Abundances of disk and bulge giants from high-resolution optical spectra

Context. Observations of the Galactic bulge suggest that the disk formed through secular evolution rather than gas dissipation and/or mergers, as previously believed. This would imply very similar chemistry in the disk and bulge. Some elements, such as the α-elements, are well studied in the bulge, but others like the neutron-capture elements are much less well explored. Stellar mass and metallicity are factors that affect the neutron-capture process. Due to this, the enrichment of the ISM and the abundance of neutron-capture elements vary with time, making them suitable probes for Galactic chemical evolution. Aims. In this work, we make a differential comparison of neutron-capture element abundances determined in the local disk(s) and the bulge, focusing on minimising possible systematic effects in the analysis, with the aim of finding possible differences/similarities between the populations. Methods. Abundances are determined for Zr, La, Ce, and Eu in 45 bulge giants and 291 local disk giants, from high-resolution optical spectra. The abundances are determined by fitting synthetic spectra using the SME-code. The disk sample is separated into thin- and thick-disk components using a combination of abundances and kinematics. Results. We find flat Zr, La, and Ce trends in the bulge, with a ~0.1 dex higher La abundance compared with the disk, possibly indicating a higher s-process contribution for La in the bulge. [Eu/Fe] decreases with increasing [Fe/H], with a plateau at around [Fe/H] ~−0.4, pointing at similar enrichment to α-elements in all populations. Conclusions. We find that the r-process dominated the neutron-capture production at early times both in the disks and bulge. Further, [La/Eu] ratios for the bulge are systematically higher than for the thick disk, pointing to either a) a different amount of SN II or b) a different contribution of the s-process in the two populations. Considering [(La+Ce)/Zr], the bulge and the thick disk follow each other closely, suggesting a similar ratio of high-to-low-mass asymptotic giant branch stars.

Abundances of disk and bulge giants from high-resolution optical spectra

Context. The formation and evolution of the Galactic bulge and the Milky Way is still a debated subject. Observations of the X-shaped bulge, cylindrical stellar motions, and the presumed existence of a fraction of young stars in the bulge have suggested that it formed through secular evolution of the disk and not through gas dissipation and/or mergers, as thought previously. Aims. Our goal was to measure the abundances of six iron-peak elements (Sc, V, Cr, Mn, Co, and Ni) in the local thin and thick disks and in the bulge. These abundances can provide additional observational constraints for Galaxy formation and chemical evolution models, and help us to understand whether the bulge has emerged from the thick disk or not. Methods. We use high-resolution optical spectra of 291 K giants in the local disk mostly obtained by the FIES at NOT (signal-to-noise ratio (S/N) of 80–100) and 45 K giants in the bulge obtained by the UVES/FLAMES at VLT (S/N of 10–80). The abundances are measured using Spectroscopy Made Easy (SME). Additionally, we apply non-local thermodynamic equilibrium corrections to the ratios [Mn/Fe] and [Co/Fe]. The thin and thick disks were separated according to their metallicity, [Ti/Fe], as well as proper motions and the radial velocities from Gaia DR2. Results. The trend of [V/Fe] vs. [Fe/H] shows a separation between the disk components, being more enhanced in the thick disk. Similarly, the [Co/Fe] vs. [Fe/H] trend shows a hint of an enhancement in the local thick disk. The trends of V and Co in the bulge appear to be even more enhanced, although within the uncertainties. The decreasing value of [Sc/Fe] with increasing metallicity is observed in all the components, while our [Mn/Fe] value steadily increases with increasing metallicity in the local disk and the bulge instead. For Cr and Ni we find a flat trend following iron for the whole metallicity range in the disk and the bulge. The ratio of [Ni/Fe] appears slightly overabundant in the thick disk and the bulge compared to the thin disk, although the difference is minor. Conclusions. The somewhat enhanced ratios of [V/Fe] and [Co/Fe] observed in the bulge suggest that the local thick disk and the bulge might have experienced different chemical enrichment and evolutionary paths. However, we are unable to predict the exact evolutionary path of the bulge solely based on these observations. Galactic chemical evolution models could, on the other hand, allow us to predict them using these results.

Minor mergers: fundamental but unexplored drivers of galaxy evolution

We use the SDSS Stripe 82 to study the stellar-mass growth that is triggered by minor mergers in local disk galaxies. Since major mergers destroy disks and create spheroids, morphologically disturbed spirals are likely remnants of minor mergers (since the disk remains intact). Disturbed spirals exhibit enhanced specific star formation rates (SSFRs), with the enhancement increasing in galaxies with ‘later’ morphological type (that have larger gas reservoirs and smaller bulges). By combining the SSFR enhancements with the fraction of time spirals in various morphological classes spend in this ‘enhanced’ mode, we estimate that ~40% of the star formation activity in local spirals is directly triggered by minor mergers. Combining our results with the star formation in local early-type galaxies – which is almost completely driven by minor mergers – suggests that around half the star formation activity at the present day is likely to be triggered by the minor-merger process.

Distributed Digital Preservation in the Cloud

The LOCKSS system is a leading technology in the field of Distributed Digital Preservation. Libraries run LOCKSS boxes to collect and preserve content published on the Web in PC servers with local disk storage. They form nodes in a network that continually audits their content and repairs any damage. Libraries wondered whether they could use cloud storage for their LOCKSS boxes instead of local disks. We review the possible configurations, evaluate their technical feasibility, assess their economic feasibility, report on an experiment in which we ran a production LOCKSS box in Amazon’s cloud service, and describe some simulations of future costs of cloud and local storage. We conclude that current cloud storage services are not cost-competitive with local hardware for long term storage, including for LOCKSS boxes.

Design and Implementation of a Remote-Based Journaling File System

The overhead brings by metadata journaling is extra space and performance degrade caused by frequent journal data flush. A journal file system based remote journal scheme was designed and implementation. The remote journal scheme removes the frequent journal I/O activities from local disk to remote server. According to the experiments in this paper, remote journal increases about 8% to 19% performance, but the penalty is light. Although remote journal does need more CPU time for network transfer, the overhead is less than 8%. And the extra network bandwidth taken by remote journal is less than 6% in metadata bound workload.

MINIMUM LOCAL DISK COVER SETS FOR BROADCASTING IN HETEROGENEOUS MULTIHOP WIRELESS NETWORKS

The concept of forwarding sets is widely adopted in many broadcast protocols for wireless multihop networks to alleviate the broadcast storm problem. In these protocols, after receiving a broadcast message, each node that is requested to relay the message instructs a subset of its 1-hop neighbors, a.k.a. the forwarding set, to further relay it. In this paper, we propose to use the Minimum Local Disk Cover Set (MLDCS) as the forwarding set in heterogeneous multihop wireless networks, where nodes may have different transmission ranges.We show that the minimum local disk cover set of a node in heterogeneous networks is equivalent to its skyline set, and then we propose a divide-and-conquer algorithm with the optimal time complexity to compute the skyline set locally and statelessly. Moreover, unlike other forwarding heuristics, the proposed algorithm requires only 1-hop neighbor information. This helps to reduce the forwarding set formation latency and thus will be more suitable for environments with a frequently changed network topology, such as vehicular ad hoc networks.