In "A Bayesian Approach to Locating the Red Giant Branch Tip Magnitude (Part I)," a new technique was introduced for obtaining distances using the tip of the red giant branch (TRGB) standard candle. ...Here we describe a useful complement to the technique with the potential to further reduce the uncertainty in our distance measurements by incorporating a matched-filter weighting scheme into the model likelihood calculations. In this scheme, stars are weighted according to their probability of being true object members. We then re-test our modified algorithm using random-realization artificial data to verify the validity of the generated posterior probability distributions (PPDs) and proceed to apply the algorithm to the satellite system of M31, culminating in a three-dimensional view of the system. Finally, a preliminary investigation into the satellite density distribution within the halo is made using the obtained distance distributions. For simplicity, this investigation assumes a single power law for the density as a function of radius, with the slope of this power law examined for several subsets of the entire satellite sample.
We use data from the Radial Velocity Experiment (RAVE) and the Tycho-Gaia astrometric solution (TGAS) catalogue to compute the velocity fields yielded by the radial (V-R), azimuthal (V-phi), and ...vertical (V-z) components of associated Galactocentric velocity. We search in particular for variation in all three velocity components with distance above and below the disc midplane, as well as how each component of V-z (line-of-sight and tangential velocity projections) modifies the obtained vertical structure. To study the dependence of velocity on proper motion and distance, we use two main samples: a RAVE sample including proper motions from the Tycho-2, PPMXL, and UCAC4 catalogues, and a RAVE-TGAS sample with inferred distances and proper motions from the TGAS and UCAC5 catalogues. In both samples, we identify asymmetries in V-R and V-z. Below the plane, we find the largest radial gradient to be partial derivative V-R/partial derivative R = -7.01 +/- 0.61 km s(-1) kpc(-1), in agreement with recent studies. Above the plane, we find a similar gradient with partial derivative V-R/partial derivative R = -9.42 +/- 1.77 km s(-1) kpc(-1). By comparing our results with previous studies, we find that the structure in V-z is strongly dependent on the adopted proper motions. Using the Galaxia Milky Way model, we demonstrate that distance uncertainties can create artificial wave-like patterns. In contrast to previous suggestions of a breathing mode seen in RAVE data, our results support a combination of bending and breathing modes, likely generated by a combination of external or internal and external mechanisms.
We undertake an investigation into the spatial structure of the M31 satellite system utilizing the distance distributions presented in a previous publication. These distances make use of the unique ...combination of depth and spatial coverage of the Pan-Andromeda Archaeological Survey to provide a large, homogeneous sample consisting of 27 of M31's satellites, as well as M31 itself. We find that the satellite distribution, when viewed as a whole, is no more planar than one would expect from a random distribution of equal size. A disk consisting of 15 of the satellites is however found to be highly significant, and strikingly thin, with an rms thickness of just (ProQuest: Formulae and/or non-USASCII text omitted) kpc. This disk is oriented approximately edge-on with respect to the Milky Way and almost perpendicular to the Milky Way disk. It is also roughly orthogonal to the disk-like structure regularly reported for the Milky Way satellite system and in close alignment with M31's Giant Stellar Stream. A similar analysis of the asymmetry of the M31 satellite distribution finds that it is also significantly larger than one would expect from a random distribution. In particular, it is remarkable that 20 of the 27 satellites most likely lie on the Milky Way side of the galaxy, with the asymmetry being most pronounced within the satellite subset forming the aforementioned disk. This lopsidedness is all the more intriguing in light of the apparent orthogonality observed between the satellite disk structures of the Milky Way and M31.
The RAdial Velocity Experiment survey, combined with proper motions and distance estimates, can be used to study in detail stellar kinematics in the extended solar neighbourhood (solar suburb). Using ...72 365 red-clump stars, we examine the mean velocity components in 3D between 6 < R < 10 kpc and −2 < Z < 2 kpc, concentrating on north-south differences. Simple parametric fits to the (R, Z) trends for V
φ and the velocity dispersions are presented. We confirm the recently discovered gradient in mean Galactocentric radial velocity, V
R, finding that the gradient is marked below the plane (δ〈V
R〉/δR = −8 km s−1 kpc−1 for Z < 0, vanishing to zero above the plane), with a Z gradient thus also present. The vertical velocity, V
Z
, also shows clear, large-amplitude (|V
Z
| = 17 km s−1) structure, with indications of a rarefaction-compression pattern, suggestive of wave-like behaviour. We perform a rigorous error analysis, tracing sources of both systematic and random errors. We confirm the north-south differences in V
R and V
Z
along the line of sight, with the V
R estimated independent of the proper motions. The complex three-dimensional structure of velocity space presents challenges for future modelling of the Galactic disc, with the Galactic bar, spiral arms and excitation of wave-like structures all probably playing a role.
We have performed integral field spectroscopy of the planetary nebulae Hen 3-1333 (PNG332.9−09.9) and Hen 2-113 (PNG321.0+03.9), which are unusual in exhibiting dual-dust chemistry and multipolar ...lobes but also ionized by late-type WC 10 central stars. The spatially resolved velocity distributions of the Hα emission line were used to determine their primary orientations. The integrated Hα emission profiles indicate that Hen 3-1333 and Hen 2-113 expand with velocities of ∼32 and 23 km s−1, respectively. The Hubble Space Telescope observations suggest that these planetary nebulae have two pairs of tenuous lobes extending upwardly from their bright compact cores. From three-dimensional geometric models, the primary lobes of Hen 3-1333 and Hen 2-113 were found to have inclination angles of about −30° and 40° relative to the line of sight, and position angles of −15° and 65° measured east of north in the equatorial coordinate system, respectively.
We report new constraints on the local escape speed of our Galaxy. Our analysis is based on a sample of high-velocity stars from the RAVE survey and two previously published data sets. We use ...cosmological simulations of disc galaxy formation to motivate our assumptions on the shape of the velocity distribution, allowing for a significantly more precise measurement of the escape velocity compared to previous studies. We find that the escape velocity lies within the range 498 < vesc < 608 km s−1 (90 per cent confidence), with a median likelihood of 544 km s−1. The fact that v2esc is significantly greater than 2v2circ (where vcirc= 220 km s−1 is the local circular velocity) implies that there must be a significant amount of mass exterior to the solar circle, that is, this convincingly demonstrates the presence of a dark halo in the Galaxy. We use our constraints on vesc to determine the mass of the Milky Way halo for three halo profiles. For example, an adiabatically contracted NFW halo model results in a virial mass of 1.42+1.14−0.54× 1012 M⊙ and virial radius of (90 per cent confidence). For this model the circular velocity at the virial radius is 142+31−21 km s−1. Although our halo masses are model dependent, we find that they are in good agreement with each other.
We analyse a sample of 82 850 stars from the RAdial Velocity Experiment (RAVE) survey, with well-determined velocities and stellar parameters, to isolate a sample of 18 026 high-probability thin-disc ...dwarfs within 600 pc of the Sun. We derive space motions for these stars, and deduce the solar space velocity with respect to the local standard of rest. The peculiar solar motion we derive is in excellent agreement in radial U⊙ and vertical W⊙ peculiar motions with other recent determinations. Our derived tangential peculiar velocity, V⊙, agrees with very recent determinations, which favour values near 13 km s−1, in disagreement with earlier studies. The derived values are not significantly dependent on the comparison sample chosen, or on the method of analysis. The local Galaxy seems very well dynamically relaxed, in a near symmetric potential.
The planetary nebula SuWt 2 (PN G311.0+02.4) is an unusual object with a prominent, inclined central emission ellipse and faint bipolar extensions. It has two A-type stars in a proven binary system ...at the centre. However, the radiation from these two central stars is too soft to ionize the surrounding material leading to a so far fruitless search for the responsible ionizing source. Such a source is clearly required and has already been inferred to exist via an observed temporal variation of the centre-of-mass velocity of the A-type stars. Moreover, the ejected nebula is nitrogen rich which raises question about the mass-loss process from a likely intermediate-mass progenitor. We use optical integral-field spectroscopy to study the emission lines of the inner nebula ring. This has enabled us to perform an empirical analysis of the optical collisionally excited lines, together with a fully three-dimensional photoionization modelling. Our empirical results are used to constrain the photoionization models, which determine the evolutionary stage of the responsible ionizing source and its likely progenitor. The time-scale for the evolutionary track of a hydrogen-rich model atmosphere is inconsistent with the dynamical age obtained for the ring. This suggests that the central star has undergone a very late thermal pulse. We conclude that the ionizing star could be hydrogen deficient and compatible with what is known as a PG 1159-type star. The evolutionary tracks for the very late thermal pulse models imply a central star mass of ∼0.64 M, which originated from an ∼3 M progenitor. The evolutionary time-scales suggest that the central star left the asymptotic giant branch about 25 000 yr ago, which is consistent with the nebula's age.
Abstract
Using the Radial Velocity Experiment (RAVE) survey, we recently brought to light a gradient in the mean galactocentric radial velocity of stars in the extended solar neighbourhood. This ...gradient likely originates from non-axisymmetric perturbations of the potential, among which a perturbation by spiral arms is a possible explanation. Here, we apply the traditional density wave theory and analytically model the radial component of the two-dimensional velocity field. Provided that the radial velocity gradient is caused by relatively long-lived spiral arms that can affect stars substantially above the plane, this analytic model provides new independent estimates for the parameters of the Milky Way spiral structure. Our analysis favours a two-armed perturbation with the Sun close to the inner ultra-harmonic 4:1 resonance, with a pattern speed and a small amplitude per cent of the background potential (14 per cent of the background density). This model can serve as a basis for numerical simulations in three dimensions, additionally including a possible influence of the Galactic bar and/or other non-axisymmetric modes.
The GALAH survey: scientific motivation De Silva, G. M; Freeman, K. C; Bland-Hawthorn, J ...
Monthly notices of the Royal Astronomical Society,
05/2015, Letnik:
449, Številka:
3
Journal Article
Recenzirano
Odprti dostop
The Galactic Archaeology with HERMES (GALAH) survey is a large high-resolution spectroscopic survey using the newly commissioned High Efficiency and Resolution Multi-Element Spectrograph (HERMES) on ...the Anglo-Australian Telescope. The HERMES spectrograph provides high-resolution (R ∼ 28 000) spectra in four passbands for 392 stars simultaneously over a 2 deg field of view. The goal of the survey is to unravel the formation and evolutionary history of the Milky Way, using fossil remnants of ancient star formation events which have been disrupted and are now dispersed throughout the Galaxy. Chemical tagging seeks to identify such dispersed remnants solely from their common and unique chemical signatures; these groups are unidentifiable from their spatial, photometric or kinematic properties. To carry out chemical tagging, the GALAH survey will acquire spectra for a million stars down to V ∼ 14. The HERMES spectra of FGK stars contain absorption lines from 29 elements including light proton-capture elements, α-elements, odd-Z elements, iron-peak elements and n-capture elements from the light and heavy s-process and the r-process. This paper describes the motivation and planned execution of the GALAH survey, and presents some results on the first-light performance of HERMES.
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