16 research outputs found
Signatures of multiple stellar populations in unresolved extragalactic globular/ young massive star clusters
We present an investigation of potential signatures of the formation of
multiple stellar populations in recently formed extragalactic star clusters.
All of the Galactic globular clusters for which good samples of individual
stellar abundances are available show evidence for multiple populations. This
appears to require that multiple episodes of star formation and light element
enrichment are the norm in the history of a globular cluster. We show that
there are detectable observational signatures of multiple formation events in
the unresolved spectra of massive, young extragalactic star clusters. We
present the results of a pilot program to search for one of the cleanest
signatures that we identify - the combined presence of emission lines from a
very recently formed population and absorption lines from a somewhat older
population. A possible example of such a system is identified in the Antennae
galaxies. This source's spectrum shows evidence of two stellar populations with
ages of 8 Myr and 80 Myr. Further investigation shows that these populations
are in fact physically separated, but only by a projected distance of 59 pc. We
show that the clusters are consistent with being bound and discuss the
possibility that their coalescence could result in a single globular cluster
hosting multiple stellar populations. While not the prototypical system
proposed by most theories of the formation of multiple populations in clusters,
the detection of this system in a small sample is both encouraging and
interesting. Our investigation suggests that expanded surveys of massive young
star clusters should detect more clusters with such signatures.Comment: 11 pages, 7 figures: accepted for publication in Ap
Distance and Reddening of the Enigmatic Gamma-ray-Detected Nova V1324 Sco
It has recently been discovered that some, if not all, classical novae emit
GeV gamma-rays during outburst. Despite using an unreliable method to determine
its distance, previous work showed that nova V1324 Sco was the most gamma-ray
luminous of all gamma-ray-detected novae. We present here a different, more
robust, method to determine the reddening and distance to V1324 Sco using
high-resolution optical spectroscopy. Using two independent methods we derived
a reddening of E(B-V) = 1.16 +/- 0.12 and a distance rD > 6.5 kpc. This
distance is >40% greater than previously estimated, meaning that V1324 Sco has
an even higher gamma-ray luminosity than previously calculated. We also use
periodic modulations in the brightness, interpreted as the orbital period, in
conjunction with pre-outburst photometric limits to show that a main-sequence
companion is strongly favored.Comment: Submitted to ApJ. 6 pages, 5 figure
1FGL J1417.7-4407: A likely gamma-ray bright binary with a massive neutron star and a giant secondary
We present multiwavelength observations of the persistent Fermi-LAT
unidentified gamma-ray source 1FGL J1417.7-4407, showing it is likely to be
associated with a newly discovered X-ray binary containing a massive neutron
star (nearly 2 M_sun) and a ~ 0.35 M_sun giant secondary with a 5.4 day period.
SOAR optical spectroscopy at a range of orbital phases reveals variable
double-peaked H-alpha emission, consistent with the presence of an accretion
disk. The lack of radio emission and evidence for a disk suggests the gamma-ray
emission is unlikely to originate in a pulsar magnetosphere, but could instead
be associated with a pulsar wind, relativistic jet, or could be due to
synchrotron self-Compton at the disk--magnetosphere boundary. Assuming a wind
or jet, the high ratio of gamma-ray to X-ray luminosity (~ 20) suggests
efficient production of gamma-rays, perhaps due to the giant companion. The
system appears to be a low-mass X-ray binary that has not yet completed the
pulsar recycling process. This system is a good candidate to monitor for a
future transition between accretion-powered and rotational-powered states, but
in the context of a giant secondary.Comment: ApJL in pres
A Detailed Observational Analysis of V1324 Sco, the Most Gamma-Ray Luminous Classical Nova to Date
It has recently been discovered that some, if not all, classical novae emit
GeV gamma rays during outburst, but the mechanisms involved in the production
of the gamma rays are still not well understood. We present here a
comprehensive multi-wavelength dataset---from radio to X-rays---for the most
gamma-ray luminous classical nova to-date, V1324 Sco. Using this dataset, we
show that V1324 Sco is a canonical dusty Fe-II type nova, with a maximum ejecta
velocity of 2600 km s and an ejecta mass of few
M. There is also evidence for complex shock interactions, including a
double-peaked radio light curve which shows high brightness temperatures at
early times. To explore why V1324~Sco was so gamma-ray luminous, we present a
model of the nova ejecta featuring strong internal shocks, and find that higher
gamma-ray luminosities result from higher ejecta velocities and/or mass-loss
rates. Comparison of V1324~Sco with other gamma-ray detected novae does not
show clear signatures of either, and we conclude that a larger sample of
similarly well-observed novae is needed to understand the origin and variation
of gamma rays in novae.Comment: 26 pages, 13 figure
Non-thermal radio emission from colliding flows in classical nova V1723 Aql
The importance of shocks in nova explosions has been highlighted by Fermi's discovery of \u3b3-ray-producing novae. Over three years of multiband Very Large Array radio observations of the 2010 nova V1723 Aql show that shocks between fast and slow flows within the ejecta led to the acceleration of particles and the production of synchrotron radiation. Soon after the start of the eruption, shocks in the ejecta produced an unexpected radio flare, resulting in a multipeaked radio light curve. The emission eventually became consistent with an expanding thermal remnant with mass 2
7 10\u207b\u2074\u2009M 99 and temperature 10\u2074 K. However, during the first two months, the 7310\u2076 K brightness temperature at low frequencies was too high to be due to thermal emission from the small amount of X-ray-producing shock-heated gas. Radio imaging showed structures with velocities of 400 km s\u207b\ub9\u2009(d/6\u2009kpc) in the plane of the sky, perpendicular to a more elongated 1500 km s\u207b\ub9\u2009(d/6\u2009kpc) flow. The morpho-kinematic structure of the ejecta from V1723 Aql appears similar to nova V959 Mon, where collisions between a slow torus and a faster flow collimated the fast flow and gave rise to \u3b3-ray-producing shocks. Optical spectroscopy and X-ray observations of V1723 Aql during the radio flare are consistent with this picture. Our observations support the idea that shocks in novae occur when a fast flow collides with a slow collimating torus. Such shocks could be responsible for hard X-ray emission, \u3b3-ray production, and double-peaked radio light curves from some classical novae.Peer reviewed: YesNRC publication: Ye