73 research outputs found
Optical observations of "hot" novae returning to quiescence
We have monitored the return to quiescence of novae previously observed in
outburst as supersoft X-ray sources, with optical photometry of the
intermediate polar (IP) V4743 Sgr and candidate IP V2491 Cyg, and optical
spectroscopy of these two and seven other systems. Our sample includes
classical and recurrent novae, short period (few hours), intermediate period
(1-2 days) and long period (symbiotic) binaries. The light curves of V4743 Sgr
and V2491 Cyg present clear periodic modulations. For V4743 Sgr, the modulation
occurs with the beat of the rotational and orbital periods. If the period
measured for V2491 Cyg is also the beat of these two periods, the orbital one
should be almost 17 hours. The recurrent nova T Pyx already shows fragmentation
of the nebular shell less than 3 years after the outburst. While this nova
still had strong [OIII] at this post-outburst epoch, these lines had already
faded after 3 to 7 years in all the others. We did not find any difference in
the ratio of equivalent widths of high ionization/excitation lines to that of
the Hbeta line in novae with short and long orbital period, indicating that
irradiation does not trigger high mass transfer rate from secondaries with
small orbital separation. An important difference between the spectra of RS Oph
and V3890 Sgr and those of many symbiotic persistent supersoft sources is the
absence of forbidden coronal lines. With the X-rays turn-off, we interpret this
as an indication that mass transfer in symbiotics recurrent novae is
intermittent.Comment: In press in Monthly Notices of the Royal Astronomical Societ
PDS 144: the first confirmed Herbig Ae-Herbig Ae wide binary
PDS 144 is a pair of Herbig Ae stars that are separated by 5.'' 35 on the sky. It has previously been shown to have an A2Ve Herbig Ae star viewed at 83 degrees inclination as its northern member and an A5Ve Herbig Ae star as its southern member. Direct imagery revealed a disk occulting PDS 144 N-the first edge-on disk observed around a Herbig Ae star. The lack of an obvious disk in direct imagery suggested PDS 144 S might be viewed face-on or not physically associated with PDS 144 N. Multi-epoch Hubble Space Telescope imagery of PDS 144 with a 5 year baseline demonstrates PDS 144 N & S are comoving and have a common proper motion with TYC 6782-878-1. TYC 6782-878-1 has previously been identified as a member of Upper Sco sub-association A at d = 145 +/- 2 pc with an age of 5-10 Myr. Ground-based imagery reveals jets and a string of Herbig-Haro knots extending 13' (possibly further) which are aligned to within 7 degrees +/- 6 degrees on the sky. By combining proper motion data and the absence of a dark mid-plane with radial velocity data, we measure the inclination of PDS 144 S to be i = 73 degrees +/- 7 degrees. The radial velocity of the jets from PDS 144 N & S indicates they, and therefore their disks, are misaligned by 25 degrees +/- 9 degrees. This degree of misalignment is similar to that seen in T Tauri wide binaries.Peer reviewe
Thomson scattering and collisional ionization in the X-ray grating spectra of the recurrent nova U Scorpii
We present a Chandra observation of the recurrent nova U Scorpii, done with the High Resolution camera-S (HRC-S) detector and the Low Energy Transmission Grating (LETG) on day 18 after the observed visual maximum of 2010, and compare it with XMM?Newton observations obtained on days 23 and 35 after maximum. The total absorbed flux was in the range 2.2?2.6 × 10−11 erg cm−2 s−1, corresponding to unabsorbed luminosity 7?8.5 × 1036 ×(d/12 kpc)2 for N(H) = 2?2.7 × 1021 cm−2. On day 18, 70 per cent of the soft X-tray flux was in a continuum typical of a very hot white dwarf (WD) atmosphere, which accounted for about 80 per cent of the flux on days 23 and 35. In addition, all spectra display very broad emission lines, due to higher ionization stages at later times. With Chandra we observed apparent P Cygni profiles. We find that these peculiar profiles are not due to blueshifted absorption and redshifted emission in photoionized ejecta, like the optical P Cyg of novae, but they are rather a superposition of WD atmospheric absorption features reflected by the already discovered Thomson scattering corona, and emission lines due to collisional ionization in condensations in the ejecta. On days 23 and 35, the absorption components were no longer measurable, having lost the initial large blueshift that displaced them from the core of the broad emission lines. We interpret this as an indication that mass-loss ceased between day 18 and day 23. On day 35, the emission line spectrum became very complex, with several different components. Model atmospheres indicate that the WD atmospheric temperature was about 730 000 K on day 18 and reached 900 000?1000 000 K on day 35. This peak temperature is consistent with a WD mass of at least 1.3 M⊙.Fil: Orio, M.. INAF Osservatorio di Padova; Italia. University of Wisconsin; Estados UnidosFil: Behar, E.. Department of Physics, Technion; IsraelFil: Gallagher, J.. University of Wisconsin; Estados UnidosFil: Bianchini, A.. Universita Di Padova; ItaliaFil: Chiosi, E.. INAF Osservatorio di Padova; ItaliaFil: Luna, Gerardo Juan Manuel. Consejo Nacional de Investigaciónes CientÃficas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de AstronomÃa y FÃsica del Espacio. - Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de AstronomÃa y FÃsica del Espacio; ArgentinaFil: Nelson, T.. University Of Minnesota; Estados UnidosFil: Rauch, T.. Eberhard Karls University; AlemaniaFil: Schaefer, B. E.. State University Of Louisiana; Estados UnidosFil: Tofflemire, B.. University of Wisconsin; Estados Unido
Correlating Changes in Spot Filling Factors with Stellar Rotation: The Case of LkCa 4
We present a multi-epoch spectroscopic study of LkCa 4, a heavily spotted
non-accreting T Tauri star. Using SpeX at NASA's Infrared Telescope Facility
(IRTF), 12 spectra were collected over five consecutive nights, spanning
1.5 stellar rotations. Using the IRTF SpeX Spectral Library, we
constructed empirical composite models of spotted stars by combining a warmer
(photosphere) standard star spectrum with a cooler (spot) standard weighted by
the spot filling factor, . The best-fit models spanned two
photospheric component temperatures, = 4100 K (K7V) and 4400 K
(K5V), and one spot component temperature, = 3060 K (M5V) with an
of 0.3. We find values of to vary between 0.77 and 0.94 with
an average uncertainty of 0.04. The variability of is periodic
and correlates with its 3.374 day rotational period. Using a mean value for
to represent the total spot coverage, we calculated spot
corrected values for and . Placing these values alongside
evolutionary models developed for heavily spotted young stars, we infer mass
and age ranges of 0.45-0.6 and 0.50-1.25 Myr, respectively. These
inferred values represent a twofold increase in the mass and a twofold decrease
in the age as compared to standard evolutionary models. Such a result
highlights the need for constraining the contributions of cool and warm regions
of young stellar atmospheres when estimating and to infer
masses and ages as well as the necessity for models to account for the effects
of these regions on the early evolution of low-mass stars.Comment: 21 pages, 9 Figures; Accepted for publication in Ap
A 2+1 + 1 quadruple star system containing the most eccentric, low-mass, short-period, eclipsing binary known
We present an analysis of a newly discovered 2+1 + 1 quadruple system with TESS containing an unresolved eclipsing binary (EB) as part of TIC 121088960 and a close neighbour TIC 121088959. The EB consists of two very low-mass M dwarfs in a highly eccentric (e = 0.709) short-period (P = 3.043 58 d) orbit. Given the large pixel size of TESS and the small separation (39) between TIC 121088959 and TIC 121088960 we used light centroid analysis of the difference image between in-eclipse and out-of-eclipse data to show that the EB likely resides in TIC 121088960, but contributes only ~10 per cent of its light. Radial velocity data were acquired with iSHELL at NASA's Infrared Facility and the Coudé spectrograph at the McDonald 2.7-m telescope. For both images, the measured RVs showed no variation over the 11 d observational baseline, and the RV difference between the two images was 8 ± 0.3 km s-1. The similar distances and proper motions of the two images indicate that TIC 121088959 and TIC 121088960 are a gravitationally bound pair. Gaia's large RUWE and astrometric_excess_noise parameters for TIC 121088960, further indicate that this image is the likely host of the unresolved EB and is itself a triple star. We carried out an SED analysis and calculated stellar masses for the four stars, all of which are in the M dwarf regime: 0.19 M⊙ and 0.14 M⊙ for the EB stars and 0.43 M⊙ and 0.39 M⊙ for the brighter visible stars, respectively. Lastly, numerical simulations show that the orbital period of the inner triple is likely the range 1-50 yr
Properties of Interstellar Turbulence from Gradients of Linear Radio Polarization Maps
Faraday rotation of linearly polarized radio signals provides a very
sensitive probe of fluctuations in the interstellar magnetic field and ionized
gas density resulting from magnetohydrodynamic (MHD) turbulence. We used a set
of statistical tools to analyze images of the spatial gradient of linearly
polarized radio emission () from the ISM for both
observational data from a test image of the Southern Galactic Plane Survey
(SGPS) and isothermal simulations of MHD turbulence. We compared the
observational data with results of synthetic observations obtained with the
simulations of 3D turbulence. Visually, in both data sets, a complex network of
filamentary structures is seen. Our analysis shows that the filaments in the
gradient can be produced by shocks as well as random fluctuations
characterizing the non-differentiable field of MHD turbulence. The latter
dominates for subsonic turbulence, while the former dominates for supersonic
turbulence. In order to quantitatively characterize these differences we use
the topology tool known as a genus curve as well as the moments of the image
distribution. We find that higher values for the moments correspond to cases of
with larger Mach numbers, but the strength of the
dependency is connected to the telescope angular resolution. In regards to the
topology, the supersonic filaments observed in have a
positive genus shift, which indicates a "swisscheese" like topology, while the
subsonic cases show a negative genus, indicating a "clump" like topology. In
the case of the genus, the dependency on the telescope resolution is not as
strong. The SGPS test region data has a distribution and morphology that
matches subsonic to transsonic type turbulence, which independently confirms
what is now expected for the WIM.Comment: Submitted to Ap
Near-Infrared Multi-Band Photometry of the Substellar Companion GJ 758 B
GJ 758 B is a cold (~600K) companion to a Sun-like star at 29 AU projected
separation, which was recently detected with high-contrast imaging. Here we
present photometry of the companion in seven photometric bands from
Subaru/HiCIAO, Gemini/NIRI and Keck/NIRC2, providing a rich sampling of the
spectral energy distribution in the 1-5 micron wavelength range. A clear
detection at 1.58 micron combined with an upper limit at 1.69 micron shows
methane absorption in the atmosphere of the companion. The mass of the
companion remains uncertain, but an updated age estimate indicates that the
most likely mass range is ~30-40 Mjup. In addition, we present an updated
astrometric analysis that imposes tighter constraints on GJ 758 B's orbit and
identifies the proposed second candidate companion, "GJ 758 C", as a background
star.Comment: 10 pages, 4 figures, accepted for publication in ApJ Letters. New
version: Corrected a few numbers in the astrometry section (which were
already correct in the print version, but were based on an outdated
simulation in the astro-ph version
A Cautionary Tale: MARVELS Brown Dwarf Candidate Reveals Itself To Be A Very Long Period, Highly Eccentric Spectroscopic Stellar Binary
We report the discovery of a highly eccentric, double-lined spectroscopic
binary star system (TYC 3010-1494-1), comprising two solar-type stars that we
had initially identified as a single star with a brown dwarf companion. At the
moderate resolving power of the MARVELS spectrograph and the spectrographs used
for subsequent radial-velocity (RV) measurements (R ~ <30,000), this particular
stellar binary mimics a single-lined binary with an RV signal that would be
induced by a brown dwarf companion (Msin(i)~50 M_Jup) to a solar-type primary.
At least three properties of this system allow it to masquerade as a single
star with a very low-mass companion: its large eccentricity (e~0.8), its
relatively long period (P~238 days), and the approximately perpendicular
orientation of the semi-major axis with respect to the line of sight (omega~189
degrees). As a result of these properties, for ~95% of the orbit the two sets
of stellar spectral lines are completely blended, and the RV measurements based
on centroiding on the apparently single-lined spectrum is very well fit by an
orbit solution indicative of a brown dwarf companion on a more circular orbit
(e~0.3). Only during the ~5% of the orbit near periastron passage does the
true, double-lined nature and large RV amplitude of ~15 km/s reveal itself. The
discovery of this binary system is an important lesson for RV surveys searching
for substellar companions; at a given resolution and observing cadence, a
survey will be susceptible to these kinds of astrophysical false positives for
a range of orbital parameters. Finally, for surveys like MARVELS that lack the
resolution for a useful line bisector analysis, it is imperative to monitor the
peak of the cross-correlation function for suspicious changes in width or
shape, so that such false positives can be flagged during the candidate vetting
process.Comment: 16 pages, 11 figures, 6 table
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