Disk Members Research Articles

Stellar Populations in the Central 0.5 pc of Our Galaxy. III. The Dynamical Substructures

We measure the 3D kinematic structures of the young stars within the central 0.5 pc of our Galactic Center using the 10 m telescopes of the W. M. Keck Observatory over a time span of 25 yr. Using high-precision measurements of positions on the sky and proper motions and radial velocities from new observations and the literature, we constrain the orbital parameters for each young star. Our results show two statistically significant substructures: a clockwise stellar disk with 18 candidate stars, as has been proposed before, but with an improved disk membership; and a second, almost edge-on plane of 10 candidate stars oriented east–west on the sky that includes at least one IRS 13 star. We estimate the eccentricity distribution of each substructure and find that the clockwise disk has 〈e〉 = 0.39 and the edge-on plane has 〈e〉 = 0.68. We also perform simulations of each disk/plane with incompleteness and spatially variable extinction to search for asymmetry. Our results show that the clockwise stellar disk is consistent with a uniform azimuthal distribution within the disk. The edge-on plane has an asymmetry that cannot be explained by variable extinction or incompleteness in the field. The orientation, asymmetric stellar distribution, and high eccentricity of the edge-on plane members suggest that this structure may be a stream associated with the IRS 13 group. The complex dynamical structure of the young nuclear cluster indicates that the star formation process involved complex gas structures and dynamics and is inconsistent with a single massive gaseous disk.

Chemical Abundances of Eight Highly-extincted Milky Way Planetary Nebulae* * Based on observations obtained with the Hobby–Eberly Telescope, which is a joint project of the University of Texas at Austin, the Pennsylvania State University, Ludwig-Maximilians-Universität München, and Georg-August-Universität Göttingen.

Low- and intermediate-mass (0.8 M ⊙ < M < 8 M ⊙) stars that evolve into planetary nebulae (PNe) play an important role in tracing and driving Galactic chemical evolution. Spectroscopy of PNe enables access to both the initial composition of their progenitor stars and products of their internal nucleosynthesis, but determining accurate ionic and elemental abundances of PNe requires high-quality optical spectra. We obtained new optical spectra of eight highly-extincted PNe with limited optical data in the literature using the Low Resolution Spectrograph 2 on the Hobby–Eberly Telescope. Extinction coefficients, electron temperatures and densities, and ionic and elemental abundances of up to 11 elements (He, N, O, Ne, S, Cl, Ar, K, Fe, Kr, and Xe) are determined for each object in our sample. Where available, astrometric data from Gaia eDR3 is used to kinematically characterize the probability that each object belongs to the Milky Way's thin disk, thick disk, or halo. Four of the PNe show kinematic and chemical signs of thin disk membership, while two may be members of the thick disk. The remaining two targets lack Gaia data, but their solar O, Ar, and Cl abundances suggest thin disk membership. Additionally, we report the detection of broad emission features from the central star of M 3–35. Our results significantly improve the available information on the nebular parameters and chemical compositions of these objects, which can inform future analyses.

Two’s company, three’s a crowd: SALT reveals the likely triple nature of the nucleus of the extreme abundance discrepancy factor planetary nebula Sp 3

Abstract The substantial number of binary central stars of planetary nebulae (CSPNe) now known ( $\sim 50$ ) has revealed a strong connection between binarity and some morphological features including jets and low-ionisation structures. However, some morphological features and asymmetries might be too complex or subtle to ascribe to binary interactions alone. In these cases, a tertiary component, that is, a triple nucleus, could be the missing ingredient required to produce these features. The only proven triple, NGC 246, is alone insufficient to investigate the shaping role of triple nuclei, but one straightforward way to identify more triples is to search for binaries in nuclei with known visual companions. Here we demonstrate this approach with the SALT HRS (High Resolution Spectrograph on the Southern African Large Telescope) discovery of a 4.81-d orbital period in the CSPN of Sp 3 which has a visual companion 0.31 arcsec away. The spectroscopic distance of the visual companion is in agreement with distance estimates to the nebula, the Gaia DR2 parallax of the central star, and the gravity distance of the central star. This supports a physical association between the visual companion and the inner 4.81 d binary, making the nucleus of Sp 3 a likely triple. We determine $T_\mathrm{eff}=68^{+12}_{-6}\ \text{kK},\ \log g=4.6\pm0.2\ \text{cm s}^{-2}$ , and $v_\mathrm{rot}=80\pm20\ \text{km s}^{-1}$ for the primary from non-local thermodynamic equilibrium model atmosphere analysis. The peculiar nebula presents an apparent bipolar morphology, jets, and an unexpected ‘extreme’ oxygen abundance discrepancy factor (adf) of $24.6^{+4.1}_{-3.4}$ . The adf is inconsistent with the purported trend for longer orbital period post-common-envelope (CE) PNe to exhibit normal adfs, further highlighting the dominant influence of selection effects in post-CE PNe. Lastly, the Type I nebular abundances of Sp 3, whose origin is often attributed to more massive progenitors, are incongruous with the likely Galactic Thick Disk membership of Sp 3, possibly suggesting that rotation and binarity may play an important role in influencing the AGB nucleosynthesis of PNe.

Confusing Binaries: The Role of Stellar Binaries in Biasing Disk Properties in the Galactic Center

Abstract The population of young stars near the supermassive black hole (SMBH) in the Galactic Center (GC) has presented an unexpected challenge to theories of star formation. Kinematic measurements of these stars have revealed a stellar disk structure (with an apparent 20% disk membership) that has provided important clues regarding the origin of these mysterious young stars. However, many of the apparent disk properties are difficult to explain, including the low disk membership fraction and the high eccentricities given the youth of this population. Thus far, all efforts to derive the properties of this disk have made the simplifying assumption that stars at the GC are single stars. Nevertheless, stellar binaries are prevalent in our Galaxy, and recent investigations suggested that they may also be abundant in the Galactic Center. Here, we show that binaries in the disk can largely alter the apparent orbital properties of the disk. The motion of binary members around each other adds a velocity component, which can be comparable to the magnitude of the velocity around the SMBH in the GC. Thus, neglecting the contribution of binaries can significantly vary the inferred stars’ orbital properties. While the disk orientation is unaffected, the apparent disk’s 2D width will be increased to about 11.°2, similar to the observed width. For a population of stars orbiting the SMBH with zero eccentricity, unaccounted for binaries will create a wide apparent eccentricity distribution with an average of 0.23. This is consistent with the observed average eccentricity of the stars’ in the disk. We suggest that this high eccentricity value, which poses a theoretical challenge, may be an artifact of binary stars. Finally, our results suggest that the actual disk membership might be significantly higher than the one inferred by observations that ignore the contribution of binaries, alleviating another theoretical challenge.

Candidate hypervelocity stars of spectral type G and K revisited

Hypervelocity stars (HVS) move so fast that they are unbound to the Galaxy. When they were first discovered in 2005, dynamical ejection from the supermassive black hole (SMBH) in the Galactic Centre (GC) was suggested as their origin. The two dozen HVSs known today are young massive B stars, mostly of 3-4 solar masses. Recently, 20 HVS candidates of low mass were discovered in the Segue G and K dwarf sample, but none of them originates from the GC. We embarked on a kinematic analysis of the Segue HVS candidate sample using the full 6D phase space information based on new proper motion measurements. Their orbital properties can then be derived by tracing back their trajectories in different mass models of our Galaxy. We present the results for 14 candidate HVSs, for which proper motion measurements were possible. Significantly lower proper motions than found in the previous study were derived. Considering three different Galactic mass models we find that all stars are bound to the Galaxy. We confirm that the stars do not originate from the GC. The distribution of their proper motions and radial velocities is consistent with predictions for runaway stars ejected from the Galactic disk by the binary supernova mechanism. However, their kinematics are also consistent with old disk membership. Moreover, most stars have rather low metallicities and strong $\alpha$-element enrichment as typical for thick disk and halo stars, whereas the metallicity of the three most metal-rich stars could possibly indicate that they are runaway stars from the thin disk. One star shows halo kinematics.

Discovery of a new Y dwarf: WISE J030449.03−270508.3

We present a new Y dwarf, WISE J030449.03-270508.3, confirmed from a candidate sample designed to pick out low temperature objects from the WISE database. The new object is typed Y0pec following a visual comparison with spectral standards, and lies at a likely distance of 10-17 pc. Its tangential velocity suggests thin disk membership, but it shows some spectral characteristics that suggest it may be metal-poor and/or older than previously identified Y0 dwarfs. Based on trends seen for warmer late type T dwarfs, the Y-band flux peak morphology is indicative of sub-solar metallicity, and the enhanced red wing of the J-band flux peak offers evidence for high gravity and/or low metallicity (with associated model trends suggesting an age closer to ~10 Gyr and mass in the range 0.02-0.03 Mo). This object may thus be extending the population parameter-space of the known Y0 dwarfs.

Two very nearby (d ~ 5 pc) ultracool brown dwarfs detected by their large proper motions from WISE, 2MASS, and SDSS data

Aims: WISE provides an infrared all-sky survey which aims at completing our knowledge on the possibly dramatically increasing number of brown dwarfs with lower temperatures. We search for the nearest representatives of the coolest brown dwarfs, which will be very interesting for detailed follow-up observations, once they haven been discovered. Methods: We have used the preliminary data release from WISE, selected bright candidates with colours typical of late-T dwarfs, tried to match them with faint 2MASS and SDSS objects, to determine their proper motions, and to follow-up them spectroscopically. Results: We have identified two new ultracool brown dwarfs, WISE J0254+0223 and WISE J1741+2553, with large proper motions of about 2.5 and 1.5 arcsec/yr, respectively. With their w1-w2~3.0 and J-w2~4.0 colour indices we expect both to have a spectral type of ~T8-T10 and absolute magnitude of M_{w2}~14. We confirm WISE J1741+2553 as a T9-T10 dwarf from near-infrared spectroscopy with LBT/LUCIFER1. From their bright WISE w2 magnitudes of 12.7 and 12.3, we estimate distances of 5.5$^{+2.3}_{-1.6}$ pc and 4.6$^{+1.2}_{-1.0}$ pc and tangential velocities of ~65 km/s and ~34 km/s indicating Galactic thick and thin disk membership, respectively.

The chemical fingerprints of the thin and the thick disk

AbstractThe past history and origin of the different Galactic stellar populations are manifested in their different chemical abundance patterns. We obtained new elemental abundances for 553 F and G dwarf stars, to more accurately quantify these patterns for the thin and thick disks. However, the exact definition of disk membership is not straightforward. Stars that have a high likelihood of belonging to the thin disk show different abundance patterns from those for the thick disk. In contrast, we show that stars for the Hercules Stream do not show unique abundance patterns, but rather follow those of the thin and thick disks. This strongly suggests that the Hercules Stream is a feature induced by internal dynamics within the Galaxy rather than the remnant of an accreted satellite.

Spectroscopic Identification of White Dwarfs in the New Luyten Two-Tenths Survey

We spectroscopically identify New Luyten Two-Tenths (NLTT) stars from the reduced proper-motion diagram of Salim & Gould. Five out of 16 objects investigated are identified as white dwarfs: four hydrogen-rich DA stars (NLTT 8581, NLTT 31748, NLTT 53177, and NLTT 56805) and one helium-rich (NLTT 8435). The properties of the stars are further constrained using Two Micron All Sky Survey photometric measurements and, in the case of NLTT 31748, using International Ultraviolet Explorer spectrophotometry. The distances range from 15 to 23 pc, and therefore the sample contributes to estimates of the local density of white dwarf stars. The newly found objects are kinematically consistent with a thick or old thin disk membership. We discuss the likelihood of further identifications.

The pulsation modes and masses of carbon-rich long period variables

Following our study of the carbon-rich giants in the HR diagram and of their luminosity function (Paper III), we investigate the pulsation data of the long period variables (LPVs) included in our sample. Pulsation modes (fundamental, over- tone(s)) for carbon LPVs are identified in the period-radius diagram, making use of observed bi-periodicity in a small subsample of those stars, and of comparison to models. Mean pulsation masses are then deduced from theoretical PMR-relations, with due attention paid to a possible bias while averaging. Mean (present) pulsation masses (0:6 4:0 M) are found to increase along the group sequence HC5 to CV6, with still larger masses possibly associated with cool extreme CV7-objects with strong mass loss and thick circumstellar shells. This is con- sistent with the 0.8-4 M range of initial masses found in Paper III for the majority of carbon-rich giants aected by mass loss during their evolution. The pulsation masses found for a few HC-stars (M 0:8 M) are consistent with their low initial masses (Mi < 1:1 M); as inferred from their thick disk membership (age' 11 Gyr?) and locus in the HR diagram. A mean pulsation mass of'0:6 M is found for the three population II Cepheids in the sample. A mass-luminosity diagram is proposed for the Galactic carbon giants. The data from observations is found consistent with theoretical predictions from AGB modeling, specially the third dredge-up (TDU) through thermal pulses (TP) with a carbon star formation line (CSFL) for TP-AGB stars. It appears that the CV-giants are close to the tip and end of their evolutionary tracks in the TP-AGB of the HR diagram. It is confirmed that this end shifts toward lower eective temperatures and higher luminosities, with increasing masses. It is shown that the C/O abundance ratios do correlate with eective temperatures, according to three distinct distributions (halo CH stars, thick disk HC-stars, and thin disk CV-stars). The mean stellar density decreases along the HC5-CV7 sequence, while the surface gravity remains nearly constant at about 0.5 CGS unit logg' 0:3; 5 10 3 SI : The nature of (thin disk) CV-stars as TP-AGB objects being confirmed, the discussion is focused on (thick disk) HC-stars since the origin of these old low-mass giants remains unclear. Unpredicted extra mixing on RGB and/or E-AGB is favored. Evolution from (old, low O/H) dwarf carbon stars is also considered since observations of metal-poor stars and recent calculations point to large supersolar (C=Fe) ratios in Population III objects and contamination through rapid cycling in the interstellar medium.

The Proper Motion of the Globular Cluster NGC 6553 and of Bulge Stars with the [ITAL]HUBBLE SPACE TELESCOPE[/ITAL][ITAL]Hubble Space Telescope[/ITAL

WFPC2 images obtained with the Hubble Space Telescope (HST) 4.16 years apart have allowed us to measure the proper motion of the metal-rich globular cluster NGC 6553 with respect to the background bulge stars. With a space velocity of (Π, Θ, W) = (-3.5, 230, -3) km s-1, NGC 6553 follows the mean rotation of both disk and bulge stars at a galactocentric distance of 2.7 kpc. While the kinematics of the cluster is consistent with either a bulge or a disk membership, the virtual identity of its stellar population with that of the bulge cluster NGC 6528 makes its bulge membership more likely. The astrometric accuracy is high enough for providing a measure of the bulge proper motion dispersion and confirming its rotation. A selection of stars based on the proper motions produced an extremely well-defined cluster color-magnitude diagram (CMD), essentially free of bulge stars. The improved turnoff definition in the decontaminated CMD confirms an old age for the cluster (~13 Gyr), indicating that the bulge underwent a rapid chemical enrichment while being built up at in the early universe. An additional interesting feature of the cluster color-magnitude diagram is a significant number of blue stragglers stars, whose membership in the cluster is firmly established from their proper motions.

The Extremely Active Single Giant 1E 1751+7046 = ET Draconis: Revised Properties and a Reevaluation of Its Evolutionary Status

New spectroscopic observations have resulted in significant revisions to several of the orginally published properties of the X-ray bright, chromospherically active star 1E 1751+7046: the spectral classification has been revised from K5 IV to K0 III, v sin i from 30-40 km s-1 to 23 km s-1, and the reported nondetection of the Li I 6707 A line to a significant log (Li) = 1.8. Chromospheric and transition region surface fluxes from IUE observations and the coronal surface flux from earlier Einstein data are close to saturation levels, and comparable only to very active binaries, pre-main-sequence stars, and FK Comae itself. IUE observations also recorded a flare. Fifteen new radial velocity measurements show no evidence for a companion and are consistent with intermediate or young disk membership. On the other hand, we show the star to be located about 250 pc above the galactic plane, suggesting an intermediate or old disk object. The new spectral class (Teff) and limits on the luminosity indicate that 1E 1751+7046 is a low-mass star on its first ascent of the giant branch. Photometry from the Four College Consortium Automatic Photometry Telescope is consistent with the recently published photometric period of 13.98 days, and the light curve is well fitted by a model consisting of two large spots at latitudes of ~30° and ~50°. There are currently only two possible evolutionary scenarios for this anomalous star: (a) coalescence from a progenitor W UMa-type contact binary; or (b) the dredge-up of both angular momentum and nuclear processed material in a low-mass (~1-2.5 M☉) giant. A space motion, obtainable once a parallax and proper motion are available from Hipparcos, may resolve the age (old disk-young disk) uncertainty: young disk motions would favor the angular momentum transfer scenario. There is no current theory that can account for the observed lithium abundance.