Mass Distribution at the Galactic Centre

In the framework of this PhD thesis, the study of the distribution of an extended mass close to the super-massive black hole (SMBH) at the centre of our Galaxy is addressed using observational data and theoretical modelling. The main emphasize is on establishing a distinction between the fraction of dark mass present in the form of a black hole and that in an extended form. Despite the significant observational and theoretical progress in the understanding of SMBHs in the last ten years, the formation of SMBHs and the interplay with their host galaxy are still poorly understood. The work presented here extends our understanding of the dynamics in the vicinity of the SMBH. Already in 1974, it was proposed that the radio source SgrA* could be a SMBH. With observations during the following years, it became more clear that the centre of our Galaxy hides an amount of dark mass close to 3 million solar masses. However, the strongest evidence for the existence of a SMBH, was only after it became possible to trace for the first time stellar orbits of fast moving stars, the so-called S-stars, around SgrA*. This was achievable using the SHARP near-infrared speckle camera at la Silla in Chile as well as the near-infrared camera NAOS/CONICA at the Very Large Telescope (VLT) at Paranal in Chile. In collaboration with the near-infrared group at the Max-Planck-Institut for Extra-terrestrial Physics in Garching by Munich, I used the imaging data on the Galactic Centre to study the stellar distribution, and used proper motion and radial velocity data to study the motion of the S-stars at the Galactic Centre. In order to investigate the distribution of mass around the black hole, in this work, stellar orbits are modelled assuming an extended mass potential, in addition to the potential of the black hole. I study the orbits in both Keplerian and non-Keplerian potentials. This is the first study of this kind combining observational data and theoretical modelling. This work shows also the first approach where the mass-to-light ratio (M/L) is considered to be varying from the outer regions to the inner regions of the galaxy. From earlier dynamical studies on galaxy evolution, it is widely agreed upon the M/L increasing in the inner regions of a galaxy. This implies that there could exist an additional quantity of hidden mass close to the centre. Here, I study explicitely possible candidates for this hidden component - faint, low-mass stars and heavier compact remnants. In practice, meaningful analysis is only possible on the star S2 which shows the shortest orbital period and therefore the most complete orbit of all other S-stars. A fourth-order Hermite integrator, which I optimized for this study, is used to model the S2-orbit in response to the SMBH potential as well as an additional extended mass distribution. A grid method is implemented to the Hermite scheme to fit the S2 orbit to the positional and velocity data available from the year 1992 till 2003. I was able to confirm the position of the black hole candidate SgrA*, to determine an upper limit on the total amount of mass that could be present in the central 20 mpc and extended into the outer regions of the central stellar cluster, as well as to deduce an upper limit on the M/L. This work leads to the results that the total central dark mass is not necessarily confined entirely in a SMBH. A fraction <20% of this mass could be present in an extended distribution. Testing different distributions for this extended mass component, it turned out that the present data do not suffice to discriminate between the different potentials. In order to study the constituents of the possible extended dark component, I investigate the K-band luminosity function (KLF) of the observed stellar cluster. The result is that, the extended mass cannot be formed only by faint, low-mass stars. The presence of stellar black holes and neutron stars is required in order to account for the possible extended mass fraction in the centre of the Milky Way. Furthermore, such a cluster of stellar remnants analysed in an approximative analytical form, is found to be stable

Default Parallels: The Science Potential Of JWST Parallel Observations During TSO Primary Observations

The James Webb Space Telescope (JWST) will observe several stars for long cumulative durations while pursuing exoplanets as primary science targets for both Guaranteed Time Observations (GTO) and very likely General Observer (GO) programs. Here we argue in favor of an automatic default parallels program to observe e.g., using the F200W/F277W filters or grism of NIRCAM/NIRISS in order to find high redshift (z >> 10) galaxies, cool red/brown dwarf-sub-stellar objects, Solar System objects, and observations of serendipitous planetary transits. We argue here the need for automated exploratory astrophysical observations with unused JWST instruments during these long duration exoplanet observations. Randomized fields that are observed in parallel mode reduce errors due to cosmic variance more effectively than single continuous fields of a typical wedding cake observing strategy (Trenti & Stiavelli 2008). Hence, we argue that the proposed automated survey will explore a unique and rich discovery space in high redshift Universe, Galactic structure, and Solar System. We show that the GTO and highly-probable GO target list of exoplanets covers the Galactic disk/halo and high redshift Universe, mostly well out of the plane of the disk of the Milky Way. Exposure times are of the order of the CEERS GTO medium deep survey in a single filter, comparable to CANDELS in HST's surveys and deep fields. The area covered by NIRISS and NIRCam combined could accumulate to a half square degree surveyed.Comment: 19 pages, 5 figures, 2 tables, accepted for publication by PAS

Default parallels: The science potential of JWST parallel observations during TSO primary observations

The James Webb Space Telescope (JWST) will observe several stars for long cumulative durations while pursuing exoplanets as primary science targets for both Guaranteed Time Observations (GTO) and very likely General Observer (GO) programs. Here we argue in favor of an automatic default parallel program to observe, e.g., using the F200W/F277W filters or grism of NIRCAM/NIRISS in order to find high redshift (z (Formula Presented) 10) galaxies, cool red/brown dwarf substellar objects, solar system objects, and observations of serendipitous planetary transits. We argue here the need for automated exploratory astrophysical observations with unused JWST instruments during these long-duration exoplanet observations. Randomized fields that are observed in parallel mode reduce errors due to cosmic variance more effectively than single continuous fields of a typical wedding cake observing strategy. Hence, we argue that the proposed automated survey will explore a unique and rich discovery space in the high-redshift universe, Galactic structure, and solar system. We show that the GTO and highly probable GO target list of exoplanets covers the Galactic disk/halo and high redshift universe, mostly well out of the plane of the disk of the Milky Way. Exposure times are of the order of the CEERS GTO medium-deep survey in a single filter, comparable to CANDELS in Hubble Space Telescopeʼs surveys and deep fields. The area covered by NIRISS and NIRCam combined could accumulate to a half square degree surveyed

Mercury's Exosphere During MESSENGER's Second Flyby: Detection of Magnesium and Distinct Distributions of Neutral Species

During MESSENGER's second Mercury flyby, the Mercury Atmospheric and Surface Composition Spectrometer observed emission from Mercury's neutral exosphere. These observations include the first detection of emission from magnesium. Differing spatial distributions for sodium, calcium, and magnesium were revealed by observations beginning in Mercury's tail region, approximately 8 Mercury radii anti-sunward of the planet, continuing past the nightside, and ending near the dawn terminator. Analysis of these observations, supplemented by observations during the first Mercury flyby as well as those by other MESSENGER instruments, suggests that the distinct spatial distributions arise from a combination of differences in source, transfer, and loss processes

Mercury's Na exosphere as seen with very high spectral resolution from the ground, and from space with MESSENGER

International audienceThe MErcury Surface, Space ENvironment, GEochemistry, and Ranging NASA&amp;#8217;s spacecraft, known as MESSENGER, flew by Mercury on September 29, 2009. It was the spacecraft&amp;#8217;s third and final flyby, before it went into orbit around the planet. The flyby presented a unique trajectory approach and perspective on the planet&amp;#8217;s exosphere, not available when in orbit. We present very high spectral resolution ground-based data obtained at the&amp;#160;University of Texas&amp;#160;McDonald 2.7-m telescope. These data were acquired within hours of the data taken with the Ultraviolet and Visible Spectrometer (UVVS) onboard MESSENGER. Both datasets targeted similar spatial regions, in the polar altitudes of Mercury. We compare the sodium emissions from both measurements in the exosphere. We find that close to the surface, both intensity measurements match, but the intensities fall off differently with altitude, with the MESSENGER data showing an exponential drop off, sharper than that of the ground-based data; an effect that we attribute to atmospheric seeing. In addition, our ground-based data provided Full Width Half Maximum (fwhm) speeds and Doppler shift speeds; our results suggest energetic processes took place in the polar regions on the dusk side of the planet, but arguably on the dawn side as well. We confirm previous conclusions of Leblanc et al. (2008, 2009) where signatures of energetic processes seem to be coupled with high fwhm speeds and intensity peaks. We compare our Doppler shift velocities with previous works, and find agreement within the uncertainties with Potter et al., (2013) on their transit velocity measurements. Although our peak emissions along the terminator vary in structure and in brightness, they do not exhibit distinctive signatures in the intensity profiles at altitudes above the poles, when compared with convolved MESSENGER space data

Messenger Observations Of Mercury\u27S Exosphere: Detection Of Magnesium And Distribution Of Constituents

Mercury is surrounded by a tenuous exosphere that is supplied primarily by the planet\u27s surface materials and is known to contain sodium, potassium, and calcium. Observations by the Mercury Atmospheric and Surface Composition Spectrometer during MESSENGER\u27s second Mercury flyby revealed the presence of neutral magnesium in the tail (anti-sunward) region of the exosphere, as well as differing spatial distributions of magnesium, calcium, and sodium atoms in both the tail and the nightside, near-planet exosphere. Analysis of these observations, supplemented by observations during the first Mercury flyby, as well as those by other MESSENGER instruments, suggests that the distinct spatial distributions arise from a combination of differences in source, transfer, and loss processes. © 2009 by the American Association for the Advancement of Science

Monte Carlo Modeling of Sodium in Mercury's Exosphere During the First Two MESSENGER Flybys

We present a Monte Carlo model of the distribution of neutral sodium in Mercury's exosphere and tail using data from the Mercury Atmospheric and Surface Composition Spectrometer (MASCS) on the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft during the first two flybys of the planet in January and September 2008. We show that the dominant source mechanism for ejecting sodium from the surface is photon-stimulated desorption (PSD) and that the desorption rate is limited by the diffusion rate of sodium from the interior of grains in the regolith to the topmost few monolayers where PSD is effective. In the absence of ion precipitation, we find that the sodium source rate is limited to approximately 10(exp 6) - 10(exp 7) per square centimeter per second, depending on the sticking efficiency of exospheric sodium that returns to the surface. The diffusion rate must be at least a factor of 5 higher in regions of ion precipitation to explain the MASCS observations during the second MESSENGER f1yby. We estimate that impact vaporization of micrometeoroids may provide up to 15% of the total sodium source rate in the regions observed. Although sputtering by precipitating ions was found not to be a significant source of sodium during the MESSENGER flybys, ion precipitation is responsible for increasing the source rate at high latitudes through ion-enhanced diffusion

Constraints on Mercury's Na Exosphere: Combined MESSENGER and Ground-Based Data

We have used observations of sodium emission obtained with the McMath-Pierce solar telescope and MESSENGER's Mercury Atmospheric and Surface Composition Spectrometer (MASCS) to constrain models of Mercury's sodium exosphere, The distribution of sodium in Mercury's exosphere during the period January 12-15. 2008. was mapped using the McMath-Pierce solar telescope with the 5" X 5" image slicer to observe the D-line emission. On January 14, 2008, the Ultraviolet and Visible Spectrometer (UVVS) channel on MASCS sampled the sodium in Mercury's anti-sunward tail region. We find that the bound exosphere has an equivalent temperature of 900-1200 K, and that this temperature can be achieved if the sodium is ejected either by photon-stimulated desorption (PSD) with a 1200 K Maxwellian velocity distribution, or by thermal accommodation of a hotter source. We were not able to discriminate between the two assumed velocity distributions of the ejected particles for the PSD. but the velocity distributions require different values of the thermal accommodation coefficient and result in different upper limits on impact vaporization, We were able to place a strong constraint on the impact vaporization rate that results in the release of neutral Na atoms with an upper limit of 2.1 x 10(exp 6) sq cm/s, The variability of the week-long ground-based observations can be explained by variations in the sources, including both PSD and ion-enhanced PSD, as well as possible temporal enhancements in meteoroid vaporization. Knowledge of both dayside and anti-sunward tail morphologies and radiances are necessary to correctly deduce the exospheric source rates, processes, velocity distribution, and surface interaction

Mercury's Complex Exosphere: Results from MESSENGER's Third Flyby

During MESSENGER's third flyby of Mercury, the Mercury Atmospheric and Surface Composition Spectrometer detected emission from ionized calcium concentrated 1 to 2 Mercury radii tailward of the planet. This measurement provides evidence for tailward magnetospheric convection of photoions produced inside the magnetosphere. Observations of neutral sodium, calcium, and magnesium above the planet's north and south poles reveal attitude distributions that are distinct for each species. A two-component sodium distribution and markedly different magnesium distributions above the two poles are direct indications that multiple processes control the distribution of even single species in Mercury's exosphere