East Shell Research Articles

Turning a tragedy into large-scale barrier island restoration in Louisiana: A three-project case study

Louisiana has successfully utilized the proceeds from the fines imposed for the Deepwater Horizon incident to significantly jump start barrier island restoration as identified in the Coastal Protection and Restoration Authority (CPRA) of Louisiana Coastal Master Plan (CPRA 2017). The Riverine Sand Mining/ Scofield Island Restoration (BA-40) project was the first to be implemented through a commitment of remaining funds in the initial emergency protective berms’ construction budget formulated into the Berms to Barrier Islands plan. The berm/ restoration conversion at Scofield Island was the first to utilize this funding mechanism. The Caminada Headland Beach and Dune Restoration–Increment II (BA-143) project was funded through the National Fish and Wildlife (NFWF) Gulf Environmental Benefit Fund and capitalized on a prior project constructed to the west completing the beach and dune restoration of the entire headland. Lastly, the Caillou Lake Headlands Restoration (TE-100) project was funded through the Natural Resource Damage Assessment (NRDA). The TE-100 project restored the entire degraded beach and dune system backed by a created marsh habitat to complement a prior restoration effort. Scofield Island is located west of the active Mississippi River bird’s foot delta in Plaquemines Parish, Louisiana. A primary objective of this project was the excavation and delivery of Mississippi riverine sand for beach and dune restoration; a first in our nation’s history. Multiple design and construction challenges arose requiring the CPRA, consulting team, and construction contractor to adapt. Construction of the beach and dune component of this project required approximately 22 miles (mi) of pipeline and four booster pumps along a sediment pipeline corridor that crossed two hurricane protection levees, went underneath two highways and a navigation channel, traversed the Empire Waterway, crossed Pelican Island, entered the Gulf of Mexico, and extended to Scofield Island. The restoration footprint length was approximately 2.4 mi, total volume placed was approximately 3.5 million cubic yards (MCY), and the benefit equaled 510 restored acres (CEC 2014). The pipeline corridor has subsequently been utilized for two other restoration projects, Shell Island East Berm Barrier Island Restoration (BA-110) and Shell Island West NRDA Restoration (BA-111). As a first in Louisiana’s restoration history, the Caminada Headland Beach and Dune Restoration–Increments I and II (BA-45 and BA-143) utilized sand dredged from Ship Shoal, an Outer Continental Shelf (OCS) sand resource located approximately 26-38 mi from the restoration areas. The 13.3 mi long headland was restored with approximately 3.7 MCY for BA-45 and 5.5 MCY for BA-143 from the borrow area (CEC 2015 and CEC 2017). A combination of cutterhead dredge/scow barges and hopper dredges were used to construct the project. A key goal of this project was restoring and protecting the fragile ecosystem which provides critical habitat for nesting shorebirds. The headland is of critical importance in serving as a defense of our national energy infrastructure. The western portion of the headland directly protects Port Fourchon, one of the nation’s most important energy ports. Caillou Lake Headlands (TE-100), known locally as Whiskey Island, is centrally located in the Isle Dernieres chain and it is a remnant of the single, larger Isle Dernieres (Last Island), which was segmented into multiple smaller islands by a major hurricane in 1856. The project included restoring the beach and dune along approximately 4.5 mi while simultaneously creating a marsh platform along approximately 5,500 feet (ft) utilizing 10.4 MCY of sand from the borrow area (CEC 2018). The borrow area lies within Ship Shoal OCS Lease Block 88 located over 10 mi along the conveyance corridor offshore of Whiskey Island. This project represents the largest barrier island restoration project to date in terms of volume per linear foot of shoreline with an average density of over 441 cubic yards per linear foot (CEC 2018).

SGR A* AND ITS ENVIRONMENT: LOW-MASS STAR FORMATION, THE ORIGIN OF X-RAY GAS AND COLLIMATED OUTFLOW

ABSTRACT We present high-resolution multiwavelength radio continuum images of the region within 150″ of Sgr A*, revealing a number of new extended features and stellar sources in this region. First, we detect a continuous 2″ east–west ridge of radio emission, linking Sgr A* and a cluster of stars associated with IRS 13 N and IRS 13E. The ridge suggests that an outflow of east–west blob-like structures is emerging from Sgr A*. In particular, we find arc-like radio structures within the ridge with morphologies suggestive of photoevaporative protoplanetary disks. We use infrared K s and L′ fluxes to show that the emission has similar characteristics to those of a protoplanetary disk irradiated by the intense radiation field at the Galactic center. This suggests that star formation has taken place within the S-cluster 2″ from Sgr A*. We suggest that the diffuse X-ray emission associated with Sgr A* is due to an expanding hot wind produced by the mass loss from B-type main sequence stars, and/or the disks of photoevaporation of low mass young stellar objects (YSOs) at a rate of ∼10−6 M ⊙ ?> yr−1. The proposed model naturally reduces the inferred accretion rate and is an alternative to the inflow–outflow style models to explain the underluminous nature of Sgr A*. Second, on a scale of 5″ from Sgr A*, we detect new cometary radio and infrared sources at a position angle PA ∼ 50° which is similar to that of two other cometary sources X3 and X7, all of which face Sgr A*. In addition, we detect a striking tower of radio emission at a PA ∼ 50°–60° along the major axis of the Sgr A East supernova remnant shell on a scale of 150″ from Sgr A*. We suggest that the cometary sources and the tower feature are tracing interaction sites of a mildly relativistic jet from Sgr A* with the atmosphere of stars and the nonthermal Sgr A East shell at a PA ∼ 50°–60° with M ˙ ∼ 1 × 10 − 7 M ⊙ yr - 1 ?> , and opening angle 10°. Lastly, we suggest that the east–west ridge of radio emission traces an outflow that is potentially associated with past flaring activity from Sgr A*. The position angle of the outflow driven by flaring activity is close to −90°.

Statistical Properties of Molecular Clumps in the Galactic Center 50 km s-1 Molecular Cloud

Abstract We present the statistical properties of molecular clumps in the Galactic center 50 km s$ ^{-1}$ molecular cloud (GCM-0.02-0.07) based on observations of the CS $ J$$ =$ 1–0 emission line with the Nobeyama Millimeter Array. In the cloud, 37 molecular clumps with local thermodynamic equilibrium (LTE) masses of 2 $ \times$ 10$ ^{2}$ –6 $ \times$ 10$ ^{3}\ M_{\odot}$ were identified by using the clumpfind algorithm. The velocity widths of the molecular clumps are about five-fold those of Galactic disk molecular clouds with the same radius. The virial-theorem masses are three-fold the LTE masses. The mass and size spectra can be described by power laws of $ dN/dM$$ \propto$$ M^{-2.6\pm0.1}$ ($ M$$ \gtrsim$ 900 $ M_{\odot}$ ) and $ dN/dR$$ \propto$$ R^{-5.9\pm0.3}$ ($ R$$ \gtrsim$ 0.35 pc), respectively. The statistical properties of the region interacting with the Sgr A East shell and those of the non-interacting part of the cloud are significantly different. The interaction probably makes the mass function steeper, from $ dN/dM$$ \propto$$ M^{-2.0\pm0.1}$ in the non-interacting part to $ dN/dM$$ \propto$$ M^{-4.0\pm0.2}$ in the interacting region. On the other hand, the interaction presumably truncates the size spectrum on the larger side of $ R$$ \sim$ 0.4 pc.

Sagittarius A Molecular Cloud Complex in H13CO+ and Thermal SiO Emission Lines

Abstract We present results of a high-resolution wide-field mapping observation of the Sagittarius A (Sgr A) molecular cloud complex in H$^{13}$CO$^+$$J$$=$ 1–0 and thermal SiO $J$$=$ 2–1 emission lines with the 25-beam receiver of the 100 GHz band operating on the Nobeyama Radio Observatory 45-m telescope. The mapping area covers a 0$^\circ\!\!\!.$5 $\times$ 0$^\circ\!\!\!.$5 area involving several named molecular clouds, for example GCM$-$0.02$-$0.07, GCM$-$0.13$-$0.08, GCM 0.11$-$0.11, the Sickle molecular cloud, the Arched filaments molecular cloud, and so on. The data have an effective angular resolution of 26$''$. The H$^{13}$CO$^+$ emission line is a famous tracer of molecular gas mass because the line is optically thin, even in the Galactic center region, and is not emphasized by shock. The emission line presents a clumpy distribution of the molecular cloud. The averaged fractional abundance in the Sgr A complex is $N$(H$^{13}$CO$^+$)$/N_{\mathrm{H_2}}$$\simeq$ (1.8$\ \pm\ $0.4) $\times$ 10$^{-11}$, comparing the LTE mass and the virial theorem mass. The SiO emission line is a famous and reliable tracer of shocked molecular gas. We find many molecular clouds that are remarkable only in the SiO emission line. Such molecular clouds have a large velocity width of up to 60 km s$^{-1}$. The brightness temperature ratio is up to $T_{\mathrm{B}}$(SiO)$/T_{\mathrm{B}}$(H$^{13}$CO$^+$) $\lesssim$ 8. The features are dominated by shock SiO-enriched gas. In such clouds, the ratio of the fractional abundance of SiO and H$^{13}$CO$^+$ molecules is $X$(SiO)$/X$(H$^{13}$CO$^+$) $\sim$ 100. The features are presumably made by supernova remnants. We found a prototypical example in GCM$-$0.02$-$0.07. It has two distinct structures. One is a ridge-like structure contacting with the Sgr A East shell; another is an expanding shell-like structure. There is a wide-velocity width ridge of SiO-enriched gas in GCM 0.11$-$0.11, which is adjacent to Vertical filaments. This suggests that the collision with the molecular cloud accelerates relativistic electrons, which illuminate the Vertical filaments–Polarized Plumes complex.

A Galactic Center 50-kms$^{-1}$ Molecular Cloud with an Expanding Shell

We performed high-resolution observations of a Galactic Center 50-km s$^{-1}$ molecular cloud in the CS $J$$=$ 1–0 line using the Nobeyama Millimeter Array (NMA). The molecular cloud mainly has three different spatial components with large velocity widths of up to 60 kms$^{-1}$. The northwest component is located at an apparent contact point to the Sgr A east shell, and elongated along the boundary of the shell. The large velocity width of the component is responsible for interaction with the Sgr A east shell. The molecular gas distribution in CS line emission is dissimilar to that observed previously in NH$_3$ line emissions. The appearance shows presumably the area of CS line emission enhanced by shock. The central and southwest components are located just out of the Sgr A east shell and far from it, respectively. However, these components have large velocity widths. We found a well-shaped circular molecular shell with expanding motion in the 50-km s$^{-1}$ molecular cloud. This is responsible for the large velocity width. The continuum source in the expanding molecular shell has a steep spectrum in mm-wavelengths, although this was not identified in a previous 5-GHz map. This source may be an SNR with an ionized sheath. From the aspect ratio of an expanding molecular shell of 1.1, the magnetic field in/around the shell is estimated to be smaller than 100$\mu$Gauss. This weak magnetic field is consistent with on-going active star formation in the 50-km s$^{-1}$ molecular cloud. A comparison among CS line emission, low-frequency continuum, and millimeter continuum toward the 50-km s$^{-1}$ molecular cloud suggests a face-on view of the Sgr A region. The molecular cloud is located in the Sgr A halo region.

The Once and Future Andromeda Stream

We study the interaction between an accreting satellite and the Andromeda galaxy (M31) analytically and numerically, using a high-resolution N-body simulation with 4 × 107 particles. For the first time, we show the self-gravitating response of the disk, the bulge, and the dark matter halo of M31 to an accreting satellite. We reproduce the stream and the shells at the east and west sides of M31 by following the evolution of the collision 4 Gyr into the future, and show that recently discovered diffuse arcs on the south side of the minor axis are the remnants of a similar collision that occurred 3-4 Gyr earlier than the stream event. The present-day integrity of the M31 disk constrains the satellite mass to be M ≤ 5 × 109 M☉. The stars that were originally in the center of the satellite are now in the east shell. Therefore, observations in this region might reveal additional clues about the nature of satellite, such as the central core and any metallicity gradient.

Interaction between the SNR Sagittarius A East and the 50-km s−1 Molecular Cloud

We performed high-resolution observations of the Galactic Center 50-km s−1 molecular cloud in the CS J = 1 - 0 line using the Nobeyama Millimeter Array. The 50-km s−1 molecular cloud corresponds to a break in the Sagittarius (Sgr) A east shell. A very broad and negative velocity wing feature is detected at an apparent contact spot between the molecular cloud and the Sgr A east shell. The velocity width of the wing feature is over 50-km s−1. The width is three times wider than those of typical Galactic Center clouds. This strongly suggests that the shell is interacting physically with the molecular cloud. The asymmetric velocity profile of the wing feature indicates that the Sgr A east shell expands and crashes into the far side of the molecular cloud. About 50 clumps are identified in the cloud using CLUMPFIND. The velocity width-size relation and the mass spectrum of clumps in the cloud are similar to those in Central Molecular Zone (CMZ).

Absorption and Emission in the Four Ground-State OH Lines Observed at 18 cm with the VLA Towards the Galactic Centre

The OH distribution in the Sgr A Complex has been observed in the 1612, 1665, 1667, and 1720 MHz OH transitions with the Very Large Array (VLA), in the BnA and DnC configurations. Spectral line maps have been produced with a channel velocity resolution of about 9 km s–1 and with angular resolutions of 4″ × 3″, and 24″ × 22″, respectively. Some clear results are highlighted here: i) the existence of an OH streamer inside the Circumnuclear Disk (CND) near Sgr A*, ii) absorption from the CND, iii) strong absorption towards the eastern and most of the western parts of the Sgr A East shell, iv) lack of absorption towards both Sgr A West and the compact H II-regions to the east of Sgr A East, v) a double-lobed structure of the High Negative Velocity Gas (HNVG) oriented to the northeast and southwest of Sgr A*, and vi) compact point-like maser emission in all four transitions. In particular, a 1720 MHz maser at –132 km s–1 in the CND as counterpart to a 1720 MHz maser at +132 km s–1 in the CND, was observed.

High Spectral and Spatial Resolution Observations of Shocked Molecular Hydrogen at the Galactic Center

The presence of OH (1720 MHz) masers and the absence of counterparts at 1665/1667 MHz has proved to be a clear diagnostic of shocked molecular gas associated with Galactic supernova remnants. This suggests that shocked molecular gas should be associated with the OH (1720 MHz) masers that have been detected in the circumnuclear disk (CND) and Sgr A East at the Galactic center. In order to test this hypothesis, we observed the H2 1-0 S(1) and Br? lines using NICMOS on the Hubble Space Telescope (HST) and the University of New South Wales Infrared Fabry-Perot (UNSWIRF) etalon at the Anglo-Australian Telescope (AAT), near the regions where OH (1720 MHz) masers are detected in the CND and Sgr A East. We present the distribution of H2 in the north and south lobes of the CND and in Sgr A East. H2 emission accompanies almost all the maser spots detected at the Galactic center. In particular, we find a striking filamentary structure near the northwest of the CND and evidence that shocked molecular gas is associated with the 70 km s-1 molecular cloud at the Galactic center. We argue that the emission from the CND could arise in gas heated by the dissipation of the random motion of clumps by collisions or the dissipation of turbulence in a more homogeneous medium. In addition, highly redshifted gas of up to 140 km s-1 close to the eastern edge of the Sgr A East shell is detected. These observations combined with OH (1720 MHz) results suggest that the H2 gas is shocked and accelerated by the expansion of Sgr A East into the 50 and the 70 km s-1 clouds and into the lobes of the CND.

The compression of the M-0.02-0.07 molecular cloud by the Sagittarius A East shell source

The «50 km s −1 » molecular cloud (M-0.02-0.07) near the Galactic center has been mapped in the CS J=7-6 and 5-4 rotational transitions. In addition, mid-infrared fine-structure lines of [Ne II], [Ar III], and [S IV] have been observed toward the compact H II regions located near the cloud's core.

HI Absorption Against the 40 km s−1Cloud and the Circum-nuclear Disk in Sgr A

HI absorption studies of the Sgr A complex based upon recent VLA2observations, show that the 40 km s−1molecular cloud liesbehindthe Sgr A West region, and that the cloud may be physically associated with the Sgr A East non-thermal shell. The absorbing gas over the range +30 to +60km s−1lies mainly between the eastern edge of the Sgr A East shell and the western boundary of a cluster of HII regions. There is a pronounced velocity gradient over this region, and at 40 km s−1similar morphology is apparent between the distribution of atomic hydrogen and the edge of the Sgr A East shell. The physical association of the 40 km s−1cloud and the Sgr A East shell, and the placing of the 40 km s−1cloud behind Sgr A West, are consistent with recent evidence which has demonstrated that Sgr A West liesinfrontof Sgr A East. The HI optical depth maps of the circum-nuclear disk plus a new HI feature associated with the northern extension of Sgr A West will be presented.