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Hajduk M.,Nicolaus Copernicus Astronomical Center | van Hoof P.A.M.,Royal Observatory of Belgium | Zijlstra A.A.,Jodrell Bank Center for Astrophysics
Monthly Notices of the Royal Astronomical Society | Year: 2013

We analyse the remnants of CK Vul (Nova Vul 1670) using optical imaging and spectroscopy. The imaging, obtained between 1991 and 2010, spans 5.6 per cent of the lifetime of the nebula. The flux of the nebula decreased during the last two decades. The central source still maintains the ionization of the innermost part of the nebula, but recombination proceeds in more distant parts of the nebula. Surprisingly, we discovered two stars located within 10 arcsec of the expansion centre of the radio emission that are characterized by pronounced long-term variations and one star with high proper motion. The high proper motion star is a foreground object, and the two variable stars are background objects. The photometric variations of two variables are induced by a dusty cloud ejected by CK Vul and passing through the line of sight to those stars. The cloud leaves strong lithium absorption in the spectra of the stars. We discuss the nature of the object in terms of recent observations. © 2013 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society.


McDonald I.,Jodrell Bank Center for Astrophysics | Zijlstra A.A.,Jodrell Bank Center for Astrophysics | Sloan G.C.,Cornell University | Kerins E.,Jodrell Bank Center for Astrophysics | And 2 more authors.
Monthly Notices of the Royal Astronomical Society | Year: 2014

Variability is examined in over 2.6 million stars covering 11 square degrees of the core of the Sagittarius dwarf spheroidal galaxy (Sgr dSph) from Visible and Infrared Survey Telescope for Astronomy Z-band observations. Generally, pulsation on the Sgr dSph giant branches appears to be excited by the internal ? mechanism. Pulsation amplitudes appear identical between red and asymptotic (red giant branch/asymptotic giant branch) giant stars, and between unreddened carbon and oxygen-rich stars at the same luminosity. The lack of correlation between infrared excess and variability among oxygen-rich stars indicates that pulsations do not contribute significantly to wind driving in oxygen-rich stars in the Sgr dSph, though the low amplitudes of these stars mean this may not apply elsewhere. The dust-enshrouded carbon stars have the highest amplitudes of the stars we observe. Only in these stars does an external ?-mechanismdriven pulsation seem likely, caused by variations in their more opaque carbon-rich molecules or dust. This may allow pulsation driving of winds to be effective in carbon stars. Variability can be simplified to a power law (A α L/T2), as in other systems. In total, we identify 3026 variable stars (with rms variability of δZ ≥ 0.015 mag), of which 176 are long-period variables associable with the upper giant branches of the Sgr dSph. We also identify 324 candidate RR Lyrae variables in the Sgr dSph and 340 in the outer Galactic bulge. © 2014 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society.


Cordero M.J.,Indiana University Bloomington | Pilachowski C.A.,Indiana University Bloomington | Johnson C.I.,Harvard - Smithsonian Center for Astrophysics | McDonald I.,Jodrell Bank Center for Astrophysics | And 2 more authors.
Astrophysical Journal | Year: 2014

47 Tuc is an ideal target to study chemical evolution and globular cluster (GC) formation in massive more metal-rich GCs, as it is the closest massive GC. We present chemical abundances for O, Na, Al, Si, Ca, Ti, Fe, Ni, La, and Eu in 164 red giant branch stars in the massive GC 47 Tuc using spectra obtained with both the Hydra multifiber spectrograph at the Blanco 4 m telescope and the FLAMES multiobject spectrograph at the Very Large Telescope. We find an average [Fe/H] = -0.79 ± 0.09 dex, consistent with literature values, as well as overabundances of alpha-elements ([α/Fe] ∼ 0.3 dex). The n-capture process elements indicate that 47 Tuc is r process-dominated ([Eu/La] = +0.24), and the light elements O, Na, and Al exhibit star-to-star variations. The Na-O anticorrelation, a signature typically seen in Galactic GCs, is present in 47 Tuc, and extends to include a small number of stars with [O/Fe] ∼ -0.5. Additionally, the [O/Na] ratios of our sample reveal that the cluster stars can be separated into three distinct populations. A Kolmogorov-Smirnov test demonstrates that the O-poor/Na-rich stars are more centrally concentrated than the O-rich/Na-poor stars. The observed number and radial distribution of 47 Tuc's stellar populations, as distinguished by their light element composition, agrees closely with the results obtained from photometric data. We do not find evidence supporting a strong Na-Al correlation in 47 Tuc, which is consistent with current models of asymptotic giant branch nucleosynthesis yields. © 2014. The American Astronomical Society. All rights reserved.


Mcdonald I.,Jodrell Bank Center for Astrophysics | Johnson C.I.,University of California at Los Angeles | Zijlstra A.A.,Jodrell Bank Center for Astrophysics
Monthly Notices of the Royal Astronomical Society: Letters | Year: 2011

We herein determine the average integrated mass-loss from stars belonging to the dominant metal-poor population ([Fe/H] ~-1.7) of the Galactic globular cluster ω Centauri (NGC 5139) during their red giant and horizontal branch (HB) evolution. Masses are empirically calculated from spectroscopic measurements of surface gravity and photometric measurements of temperature and luminosity. Systematic uncertainties prevent an absolute measurement of masses at a phase of evolution. However, the relative masses of early asymptotic giant branch (AGB) stars and central red giant branch (RGB) stars can be measured, and used to derive the mass-loss between these two phases. This can then be used as a physical check of models of HB stars. For ω Centauri, the average difference is found to be 26 ± 4 per cent. Assuming initial and final masses of 0.83 and 0.53 M⊙, we determine that 0.21 ± 0.03 M⊙ is lost on the RGB and 0.09 ±~0.05 M⊙ is lost on the AGB. The implied HB stellar mass of 0.62 ± 0.04 M⊙ is commensurate with literature determinations of the masses of the cluster's HB stars. The accuracy of this measurement can be improved through better selection of stars and spectral coverage, and applied to other clusters where HB models do not currently agree. © 2011 The Authors Monthly Notices of the Royal Astronomical Society © 2011 RAS.


McDonald I.,Jodrell Bank Center for Astrophysics | Boyer M.L.,STS Inc | Van Loon J.Th.,Keele University | Zijlstra A.A.,Jodrell Bank Center for Astrophysics
Astrophysical Journal | Year: 2011

Dust production among post-main-sequence stars is investigated in the Galactic globular cluster 47 Tucanae (NGC104) based on infrared photometry and spectroscopy. We identify metallic iron grains as the probable dominant opacity source in these winds. Typical evolutionary timescales of asymptotic giant branch stars suggest the mass-loss rates we report are too high. We suggest that this is because the iron grains are small or elongated and/or that iron condenses more efficiently than at solar metallicity. Comparison to other works suggests metallic iron is observed to be more prevalent toward lower metallicities. The reasons for this are explored, but remain unclear. Meanwhile, the luminosity at which dusty mass loss begins is largely invariant with metallicity, but its presence correlates strongly with long-period variability. This suggests that the winds of low-mass stars have a significant driver that is not radiation pressure, but may be acoustic driving by pulsations. © 2011. The American Astronomical Society. All rights reserved.


Mcdonald I.,Jodrell Bank Center for Astrophysics | Zijlstra A.A.,Jodrell Bank Center for Astrophysics | Boyer M.L.,STS Inc
Monthly Notices of the Royal Astronomical Society | Year: 2012

We derive the fundamental parameters (temperature and luminosity) of 107619 Hipparcos stars and place these stars on a true Hertzsprung-Russell diagram. This is achieved by comparing bt-settl model atmospheres to spectral energy distributions (SEDs) created from Hipparcos, Tycho, Sloan Digital Sky Survey, DENIS, Two Micron All Sky Survey, MSX, AKARI, IRAS and Wide-field Infrared Survey Explorer data. We also identify and quantify from these SEDs any infrared excesses attributable to circumstellar matter. We compare our results to known types of objects, focusing on the giant branch stars. Giant star dust production (as traced by infrared excess) is found to start in earnest around 680 L ⊙. © 2012 The Authors Monthly Notices of the Royal Astronomical Society. © 2012 RAS.


McDonald I.,Jodrell Bank Center for Astrophysics | Sloan G.C.,Cornell University | Zijlstra A.A.,Jodrell Bank Center for Astrophysics | Matsunaga N.,University of Tokyo | And 4 more authors.
Astrophysical Journal Letters | Year: 2010

Iron, the universe's most abundant refractory element, is highly depleted in both circumstellar and interstellar environments, meaning it exists in solid form. The nature of this solid is unknown. In this Letter, we provide evidence that metallic iron grains are present around oxygen-rich asymptotic giant branch stars, where it is observationally manifest as a featureless mid-infrared excess. This identification is made using Spitzer Space Telescope observations of evolved globular cluster stars, where iron dust production appears ubiquitous and in some cases can be modeled as the only observed dust product. In this context, FeO is examined as the likely carrier for the 20 μm feature observed in some of these stars. Metallic iron appears to be an important part of the dust condensation sequence at low metallicity, and subsequently plays an influential role in the interstellar medium. We explore the stellar metallicities and luminosities at which iron formation is observed, and how the presence of iron affects the outflow and its chemistry. The conditions under which iron can provide sufficient opacity to drive a wind remain unclear. © 2010. The American Astronomical Society. All rights reserved..


McDonald I.,Jodrell Bank Center for Astrophysics | Zijlstra A.A.,Jodrell Bank Center for Astrophysics
Monthly Notices of the Royal Astronomical Society | Year: 2014

Ultraviolet radiation fromwhite dwarfs can efficiently clear Galactic globular clusters (GCs) of their intraclustermedium (ICM). This solves the problem of themissing ICM in clusters, which is otherwise expected to build up to easily observable quantities. To show this, we recreate the ionizing flux in 47 Tuc, following randomly generated stars through their asymptotic giant branch (AGB), post-AGB and white dwarf evolution. Each white dwarf can ionize all the material injected into the cluster by stellar winds for ~3 Myr of its evolution: ~40 such white dwarfs exist at any point. Every GC's ICM should be ionized. The neutral cloud in M15 should be caused by a temporary overdensity. A pressure-supported ICM will expand over the cluster's tidal radius, where it will be truncated, allowing Jeans escape. The modelled Jeans mass-loss rate approximates the total stellar mass-loss rate, allowing efficient clearing of ICM. Any cluster's ICM mass should equal the mass injected by its stars over the sound travel time between the cluster core and tidal radius. We predict ~11.3 M⊙ of ICM within 47 Tuc, cleared over ~4 Myr, compared to a dynamical time-scale of 4.3 Myr. We present a new mass hierarchy, discussing the transition between GCs dwarf galaxies. © 2014 The Authors.


McDonald I.,Jodrell Bank Center for Astrophysics | Zijlstra A.A.,Jodrell Bank Center for Astrophysics
Astrophysical Journal Letters | Year: 2016

Low- and intermediate-mass stars eject much of their mass during the late, red giant branch (RGB) phase of evolution. The physics of their strong stellar winds is still poorly understood. In the standard model, stellar pulsations extend the atmosphere, allowing a wind to be driven through radiation pressure on condensing dust particles. Here, we investigate the onset of the wind, using nearby RGB stars drawn from the Hipparcos catalog. We find a sharp onset of dust production when the star first reaches a pulsation period of 60 days. This approximately coincides with the point where the star transitions to the first overtone pulsation mode. Models of the spectral energy distributions show stellar mass-loss rate suddenly increasing at this point, by a factor of ∼10 over the existing (chromospherically driven) wind. The dust emission is strongly correlated with both pulsation period and amplitude, indicating stellar pulsation is the main trigger for the strong mass loss, and determines the mass-loss rate. Dust emission does not strongly correlate with stellar luminosity, indicating radiation pressure on dust has little effect on the mass-loss rate. RGB stars do not normally appear to produce dust, whereas dust production by asymptotic giant branch stars appears commonplace, and is probably ubiquitous above the RGB-tip luminosity. We conclude that the strong wind begins with a step change in mass-loss rate and is triggered by stellar pulsations. A second rapid mass-loss-rate enhancement is suggested when the star transitions to the fundamental pulsation mode at a period of ∼300 days. © 2016. The American Astronomical Society. All rights reserved..


McDonald I.,Jodrell Bank Center for Astrophysics | Zijlstra A.A.,Jodrell Bank Center for Astrophysics | Rajoelimanana A.F.,South African Astronomical Observatory | Rajoelimanana A.F.,University of Cape Town | Johnson C.I.,University of California at Los Angeles
Monthly Notices of the Royal Astronomical Society: Letters | Year: 2013

The asymptotic giant branch (AGB) of the globular cluster NGC 4372 appears to extend to unexpectedly high luminosities. We show, on the basis of proper motions and spatial distribution, that the extended AGB is indeed a likely part of the cluster. We also present the first spectra of the very cool (2600 K), very luminous (8000 L⊙), very dusty, oxygen-rich, purported long-period variable stars V1 and V2 that define the AGB tip. In particular, on the basis of their radial velocities, we conclude that V1 and V2 are probably members. We find that V1 and V2 are likely undergoing the superwind phase that terminates their nuclearburning evolution. We hypothesize that the mass-loss processes that terminate the AGB are inhibited in NGC 4372 due to a lack of atmospheric pulsation and the high gas-to-dust ratio in the ejecta, leading to a delay in the associated enhanced mass loss and dust production. Previously predicted, but never observed, this explains the high mass of the white dwarf in Pease 1 in M15 without the need to invoke a stellar merger. If commonplace, this phenomenon has implications for the mass return from stars, the production of carbon stars and supernovae through the Universe's history, and the AGB contribution to light from unresolved metal-poor populations.©2012 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society.

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