APOGEE Data Research Articles

APOGEE CHEMICAL TAGGING CONSTRAINT ON THE MAXIMUM STAR CLUSTER MASS IN THE α-ENHANCED GALACTIC DISK

ABSTRACT Stars born from the same molecular cloud should be nearly homogeneous in their element abundances. The concept of chemical tagging is to identify members of disrupted clusters by their clustering in element abundance space. Chemical tagging requires large samples of stars with precise abundances for many individual elements. With uncertainties of &sgr; [ X / Fe ] ?> and &sgr; [ Fe / H ] ≃ 0.05 ?> for 10 elements measured for > 10 4 ?> stars, the APOGEE DR12 spectra may be the first well-suited data set to put this idea into practice. We find that even APOGEE data offer only ∼500 independent volume elements in the 10-dimensional abundance space, when we focus on the α-enhanced Galactic disk. We develop and apply a new algorithm to search for chemically homogeneous sets of stars against a dominant background. By injecting star clusters into the APOGEE data set, we show that chemically homogeneous clusters with masses ≳ 3 × 10 7 M ⊙ ?> would be easily detectable and yet no such signal is seen in the data. By generalizing this approach, we put a first abundance-based constraint on the cluster mass function for the old disk stars in the Milky Way.

Carbon, nitrogen and α-element abundances determine the formation sequence of the Galactic thick and thin discs

Using the DR12 public release of APOGEE data, we show that thin and thick disk separate very well in the space defined by [$\alpha$/Fe], [Fe/H] and [C/N]. Thick disk giants have both higher [C/N] and higher [$\alpha$/Fe] than do thin disk stars with similar [Fe/H]. We deduce that the thick disk is composed of lower mass stars than the thin disk. Considering the fact that at a given metallicity there is a one-to-one relation between stellar mass and age, we are then able to infer the chronology of disk formation. Both the thick and the thin disks - defined by [$\alpha$/Fe] -- converge in their dependance on [C/N] and [C+N/Fe] at [Fe/H]$\approx$-0.7. We conclude that 1) the majority of thick disk stars formed earlier than did the thin disk stars 2) the formation histories of the thin and thick disks diverged early on, even when the [Fe/H] abundances are similar 3) that the star formation rate in the thin disk has been lower than in the thick disk, at all metallicities. Although these general conclusions remain robust, we also show that current stellar evolution models cannot reproduce the observed C/N ratios for thick disk stars. Unexpectedly, reduced or inhibited canonical extra-mixing is very common in field stars. While subject to abundance calibration zeropoint uncertainties, this implies a strong dependence of non canonical extra-mixing along the red giant branch on the initial composition of the star and in particular on the $\alpha$ elemental abundance.

Vertical velocities from proper motions of red clump giants

We derive the vertical velocities of disk stars in the range of Galactocentric radii of R=5-16 kpc within 2 kpc in height from the Galactic plane. This kinematic information is connected to dynamical aspects in the formation and evolution of the Milky Way, such as the passage of satellites and vertical resonance and determines whether the warp is a long-lived or a transient feature. We used the proper motions of the PPMXL survey, correcting of systematic errors with the reference of quasars. From the color-magnitude diagram K versus (J-K) we selected the standard candles corresponding to red clump giants and used the information of their proper motions to build a map of the vertical motions of our Galaxy. We derived the kinematics of the warp both analytically and through a particle simulation to fit these data. Complementarily, we also carried out the same analysis with red clump giants spectroscopically selected with APOGEE data, and we predict the improvements in accuracy that will be reached with future Gaia data. A simple model of warp with the height of the disk z_w(R,phi)=gamma (R-R_sun) sin(phi-phi_w) fits the vertical motions if d(gamma)/dt/gamma=-34+/-17 Gyr^{-1}; the contribution to d(gamma)/dt comes from the southern warp and is negligible in the north. The vertical motion in the warp apparently indicates that the main S-shaped structure of the warp is a long-lived feature, whereas the perturbation that produces an irregularity in the southern part is most likely a transient phenomenon. With the use of the Gaia end-of-mission products together with spectroscopically classified red clump giants, the precision in vertical motions can be increased by an order of magnitude at least.

Gas and stellar motions and observational signatures of corotating spiral arms

We have observed a snapshot of our N-body/Smoothed Particle Hydrodynamics simulation of a Milky Way-sized barred spiral galaxy in a similar way to how we can observe the Milky Way. The simulated galaxy shows a co-rotating spiral arm, i.e. the spiral arm rotates with the same speed as the circular speed. We observed the rotation and radial velocities of the gas and stars as a function of the distance from our assumed location of the observer at the three lines of sight on the disc plane, (l, b) = (90, 0), (120, 0) and (150,0) deg. We find that the stars tend to rotate slower (faster) behind (at the front of) the spiral arm and move outward (inward), because of the radial migration. However, because of their epicycle motion, we see a variation of rotation and radial velocities around the spiral arm. On the other hand, the cold gas component shows a clearer trend of rotating slower (faster) and moving outward (inward) behind (at the front of) the spiral arm, because of the radial migration. We have compared the results with the velocity of the maser sources from Reid et al. (2014), and find that the observational data show a similar trend in the rotation velocity around the expected position of the spiral arm at l = 120 deg. We also compared the distribution of the radial velocity from the local standard of the rest, V_LSR, with the APOGEE data at l = 90 deg as an example.

What drives the evolution of the Milky Way's disk?

Data from the recently launched Gaia satellite in combination with data from large spectroscopic and asteroseismology surveys will allow great progress in unraveling the evolutionary history of the Milky Way disk over the next decade. Here, I briefly review the progress made with observations in the solar neighborhood in constraining the main processes involved in the formation and evolution of the Galactic disk. I then focus on recent new insights on the chemo-dynamical structure of the disk from SEGUE and APOGEE data. I conclude by summarizing the current state of knowledge and what evolutionary actors best explain the data that we have now, and by looking forward to the most informative measurements that Gaia and other surveys can make.

Association of electron precipitation with auroral kilometric radiation

An examination is made of a large body of auroral X ray and auroral kilometric radiation (AKR) data acquired on the Polar satellite in order to investigate the relationship between these two phenomena on a global scale. A detailed study was made of 14 cases selected with strong X ray and AKR emissions and appreciable time variations. In nearly all cases a maximum in the cross‐correlation coefficient of auroral X rays and AKR occurred for X rays emitted slightly before local midnight, indicating that the AKR sources were concentrated in this magnetic local time (MLT) sector. The correlation is quite strong with coefficients between 0.29 and 0.82. The enhancement in the correlation coefficient generally extends over an MLT interval of 6 hours or less, even though electron precipitation above 2 keV extends over a longer MLT interval. This behavior is consistent with the importance of another factor, such as the presence of plasma cavities, being necessary for AKR production. Owing to statistical uncertainties, it was not possible from apogee data to establish whether the X rays or AKR occurred first during the short enhancements. Higher time resolution was achievable when Polar was near perigee, but AKR propagation to Polar was not assured at this location. During a perigee pass on January 13, 1997, Polar observed an auroral X‐ray enhancement in time coincidence with inverted‐V electrons, upward streaming ions, and enhanced AKR. In this event the inverted‐V potential structure extended over at least 2 hours of MLT and lasted for only l min.

On the low latitude topside models: II. Electron temperature

Electron temperature measured within the scope of the ACTIVE Project was analyzed for range of ±30 invariant latitude. Apogee (2500 km) and perigee (500 km) electron temperature data of December 1989 and March 1990 are in general agreement with the IRI. In morning hours there appears in high altitudes a longitude effect that is due to the variable illumination of the zone of main electron production. Geomagnetic activity causes an increase of electron temperature near 500 km, a decrease near 2500 km.

On the low latitude topside models: I. Electron density

Ion density measured within the scope of the ACTIVE Project was analyzed for range of ±30 invariant latitude. Apogee (2500 km) and perigee (500 km) data of December 1989 and March 1990, though in general agreement with the IRI, show a distinct equatorial anomaly yet at altitude of 500 km. A small longitude effect appears at local early morning at apogee height. Geomagnetic activity causes an increase of density, particularly in apogee.

Sputnik as a tool for securing geodetic information

A simplified picture is employed for utilizing Sputnik tracking data (perigee, apogee, period, maximum and minimum orbital velocity) in the determination of the Earth's mass and radius. For this purpose, basic characteristics of the motion are conveniently re-expressed in terms of the perigee and apogee parameters. Means for deducing the launching conditions are also provided and from reported perigee, apogee data for Sputnik I it is inferred that horizontal launching took place at a firing velocity slightly more than the lower bound value.