We'll use this page to post material for our morning coffee discussions (currently MWF, 11:30am in the library). If you wish to discuss a particular paper, or just find the abstract interesting, please post it here under the appropriate date. If you are creating today's abstract list, please begin a new header so that we can maintain an archive of all abstracts of interest.

March 6 Discussion


DECIPHERING THE 3D STRUCTURE OF THE OLD GALACTIC BULGE FROM THE OGLE RR LYRAE STARS
http://arxiv.org/pdf/1412.4121v1.pdf

We have analyzed a sample of 27,258 fundamental-mode RR Lyrae variable stars (type RRab) detected recently toward the Galactic bulge by the OGLE survey. The data support our earlier claim that these metal-poor stars trace closely the barred structure formed of intermediate-age red clump giants. The distance to the Galactic center (GC) inferred from the bulge RR Lyrae stars is R0= 8.27±0.01(stat)±0.40(sys) kpc. We show that their spatial distribution has the shape of a triaxial ellipsoid with the major axis located in the Galactic plane and inclined at an angle i= 20◦±3◦to the Sun–GC line of sight. The obtained scale-length ratio of the major axis to the minor axis in the Galactic plane, and to the axis vertical to the plane is 1 : 0.49(2) : 0.39(2). We do not see the evidence for the bulge RR Lyrae stars forming an X-shaped structure. Based on the light curve parameters, we derive metallicities of the RRab variables and show that there is a very mild but statistically significant radial metallicity gradient. About 60% of the bulge RRab stars form two very close sequences on the period-amplitude (or Bailey) diagram, which we interpret as two major old bulge populations: A and B. Their metallicities likely differ. Population A is about four times less abundant than the slightly more metal-poor population B. Most of the remaining stars seem to represent other, even more metal-poor populations of the bulge. The presence of multiple old populations indicates that the Milky Way bulge was initially formed through mergers.

February 25th Discussion


We're going to mix things up a bit and talk about really massive stars (not ~2 Msun, but instead >20 Msun!). The Wright et al. 2015 paper presents an updated compilation of O & B stars in Cyg OB2, one of the most massive nearby associations in our galaxy. Using this new catalog, the authors 1) construct an extinction map towards Cyg OB2, 2) estimate stellar masses and ages using isochrones, 3) infer the recent SFH, 4) derive the high-mass IMF slope, and 5) evaluate the total mass of the system. If we have time we can look through Weisz et al. 2015, where they measure the distribution of high-mass IMF slopes in 85 young M31 clusters from the PHAT survey.

The Massive Star Population of Cygnus OB2

Wright et al., 2015
arXiv PDF

We have compiled a significantly updated and comprehensive census of massive stars in the nearby Cygnus OB2 association by gathering and homogenising data from across the literature. The census contains 169 primary OB stars, including 52 O-type stars and 3 Wolf-Rayet stars. Spectral types and photometry are used to place the stars in a Hertzprung-Russell diagram, which is compared to both non-rotating and rotating stellar evolution models, from which stellar masses and ages are calculated. The star formation history and mass function of the association are assessed, and both are found to be heavily influenced by the evolution of the most massive stars to their end states. We find that the mass function of the most massive stars is consistent with a `universal' power-law slope of Gamma = 1.3. The age distribution inferred from stellar evolutionary models with rotation and the mass function suggest the majority of star formation occurred more or less continuously between 1 and 7 Myr ago, in agreement with studies of low- and intermediate mass stars in the association. We identify a nearby young pulsar and runaway O-type star that may have originated in Cyg OB2 and suggest that the association has already seen its first supernova. Finally we use the census and mass function to calculate the total mass of the association of 16500^+3800_-2800 Msun, at the low end, but consistent with, previous estimates of the total mass of Cyg OB2. Despite this Cyg OB2 is still one of the most massive groups of young stars known in our Galaxy making it a prime target for studies of star formation on the largest scales.


The High-Mass Stellar Initial Mass Function in M31 Clusters

Weisz et al., 2015
arXiv PDF


We have undertaken the largest systematic study of the high-mass stellar initial mass function (IMF) to date using the optical color-magnitude diagrams (CMDs) of 85 resolved, young (4 Myr < t < 25 Myr), intermediate mass star clusters (10^3-10^4 Msun), observed as part of the Panchromatic Hubble Andromeda Treasury (PHAT) program. We fit each cluster's CMD to measure its mass function (MF) slope for stars >2 Msun. For the ensemble of clusters, the distribution of stellar MF slopes is best described by Γ=+1.45+0.03−0.06 with a very small intrinsic scatter. The data also imply no significant dependencies of the MF slope on cluster age, mass, and size, providing direct observational evidence that the measured MF represents the IMF. This analysis implies that the high-mass IMF slope in M31 clusters is universal with a slope (Γ=+1.45+0.03−0.06) that is steeper than the canonical Kroupa (+1.30) and Salpeter (+1.35) values. Using our inference model on select Milky Way (MW) and LMC high-mass IMF studies from the literature, we find ΓMW∼+1.15±0.1 and ΓLMC∼+1.3±0.1, both with intrinsic scatter of ~0.3-0.4 dex. Thus, while the high-mass IMF in the Local Group may be universal, systematics in literature IMF studies preclude any definitive conclusions; homogenous investigations of the high-mass IMF in the local universe are needed to overcome this limitation. Consequently, the present study represents the most robust measurement of the high-mass IMF slope to date. We have grafted the M31 high-mass IMF slope onto widely used sub-solar mass Kroupa and Chabrier IMFs and show that commonly used UV- and Halpha-based star formation rates should be increased by a factor of ~1.3-1.5 and the number of stars with masses >8 Msun are ~25% fewer than expected for a Salpeter/Kroupa IMF.

February 13th Discussion


The pulsation modes, masses and evolution of luminous red giants
P. R. Wood
[ arXiv:1502.03137v1 | PDF File ]
The period-luminosity sequences and the multiple periods of luminous red giant stars are examined using the OGLE III catalogue of long-period variables in the Large Magellanic Cloud. It is shown that the period ratios in individual multimode stars are systematically different from the ratios of the periods at a given luminosity of different period-luminosity sequences. This leads to the conclusion that the masses of stars at the same luminosity on the different period-luminosity sequences are different. An evolutionary scenario is used to show that the masses of stars on adjacent sequences differ by about 16-26% at a given luminosity, with the shorter period sequence being more massive. The mass is also shown to vary across each sequence by a similar percentage, with the mass increasing to shorter periods. On one sequence, sequence B, the mass distribution is shown to be bimodal. It is shown that the small amplitude variables on sequences A', A and B pulsate in radial and nonradial modes of angular degree l=0, 1 and 2, with the l=1 mode being the most common. The stars on sequences C' and C are predominantly radial pulsators (l=0). Matching period ratios to pulsation models shows that the radial pulsation modes associated with sequences A', A, B, C' and C are the 4th, 3rd, 2nd and 1st overtones and the fundamental mode, respectively.

A higher-than-predicted measurement of iron opacity at solar interior temperatures

J.E. Bailey et al.

http://www.nature.com/nature/journal/v517/n7532/full/nature14048.html

Nearly a century ago it was recognized1 that radiation absorption by stellar matter controls the internal temperature profiles within stars. Laboratory opacity measurements, however, have never been performed at stellar interior conditions, introducing uncertainties in stellar models2, 3, 4, 5. A particular problem arose2, 3, 6, 7, 8 when refined photosphere spectral analysis9, 10 led to reductions of 30–50 per cent in the inferred amounts of carbon, nitrogen and oxygen in the Sun. Standard solar models11 using the revised element abundances disagree with helioseismic observations that determine the internal solar structure using acoustic oscillations. This could be resolved if the true mean opacity for the solar interior matter were roughly 15 per cent higher than predicted2, 3, 6, 7, 8, because increased opacity compensates for the decreased element abundances. Iron accounts for a quarter of the total opacity2, 12 at the solar radiation/convection zone boundary. Here we report measurements of wavelength-resolved iron opacity at electron temperatures of 1.9–2.3 million kelvin and electron densities of (0.7–4.0) × 1022 per cubic centimetre, conditions very similar to those in the solar region that affects the discrepancy the most: the radiation/convection zone boundary. The measured wavelength-dependent opacity is 30–400 per cent higher than predicted. This represents roughly half the change in the mean opacity needed to resolve the solar discrepancy, even though iron is only one of many elements that contribute to opacity.

February 11th Discussion


I would like to discuss a possible detection of a new, potentially still star-forming, Milky Way dSph on the other side of the Galactic center.

Clustered Cepheid Variables 90 kpc from the Galactic Center


http://adsabs.harvard.edu/abs/2015arXiv150201358C and http://adsabs.harvard.edu/abs/2009MNRAS.399L.118C
Chakrabarti, Sukanya; Saito, Roberto; Quillen, Alice; Gran, Felipe; Klein, Christopher; Blitz, Leo

Distant regions close to the plane of our Galaxy are largely unexplored by optical surveys as they are hidden by dust. We have used near-infrared data (that minimizes dust obscuration) from the ESO Public survey VISTA Variables of the Via Lactea (VVV) (Minniti et al. 2011; Saito et al. 2012; henceforth S12) to search for distant stars at low latitudes. We have discovered four Cepheid variables within an angular extent of one degree centered at Galactic longitude of $l = -27.4^\circ$ and Galactic latitude of $b = -1.08 ^\circ$. We use the tightly constrained period-luminosity relationship that these pulsating stars obey (Persson et al. 2004; Matsunaga et al. 2011) to derive distances. We infer an average distance to these Cepheid variables of 90 kpc. The Cepheid variables are highly clustered in angle (within one degree) and in distance (the standard deviation of the distances is 12 kpc). They are at an average distance of $\sim 2~\rm kpc$ from the plane and their maximum projected separation is $\sim 1~ \rm kpc$. These young ($\sim$ 100 Myr old), pulsating stars (Bono et al. 2005) are unexpected at such large distances from the Galactic disk, which terminates at $\sim$ 15 kpc (Minniti et al. 2011). The highly clustered nature in distance and angle of the Cepheid variables suggests that the stars may be associated with a dwarf galaxy, one that was earlier predicted by a dynamical analysis (Chakrabarti \& Blitz 2009).


January 28th Discussion


We will discuss some recent controversy about formation scenarios of the MW stellar halo.

The Progenitors of the Milky Way Stellar Halo: Big Bricks Favoured over Little Bricks

http://arxiv.org/abs/1501.02806
A. J. Deason (UCSC), V. Belokurov (Cambridge), D. R. Weisz (UW)
(Submitted on 12 Jan 2015)
  • We present a census of blue horizontal branch (BHB) and blue straggler (BS) stars belonging to dwarf galaxies and globular clusters, and compare these counts to that of the Milky Way stellar halo. We find, in agreement with earlier studies, that the ratio of BS-to-BHB stars in these satellite populations is dependent on stellar mass. Dwarf galaxies show an increasing BS-to-BHB ratio with luminosity. In contrast, globular clusters display the reverse trend, with N_BS/N_BHB (< 1) decreasing with luminosity. The faintest (L < 10^5 L_Sun) dwarfs have similar numbers of BS and BHB stars (N_BS/N_BHB ~ 1), whereas more massive dwarfs tend to be dominated by BS stars (N_BS/N_BHB ~ 2-40). We find that the BS-to-BHB ratio in the stellar halo is relatively high (N_BS/N_BHB ~ 5-6), and thus inconsistent with the low ratios found in both ultra-faint dwarfs and globular clusters. Our results favour more massive dwarfs as the dominant "building blocks" of the stellar halo, in good agreement with current predictions from LambdaCDM models.

The Frequency of Field Blue-Straggler Stars in the Thick Disk and Halo System of the Galaxy

http://arxiv.org/abs/1501.03559
Rafael M. Santucci, Vinicius M. Placco, Silvia Rossi,Timothy C. Beers, Henrique M. Reggiani, Young Sun Lee, Xiang-Xiang Xue, Daniela Carollo
(Submitted on 15 Jan 2015)
  • We present an analysis of a new, large sample of field blue-straggler stars (BSSs) in the thick disk and halo system of the Galaxy, based on stellar spectra obtained during the Sloan Digital Sky Survey (SDSS) and the Sloan Extension for Galactic Understanding and Exploration (SEGUE). Using estimates of stellar atmospheric parameters obtained from application of the SEGUE Stellar Parameter Pipeline, we obtain a sample of some 8000 BSSs, which are considered along with a previously selected sample of some 4800 blue horizontal-branch (BHB) stars. We derive the ratio of BSSs to BHB stars, FBSS/BHB, as a function of Galactocentric distance and distance from the Galactic plane. The maximum value found for FBSS/BHB is 4.0 in the thick disk (at 3 kpc < |Z| < 4 kpc), declining to on the order of 1.52.0 in the inner-halo region; this ratio continues to decline to 1.0 in the outer-halo region. We associate a minority of field BSSs with a likely extragalactic origin; at least 5% of the BSS sample exhibit radial velocities, positions, and distances commensurate with membership in the Sagittarius Stream.




January 26th Discussion


arXiv:1501.05651 [pdf]
A spin-down clock for cool stars from observations of a 2.5-billion-year-old clusterSoren Meibom, Sydney A. Barnes, Imants Platais, Ronald L. Gilliland, David W. Latham, Robert D. Mathieu
Comments: 24 pages, 12 figures, 1 table, published in Nature January 2015Subjects: Solar and Stellar Astrophysics (astro-ph.SR)
The ages of the most common stars - low-mass (cool) stars like the Sun, and smaller - are difficult to derive because traditional dating methods use stellar properties that either change little as the stars age or are hard to measure. The rotation rates of all cool stars decrease substantially with time as the stars steadily lose their angular momenta. If properly calibrated, rotation therefore can act as a reliable determinant of their ages based on the method of gyrochronology. To calibrate gyrochronology, the relationship between rotation period and age must be determined for cool stars of different masses, which is best accomplished with rotation period measurements for stars in clusters with well-known ages. Hitherto, such measurements have been possible only in clusters with ages of less than about one billion years, and gyrochronology ages for older stars have been inferred from model predictions. Here we report rotation period measurements for 30 cool stars in the 2.5-billion-year-old cluster NGC 6819. The periods reveal a well-defined relationship between rotation period and stellar mass at the cluster age, suggesting that ages with a precision of order 10 per cent can be derived for large numbers of cool Galactic field stars.

We may also briefly touch on the SMC's star formation history (http://arxiv.org/abs/1501.05347).

January 23rd Discussion


We'll start with a discussion of the recent Holtzman et al. APOGEE paper:
http://arxiv.org/abs/1501.04110

Uncertainties in The Interstellar Extinction Curve and the Cepheid Distance to M101
Authors: David M. Nataf
Categories: astro-ph.GA astro-ph.CO
Comments: 6 pages, 2 figures, 4 tables. Accepted for publication in Monthly
Notices of the Royal Astronomical Society Main Journal
\\
I revisit the Cepheid-distance determination to the nearby spiral galaxy M101 (Pinwheel Galaxy) of Shappee & Stanek (2011), in light of several recent
investigations questioning the shape of the interstellar extinction curve at $\lambda \approx 8,000$ \AA (i.e. I-band). I find that the relatively steep
extinction ratio $A_{I}/E(V-I)=1.1450$ (Fitzpatrick & Massa 2007) is slightly favoured relative to $A_{I}/E(V-I)=1.2899$ (Fitzpatrick 1999) and significantly
favoured relative the historically canonical value of $A_{I}/E(V-I)=1.4695$ (Cardelli et al. 1989). The steeper extinction curves, with lower values of
$A_{I}/E(V-I)$, yield fits with reduced scatter, metallicity-dependences to the dereddened Cepheid luminosities that are closer to values inferred in the local
group, and that are less sensitive to the choice of reddening cut imposed in the sample selection. The increase in distance modulus to M101 when using the
preferred extinction curve is ${\Delta}{\mu} \sim 0.06$ mag, resulting in an estimate of the distance modulus to M101 relative to the LMC of $
{\Delta}\mu_{\rm{LMC}} \approx 10.72 \pm 0.03$ (stat). The best-fit metallicity-dependence is $dM_{I}/d\rm{[O/H]} \approx (-0.38 \pm 0.14$ (stat))
mag dex$^{-1}$.
\\ ( http://arxiv.org/abs/1501.05311 , 252kb)

Confronting uncertainties in stellar physics: calibrating convective overshooting with eclipsing binaries
Authors: Richard J. Stancliffe (1), Luca Fossati (1), Jean-Claude Passy (1),
Fabian R. N. Schneider (1 and 2) ((1) AIfA, (2) Oxford)
Categories: astro-ph.SR
Comments: 9 pages, 15 figures. Accepted for publication in A&A (20/01/2015)
\\
As part of a larger program aimed at better quantifying the uncertainties in stellar computations, we attempt to calibrate the extent of convective
overshooting in low to intermediate mass stars by means of eclipsing binary systems. We model 12 such systems, with component masses between 1.3 and 6.2
solar masses, using the detailed binary stellar evolution code STARS, producing grids of models in both metallicity and overshooting parameter. From these, we
determine the best fit parameters for each of our systems. For three systems, none of our models produce a satisfactory fit. For the remaining systems, no
single value for the convective overshooting parameter fits all the systems, but most of our systems can be well described with an overshooting parameter
between 0.09 and 0.15, corresponding to an extension of the mixed region above the core of about 0.1-0.3 pressure scale heights. Of the nine systems where we
are able to obtain a good fit, seven can be reasonably well fit with a single parameter of 0.15. We find no evidence for a trend of the extent of
overshooting with either mass or metallicity, though the data set is of limited size. We repeat our calculations with a second evolution code, MESA, and we
find general agreement between the two codes. For the extension of the mixed region above the convective core required by the MESA models is about 0.15-0.4
pressure scale heights. For the system EI Cep, we find that MESA gives an overshooting region that is larger than the STARS one by about 0.1 pressure
scale heights for the primary, while for the secondary the difference is only 0.05 pressure scale heights.
\\ ( http://arxiv.org/abs/1501.05322 , 163kb)

Spectro-photometric distances to stars: a general-purpose Bayesian approach
Authors: Bas\'ilio X. Santiago, Doroth\'ee E. Brauer, Friedrich Anders,
Cristina Chiappini, L\'eo Girardi, Helio J. Rocha-Pinto, Eduardo Balbinot,
Luiz N. da Costa, Marcio A.G. Maia, Mathias Schultheis, Matthias Steinmetz,
Andrea Miglio, Josefina Montalb\'an, Donald P. Schneider, Timothy C. Beers,
Peter M. Frinchaboy, Young Sun Lee, and Gail Zasowski
Categories: astro-ph.IM astro-ph.GA astro-ph.SR
Comments: 20 pages, 11 figures
\\
We have developed a procedure that estimates distances to stars using measured spectroscopic and photometric quantities. It employs a Bayesian
approach to build the probability distribution function over stellar evolutionary models given the data, delivering estimates of expected distance
for each star individually. Our method provides several alternative distance estimates for each star in the output, along with their associated
uncertainties. The code was first tested on simulations, successfully recovering input distances to mock stars with errors that scale with the
uncertainties in the adopted spectro-photometric parameters, as expected. The code was then validated by comparing our distance estimates to parallax
measurements from the Hipparcos mission for nearby stars (< 60 pc), to asteroseismic distances of CoRoT red giant stars, and to known distances of
well-studied open and globular clusters. The photometric data of these reference samples cover both the optical and near infra-red wavelengths. The
spectroscopic parameters are also based on spectra taken at various wavelengths, with varying spectral coverage and resolution: the Radial Velocity
Experiment, the Sloan Digital Sky Survey programs SEGUE and APOGEE, and the ESO HARPS instrument. For Hipparcos and CoRoT samples, the typical random distance
scatter is 20% or less, both for the nearby and farther data. There is a trend towards underestimating the distances by < 10%. The comparison to star clusters
from SEGUE and APOGEE has led to systematic differences < 5% for most cluster stars although with significant scatter. Finally, we tested our distances
against those previously determined for a high quality sample of giant stars from the RAVE survey, again finding a reasonable agreement, with only a small
systematic trend. Efforts are underway to provide our code to the community by running it on a public server.
http://arxiv.org/abs/1501.05500


January 21st Discussion

arXiv:1501.04846 [pdf, other]
Active star formation at intermediate Galactic latitude: the case of IRAS 06345-3023J. L. Yun, P. M. Palmeirim

Comments: 6 pages, 5 figures, 2 tablesSubjects: Solar and Stellar Astrophysics (astro-ph.SR); Astrophysics of Galaxies (astro-ph.GA)
We report the discovery of a small aggregate of young stars seen in high-resolution, deep near-infrared ($JHK_S$) images towards IRAS 06345-3023 in the outer Galaxy and well below the mid-plane of the Galactic disc. The group of young stars is likely to be composed of low-mass stars, mostly Class I young stellar objects. The stars are seen towards a molecular cloud whose CO map peaks at the location of the IRAS source. The near-infrared images reveal, additionally, the presence of nebular emission with rich morphological features, including arcs in the vicinity of embedded stars, wisps and bright rims of a butterfly-shaped dark cloud. The location of this molecular cloud as a new star formation site well below the Galactic plane in the outer Galaxy indicates that active star formation is taking place at vertical distances larger than those typical of the (thin) disc.

Formation of In Situ Stellar Haloes in Milky Way-Mass Galaxies
Authors: Andrew P. Cooper (1), Owen H. Parry (2), Ben Lowing (1), Shaun Cole
(1) and Carlos Frenk (1) ((1) ICC Durham, (2) University of Maryland)
Categories: astro-ph.GA
Comments: 12 pages, 9 figures, submitted to MNRAS
Report-no: DURAST/2015/0001
\\
We study the formation of stellar haloes in three Milky Way-mass galaxies using cosmological SPH simulations, focusing on the subset of halo stars that
form in situ, as opposed to those accreted from satellites. In situ stars in our simulations dominate the stellar halo out to 20 kpc and account for 30 - 40
per cent of its total mass. We separate in situ halo stars into three straightforward, physically distinct categories according to their origin:
stars scattered from the disc of the main galaxy ("heated disc"), stars formed from gas smoothly accreted onto the halo ("smooth"-gas) and stars formed in
streams of gas stripped from infalling satellites ("stripped"-gas). We find that most belong to this latter category. Those originating in smooth gas
outside the disc tend to form at the same time and place as the stripped-gas population, suggesting that their formation is associated with the same
gas-rich accretion events. The scattered disc star contribution is negligible overall but significant in the Solar neighbourhood, where ~90 per cent of stars
on eccentric orbits once belonged to the disc. However, the distinction between halo and thick disc in this region is highly ambiguous. The chemical and
kinematic properties of the different components are very similar at the present day, but the global properties of the in situ halo differ substantially
between the three galaxies in our study. We conclude that, in our simulations, the hierarchical buildup of structure is the driving force behind not only the
accreted stellar halo, but also those halo stars formed in-situ.
\\ ( http://arxiv.org/abs/1501.04630 , 1346kb)

Integrated Light Chemical Tagging Analyses of Seven M31 Outer Halo
Globular Clusters from the Pan-Andromeda Archaeological Survey
Authors: Charli M. Sakari, Kim A. Venn, Dougal Mackey, Matthew D. Shetrone, Aaron Dotter, Annette M.N. Ferguson, and Avon Huxor
Categories: astro-ph.GA
Comments: Accepted for publication in MNRAS
\\
Detailed chemical abundances are presented for seven M31 outer halo globular clusters (with projected distances from M31 greater than 30 kpc), as derived
from high resolution integrated light spectra taken with the Hobby Eberly Telescope. Five of these clusters were recently discovered in the Pan-Andromeda
Archaeological Survey (PAndAS)---this paper presents the first determinations of integrated Fe, Na, Mg, Ca, Ti, Ni, Ba, and Eu abundances for these clusters.
Four of the target clusters (PA06, PA53, PA54, and PA56) are metal-poor ([Fe/H] < -1.5), alpha-enhanced (though they are possibly less alpha-enhanced than
Milky Way stars at the 1 sigma level), and show signs of star-to-star Na and Mg variations. The other three globular clusters (H10, H23, and PA17) are more
metal rich, with metallicities ranging from [Fe/H] = -1.4 to -0.9. While H23 is chemically similar to Milky Way field stars, Milky Way globular clusters, and
other M31 clusters, H10 and PA17 have moderately low [Ca/Fe], compared to Milky Way field stars and clusters. Additionally, PA17's high [Mg/Ca] and [Ba/Eu]
ratios are distinct from Milky Way stars, and are in better agreement with the stars and clusters in the Large Magellanic Cloud (LMC). None of the clusters
studied here can be conclusively linked to any of the identified streams from PAndAS; however, based on their locations, kinematics, metallicities, and
detailed abundances, the most metal-rich PAndAS clusters H23 and PA17 may be associated with the progenitor of the Giant Stellar Stream, H10 may be
associated with the SW Cloud, and PA53 and PA56 may be associated with the Eastern Cloud.
\\ ( http://arxiv.org/abs/1501.04626 , 1327kb)

January 20th Discussion

Bayesian Ages for Early-Type Stars from Isochrones Including Rotation, and a Possible Old Age for the Hyades

Timothy D. Brandt, Chelsea X. Huang


(Submitted on 19 Jan 2015)

http://arxiv.org/abs/1501.04404


  • We combine recently computed models of stellar evolution using a new treatment of rotation with a Bayesian statistical framework to constrain the ages and other properties of early-type stars. We find good agreement for early-type stars and clusters with known young ages, including beta Pictoris and the Pleiades. However, we derive a slightly older age for the Ursa Majoris moving group (600+/-100 Myr compared to 500+/-100 Myr), and a much older age for the Hyades open cluster (950+/-100 Myr compared to 625+/-50 Myr). These older ages result from both the increase in main-sequence lifetime with stellar rotation and from the fact that rotating models near the main-sequence turnoff are more luminous, overlapping with slightly more massive (and shorter-lived) nonrotating ones. The dramatically older age inferred for the Hyades requires a major reevaluation either of the cluster age or of the rotating stellar models. Our method uses a large grid of nonrotating models to interpolate between a much sparser rotating grid, and also includes a detailed calculation of synthetic magnitudes as a function of orientation. We provide a web interface at www.bayesianstellarparameters.info, where the results of our analysis may be downloaded for individual early-type (B-V < 0.25) Hipparcos stars. The web interface accepts user-supplied parameters for a Gaussian metallicity prior and returns posterior probability distributions on mass, age, and orientation.






Abundances, Stellar Parameters, and Spectra From the SDSS-III/APOGEE Survey


Jon A. Holtzman, Matthew Shetrone, Jennifer A. Johnson, Carlos Allende Prieto, Friedrich Anders, Brett Andrews, Timothy C. Beers, Dmitry Bizyaev, Michael R. Blanton, Jo Bovy, Ricardo Carrera, Katia Cunha, Daniel J. Eisenstein, Diane Feuillet, Peter M. Frinchaboy, Jessica Galbraith-Frew, Ana E. Garcia Perez, D. Anibal Garcia Hernandez, Sten Hasselquist, Michael R. Hayden, Fred R. Hearty, Inese Ivans, Steven R. Majewski, Sarah Martell, Szabolcs Meszaros, Demitri Muna, David L. Nidever, Duy Cuong Nguyen, Robert W. O'Connell, Kaike Pan, Marc Pinsonneault, Annie C. Robin, Ricardo P. Schiavon, Neville Shane, Jennifer Sobeck, Verne V. Smith, Nicholas Troup, David H. Weinberg, John C. Wilson, W. M. Wood-Vasey, Olga Zamora, Gail Zasowski


(Submitted on 16 Jan 2015)

http://arxiv.org/abs/1501.04110


  • The SDSS-III/APOGEE survey operated from 2011-2014 using the APOGEE spectrograph, which collects high-resolution (R~22,500), near-IR (1.51-1.70 microns) spectra with a multiplexing (300 fiber-fed objects) capability. We describe the survey data products that are publicly available, which include catalogs with radial velocity, stellar parameters, and 15 elemental abundances for over 150,000 stars, as well as the more than 500,000 spectra from which these quantities are derived. Calibration relations for the stellar parameters (Teff, log g, [M/H], [alpha/M]) and abundances (C, N, O, Na, Mg, Al, Si, S, K, Ca, Ti, V, Mn, Fe, Ni) are presented and discussed. The internal scatter of the abundances within clusters indicates that abundance precision is generally between 0.05 and 0.09 dex across a broad temperature range; within more limited ranges and at high S/N, it is smaller for some elemental abundances. We assess the accuracy of the abundances using comparison of mean cluster metallicities with literature values, APOGEE observations of the solar spectrum and of Arcturus, comparison of individual star abundances with other measurements, and consideration of the locus of derived parameters and abundances of the entire sample, and find that it is challenging to determine the absolute abundance scale; external accuracy may be good to 0.1-0.2 dex. Uncertainties may be larger at cooler temperatures (Teff<4000K). Access to the public data release and data products is described, and some guidance for using the data products is provided.



January 16th Discussion

The Frequency of Field Blue-Straggler Stars in the Thick Disk and Halo System of the Galaxy

arXiv:1501.03559 [pdf, ps, other]
Rafael M. Santucci, Vinicius M. Placco, Silvia Rossi, Timothy C. Beers, Henrique M. Reggiani, Young Sun Lee, Xiang-Xiang Xue, Daniela Carollo
Comments: 13 pages, 19 figures; accepted for publication in ApJ
Subjects: Solar and Stellar Astrophysics (astro-ph.SR); Astrophysics of Galaxies (astro-ph.GA)
We present an analysis of a new, large sample of field blue-straggler stars (BSSs) in the thick disk and halo system of the Galaxy, based on stellar spectra obtained during the Sloan Digital Sky Survey (SDSS) and the Sloan Extension for Galactic Understanding and Exploration (SEGUE). Using estimates of stellar atmospheric parameters obtained from application of the SEGUE Stellar Parameter Pipeline, we obtain a sample of some 8000 BSSs, which are considered along with a previously selected sample of some 4800 blue horizontal-branch (BHB) stars. We derive the ratio of BSSs to BHB stars, F$_{\rm BSS/BHB}$, as a function of Galactocentric distance and distance from the Galactic plane. The maximum value found for F$_{\rm BSS/BHB}$ is $\sim~$4.0 in the thick disk (at 3 kpc $<$ $|$Z$|$ $<$ 4 kpc), declining to on the order of $\sim~1.5-2.0$ in the inner-halo region; this ratio continues to decline to $\sim~$1.0 in the outer-halo region. We associate a minority of field BSSs with a likely extragalactic origin; at least 5$\%$ of the BSS sample exhibit radial velocities, positions, and distances commensurate with membership in the Sagittarius Stream.

The Ages of Early-Type Stars: Strömgren Photometric Methods Calibrated, Validated, Tested, and Applied to Hosts and Prospective Hosts of Directly Imaged Exoplanets

Trevor J. David, Lynne A. Hillenbrand


  • Age determination is undertaken for nearby early-type (BAF) stars, which constitute attractive targets for high-contrast debris disk and planet imaging surveys. Our analysis sequence consists of: acquisition of uvby{\beta} photometry from catalogs, correction for the effects of extinction, interpolation of the photometry onto model atmosphere grids from which atmospheric parameters are determined, and finally, comparison to the theoretical isochrones from pre-main sequence through post-main sequence stellar evolution models, accounting for the effects of stellar rotation. We calibrate and validate our methods at the atmospheric parameter stage by comparing our results to fundamentally determined Teff and log g values. We validate and test our methods at the evolutionary model stage by comparing our results on ages to the accepted ages of several benchmark open clusters (IC 2602, {\alpha} Persei, Pleiades, Hyades). Finally, we apply our methods to estimate stellar ages for 3493 field stars, including several with directly imaged exoplanet candidates.

arXiv:1501.03161 [pdf, ps, other]Potassium: a new actor on the globular cluster chemical evolution stage. The case of NGC 2808A. Mucciarelli, M. Bellazzini, T. Merle, B. Plez, E. Dalessandro, R. Ibata
Comments: Accepted for publication by ApJ, 5 pages, 3 figures
Subjects: Solar and Stellar Astrophysics (astro-ph.SR)
We derive [K/Fe] abundance ratios for 119 stars in the globular cluster NGC 2808, all of them having O, Na, Mg and Al abundances homogeneously measured in previous works. We detect an intrinsic star-to-star spread in the Potassium abundance. Moreover [K/Fe] abundance ratios display statistically significant correlations with [Na/Fe] and [Al/Fe], and anti-correlations with [O/Fe] and [Mg/Fe]. All the four Mg deficient stars ([Mg/Fe]<0.0) discovered so far in NGC 2808 are enriched in K by ~0.3 dex with respect to those with normal [Mg/Fe]. NGC 2808 is the second globular cluster, after NGC 2419, where a clear Mg-K anti-correlation is detected, albeit of weaker amplitude. The simultaneous correlation/anti-correlation of [K/Fe] with all the light elements usually involved in the chemical anomalies observed in globular cluster stars, strongly support the idea that these abundance patterns are due to the same self-enrichment mechanism that produces Na-O and Mg-Al anti-correlations. This finding suggests that detectable spreads in K abundances may be typical in the massive globular clusters where the self-enrichment processes are observed to produce their most extreme manifestations.

January 14th Discussion


Hubble Space Telescope observations of the Kepler-field cluster NGC 6819. I. The bottom of the white dwarf cooling sequence

arXiv:1501.02953 [pdf, ps, other]
L. R. Bedin (1), M. Salaris (2), J. Anderson (3), S. Cassisi (4), A. P. Milone (5), G. Piotto (6), I. R. King (7), P. Bergeron (8). ((1) INAF-OAPD, (2) J.M.Univ.Liverpool, (3) STScI, (4) INAF-OATE, (5) RSAA-ANU-MSO, (6) Univ.PD, (7) Univ. of Washington Seattle, and (8) Univ. of Montreal)
Comments: 11 pages, 7 figures (4 at low resolution). Accepted for publication in MNRAS on January 12, 2015
Subjects: Solar and Stellar Astrophysics (astro-ph.SR)
We use Hubble Space Telescope (HST) to reach the end of the white dwarf (WD) cooling sequence (CS) in the solar-metallicity open cluster NGC 6819. Our photometry and completeness tests show a sharp drop in the number of WDs along the CS at magnitudes fainter than mF606W = 26.050+/- 0.075. This implies an age of 2.25+/-0.20 Gyr, consistent with the age of 2.25+/-0.30 Gyr obtained from fits to the main-sequence turn-off. The use of different WD cooling models and initial-final-mass relations have a minor impact the WD age estimate, at the level of ~0.1 Gyr. As an important by-product of this investigation we also release, in electronic format, both the catalogue of all the detected sources and the atlases of the region (in two filters). Indeed, this patch of sky studied by HST (of size ~70 arcmin sq.) is entirely within the main Kepler-mission field, so the high-resolution images and deep catalogues will be particularly useful.

How to Cons-Train Your M Dwarf: measuring effective temperature, bolometric luminosity, mass, and radius

arXiv:1501.01635
Andrew W. Mann, Gregory A. Feiden, Eric Gaidos, Tabetha Boyajian
Comments: 37 pages, 24 figures, 7 tables. Submitted to ApJ
Subjects: Solar and Stellar Astrophysics (astro-ph.SR)
Precise and accurate parameters for late-type (late K and M) dwarf stars are important for proper characterization of any planets they host, but studies have been hampered by these stars' complex spectra and dissimilarity to the Sun. We exploited a calibrated method of spectroscopic effective temperature (Teff) estimation and the Stefan-Boltzmann law to determine radii with an accuracy of 2-5% and expand the sample to 161 nearby K7-M7 dwarf stars covering a wider range of Teff and metallicity. We developed improved relations between Teff, radius, and luminosity, as well as between Teff and color. Our Teff-radius relation depends strongly on [Fe/H], as predicted by theory. We derived a relation between absolute KS magnitude and radius that is accurate to better than 3%. We derived bolometric correction to the VRCICgrizJHKS and Gaia passbands as a function of color, accurate to 1-3%. We confronted the reliability of predictions from Dartmouth stellar evolution models using a Monte Carlo Markov Chain (MCMC) to find the values of unobservable model parameters (mass, age) that best reproduce the observed effective temperature and bolometric flux while satisfying constraints on distance and metallicity as Bayesian priors. Theoretical masses were related to KS-band luminosities and compared to a relation developed from astrometric binaries. Model masses agree well with the empirical relation, with a notable offset at M⋆>0.55M⊙. The best-agreement models over-predict stellar Teffs by an average of 2.2% and under-predict stellar radii by 4.6%, similar to differences with values from low-mass eclipsing binaries. These differences are not correlated with metallicity, mass, or activity, suggesting issues with the underlying model assumptions e.g., opacities, helium mass fraction, or convective mixing length.



MHP picks:
[7] arXiv:1501.02806 [pdf, ps, other]
The Progenitors of the Milky Way Stellar Halo: Big Bricks Favoured over Little Bricks
A. J. Deason (UCSC), V. Belokurov (Cambridge), D. R. Weisz (UW)
Comments: 5 pages, 2 figures. Accepted for publication in MNRAS Letters
Subjects: Astrophysics of Galaxies (astro-ph.GA)
We present a census of blue horizontal branch (BHB) and blue straggler (BS) stars belonging to dwarf galaxies and globular clusters, and compare these counts to that of the Milky Way stellar halo. We find, in agreement with earlier studies, that the ratio of BS-to-BHB stars in these satellite populations is dependent on stellar mass. Dwarf galaxies show an increasing BS-to-BHB ratio with luminosity. In contrast, globular clusters display the reverse trend, with N_BS/N_BHB (< 1) decreasing with luminosity. The faintest (L < 10^5 L_Sun) dwarfs have similar numbers of BS and BHB stars (N_BS/N_BHB ~ 1), whereas more massive dwarfs tend to be dominated by BS stars (N_BS/N_BHB ~ 2-40). We find that the BS-to-BHB ratio in the stellar halo is relatively high (N_BS/N_BHB ~ 5-6), and thus inconsistent with the low ratios found in both ultra-faint dwarfs and globular clusters. Our results favour more massive dwarfs as the dominant "building blocks" of the stellar halo, in good agreement with current predictions from LambdaCDM models.
[38] arXiv:1501.03062 [pdf, ps, other]
Spectroscopic analysis of metal-poor stars from LAMOST: early results
Hai-Ning Li, Gang Zhao, Norbert Christlieb, Liang Wang, Wei Wang, Yong Zhang, Yonghui Hou, Hailong Yuan
Comments: 14 pages, 10 figures, 5 tables, published in ApJ
Journal-ref: 2015, ApJ, 798, 110
Subjects: Solar and Stellar Astrophysics (astro-ph.SR)
We report on early results from a pilot program searching for metal-poor stars with LAMOST and follow-up high-resolution observation acquired with the MIKE spectrograph attached to the Magellan~II telescope. We performed detailed abundance analysis for eight objects with iron abundances [Fe/H] < -2.0, including five extremely metal-poor (EMP; [Fe/H] < -3.0) stars with two having [Fe/H] < -3.5. Among these objects, three are newly discovered EMP stars, one of which is confirmed for the first time with high-resolution spectral observations. Three program stars are regarded as carbon-enhanced metal-poor (CEMP) stars, including two stars with no enhancement in their neutron-capture elements, which thus possibly belong to the class of CEMP-no stars; one of these objects also exhibits significant enhancement in nitrogen, and is thus a potential carbon and nitrogen-enhanced metal-poor star. The [X/Fe] ratios of the sample stars generally agree with those reported in the literature for other metal-poor stars in the same [Fe/H] range. We also compared the abundance patterns of individual program stars with the average abundance pattern of metal-poor stars, and find only one chemically peculiar object with abundances of at least two elements (other than C and N) showing deviations larger than 0.5dex. The distribution of [Sr/Ba] versus [Ba/H] agrees that an additional nucleosynthesis mechanism is needed aside from a single r-process. Two program stars with extremely low abundances of Sr and Ba support the prospect that both main and weak r-process may have operated during the early phase of Galactic chemical evolution. The distribution of [C/N] shows that there are two groups of carbon-normal giants with different degrees of mixing. However, it is difficult to explain the observed behavior of the [C/N] of the nitrogen-enhanced unevolved stars based on current data.
[41] arXiv:1501.03101 [pdf, other]
The formation of the solar system
S. Pfalzner, M. B. Davies, M. Gounelle, A. Johansen, C. Muenker, P. Lacerda, S. Portegies Zwart, L. Testi, M. Trieloff, D. Veras
Comments: 36 pages, 7 figures, invited review in Physica Scripta
Subjects: Astrophysics of Galaxies (astro-ph.GA); Earth and Planetary Astrophysics (astro-ph.EP); Solar and Stellar Astrophysics (astro-ph.SR)
The solar system started to form about 4.56 Gyr ago and despite the long intervening time span, there still exist several clues about its formation. The three major sources for this information are meteorites, the present solar system structure and the planet-forming systems around young stars. In this introduction we give an overview of the current understanding of the solar system formation from all these different research fields. This includes the question of the lifetime of the solar protoplanetary disc, the different stages of planet formation, their duration, and their relative importance. We consider whether meteorite evidence and observations of protoplanetary discs point in the same direction. This will tell us whether our solar system had a typical formation history or an exceptional one. There are also many indications that the solar system formed as part of a star cluster. Here we examine the types of cluster the Sun could have formed in, especially whether its stellar density was at any stage high enough to influence the properties of today's solar system. The likelihood of identifying siblings of the Sun is discussed. Finally, the possible dynamical evolution of the solar system since its formation and its future are considered.