementary follow-up spectroscopy observations are consistent with a variable young stellar object illuminating its surrounding nebula. In this case the source periodic variation along the cycles produces an unprecedented light echo in the different regions of the nebula.Peer reviewe
NICER X-ray Observations of Eta Carinae During its Most Recent Periastron Passage
We report high-precision X-ray monitoring observations in the 0.4-10 keV band
of the luminous, long-period colliding-wind binary Eta Carinae up to and
through its most recent X-ray minimum/periastron passage in February 2020. Eta
Carinae reached its observed maximum X-ray flux on 7 January 2020, at a flux
level of ergs s cm, followed by a rapid
plunge to its observed minimum flux, ergs s
cm near 17 February 2020. The NICER observations show an X-ray recovery
from minimum of only 16 days, the shortest X-ray minimum observed so far.
We provide new constraints of the "deep" and "shallow" minimum intervals.
Variations in the characteristic X-ray temperature of the hottest observed
X-ray emission indicate that the apex of the wind-wind "bow shock" enters the
companion's wind acceleration zone about 81 days before the start of the X-ray
minimum. There is a step-like increase in column density just before the X-ray
minimum, probably associated with the presence of dense clumps near the shock
apex. During recovery and after, the column density shows a smooth decline,
which agrees with previous measurements made by SWIFT at the same
orbital phase, indicating that changes in mass-loss rate are only a few percent
over the two cycles. Finally, we use the variations in the X-ray flux of the
outer ejecta seen by NICER to derive a kinetic X-ray luminosity of the ejecta
of ergs s near the time of the "Great Eruption'
Eta Carinae: an evolving view of the central binary, its interacting winds and its foreground ejecta
FUV spectra of Eta 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 ten-fold which has led to increased
ionization of the multiple shells within the Homunculus and photo-destruction
of molecular hydrogen. 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 bowshock that is newly generated every orbital period. There is no
evidence of changes in the properties of the binary winds.Comment: 36 pages, 22 figures, accepted Astrophysical Journa
A lanthanide-rich kilonova in the aftermath of a long gamma-ray burst
Kilonovae are a rare class of astrophysical transients powered by the
radioactive decay of nuclei heavier than iron, synthesized in the merger of two
compact objects. Over the first few days, the kilonova evolution is dominated
by a large number of radioactive isotopes contributing to the heating rate. On
timescales of weeks to months, its behavior is predicted to differ depending on
the ejecta composition and merger remnant. However, late-time observations of
known kilonovae are either missing or limited. Here we report observations of a
luminous red transient with a quasi-thermal spectrum, following an unusual
gamma-ray burst of long duration. We classify this thermal emission as a
kilonova and track its evolution up to two months after the burst. At these
late times, the recession of the photospheric radius and the rapidly-decaying
bolometric luminosity () support the
recombination of lanthanide-rich ejecta as they cool.Comment: 47 pages, 14 figures, 9 tables; submitted; a minor typo fixe