Osservatorio Astronomico di Roma

Electromagnetic Counterparts of Gravitational Waves
2018-02-13 11:45 - 12:45 (Europe/Rome) | Aula Gratton
Compact binary systems composed by neutron stars (NS) and black holes (BH) are one of the principal sources of gravitational waves (GW) in the terrestrial interferometers frequency range (10 Hz -1 kHz). While BH binary coalescences are not expected in general to be accompanied by any electromagnetic signal, detectable electromagnetic counterparts, such as Short Gamma-Ray Bursts (SGRB) and Kilonovae, could follow binary NS and NS-BH mergers.
The recent join GW and electromagnetic observation of a binary NS coalescence (GW170817) marked the down of multimessenger astronomy era that open a new way to explore the high energetic universe and test the fundamental physics under extreme conditions.
The coincident detection of gravitational and electromagnetic radiation is important because these messengers carry different and complementary informations: while from a GW detection it is possible to infer the masses and the spins of the binary progenitors, electromagnetic emission is essential to localize the source, thus enabling the host galaxy identification.
In this talk I will show how a joint detection of SGRB and GW from a NS-BH progenitor could be used to measure the neutron star radius and thus constrain the dense matter equation of state.
I will also provide a brief description of isotropic electromagnetic counterpart of compact binary coalescence such as Kilonova and Spindown powered transients.

A Panchromatic View of High-Redshift Quasar Host Galaxies
2018-02-06 11:45 - 12:45 | Aula Gratton
While it is now widely accepted that the growth of supermassive black-holes in galaxies is fundamentally linked to the process of galaxy formation, the host galaxies of the most luminous and massive accreting black-holes or quasars are still poorly understood. Quasars out-shine their host galaxies at most wavelengths making these studies difficult, and the space-density of luminous quasars is also such that very wide-field surveys are required to be able to assemble statistical samples. I will present new observations of the host galaxies of a population of heavily reddened, luminous quasars at redshifts of 2-3. Exploiting wide-field optical surveys together with the dust obscuration towards the quasar line-of-sight, we are, for the first time, able to probe the rest-frame UV emission from the star-forming hosts of these high-luminosity quasars. Observations of the quasar host galaxies at sub-millimetre and millimetre wavelengths with SCUBA2, ALMA and the VLA are also revealing complex gas morphologies, diverse interstellar medium properties and evidence for significant over-densities and major-mergers.

The wee free magnetic elements on the photosphere: a turbulent story
2018-01-30 08:00 - 17:00 | Aula Gratton
The study of the dynamics of the photosphere allow us to investigate the physical processes occurring in both the interior of the Sun and in the higher layers of the solar atmosphere due the magnetic coupling between the photosphere and the corona. This field concerns many basic stellar processes such as: global dynamo, turbulent convection and super-hot corona.
In this talk, we report on the recent results on the transport of small scale magnetic field by advection/diffusion as seen on different data-sets from ground-based and space telescopes. We compare those results with simplified advective models that mimic the motion scales observed on the solar surface.
We will digress on how the advection of the smallest magnetic elements on the solar surface affects the toroidal-to-poloidal conversion in the solar cycle, on how it can be used to estimate the possible nano-flare contribution to the coronal heating and on its consequences on transiting exo-planet detection.

Modern core-collapse supernova nucleosynthesis
2018-01-25 11:45 - 12:45 | Aula Gratton
Massive stars and their core-collapse supernovae are key contributors to the evolution and chemical enrichment of galaxies. Despite over 50 years of research, the explosion mechanism of core-collapse supernovae is still not fully understood, which hinders nucleosynthesis predictions. In a previous, proof-of-principle study, we have established a modern method to induced explosions in spherically symmetric (otherwise non-exploding) simulations to calculate the nucleosynthesis. This method (PUSH method) includes two aspects that are crucial for nucleosynthesis: One, the mass cut is emerging from the simulation, consistently with the explosion energy. And two, neutrino-interactions with matter are included. In this talk, I will discuss the nucleosynthesis predictions from our PUSH models and put the results in the context of other nucleosynthesis predictions

Deep, multi-band photometry of low-mass stars to reveal young clusters: a blind study of the NGC 2264 region
2018-01-23 11:45 - 12:45 | Aula Gratton
The LSST survey will provide multi-epoch, multi-wavelength (u,g,r,i,z,y) mapping of the Southern Hemisphere, with a single-visit depth of r~24.5 and a gain of three magnitudes by the end of the program. This unprecedented spatial coverage will enable detection of young, pre-main sequence stars and stellar clusters down to distances of 5-10 kpc. A crucial and challenging step for spatial analyses of large stellar populations is measuring the extinction Av of individual objects. Multi-color photometry on a (r-i, g-r) or (i-J, r-i) diagram offers a direct solution to this issue for M-type stars: indeed, while the color locus of early-type (< K7) stars on these diagrams is parallel to the reddening vector, the color locus traced by M-type stars is tilted with respect to the reddening vector, which enables a straightforward and empirical measurement of their Av. By investigating the correlation between extinction and spatial properties of M-type stars in a given field, it is therefore possible to reconstruct the structure of the region and probe the nature of its population. In this study, we test the method on the NGC 2264 field. We selected a 2°x2° area centered on the NGC 2264 cluster, and collected the available r,i,J photometry from existing large-scale surveys (notably Pan-STARRS and UKIDSS). Then, assuming no prior knowledge on the nature of stars in the field, we used the (i-J, r-i) diagram to identify and deredden M-type stars in the sample, and the (r-i, r) + (RA, Dec) diagrams to investigate the nature and spatial distribution of stars as a function of their Av. We derived a non-uniform distribution of Av across the region, and could distinguish between a diffuse field population and a clustered stellar population toward the center of the field. An a posteriori comparison between the inferred spatial density map of the clustered population and the literature census of the NGC 2264 cluster enabled us to assess the performance of the method and its predictive capability.

Rossby number calculations for fully convective stars
2018-01-16 11:45 - 12:45 | Aula Gratton
The Rossby number is a quantity that gauges the relative importance of
rotation in astrophysical flows. In particular, it plays a significant role
in mean-field, alpha-Omega dynamo models as it is directly related to the
“dynamo number” which, in turn, measures the efficiency of the magnetic field
generation. From an observational standpoint, the Rossby number has been
widely used to correlate either coronal or chromospheric magnetic activity of
low-mass stars with their rotation. A fundamental problem with this
rotation-activity relationship is the determination of the convective turnover
time tau_c, on which the Rossby number depends, and that can be obtained
from some empirical relations or from stellar models. The ATON stellar
evolution code has been used in the near past to calculate Rossby numbers
for solar-type stars and now, in view of the recent discovery of fully
convective stars in the non-saturated region of the rotation-activity
relationship, is being used to extend those calculations to very low mass
stars.

Not all stars are the Sun: rethinking the Mixing Length
2018-01-12 11:45 - 12:45 | Aula Gratton
Characterizing heat transport in stars is notoriously complicated, and the task of reproducing the physics involved with high precision on stellar evolutionary timescales is a long-standing problem in stellar modeling. For this reason, convection in 1-D stellar evolution codes is addressed primarily through a framework known as mixing length theory (MLT). In this simplified picture, the efficiency of convective heat transport is encapsulated by the mixing length parameter alpha_{MLT}, measured in pressure scale heights.
While it is well understood that our nearest star is not a valid representation of stars in general, it remains the standard procedure to calibrate alpha_{MLT} according to solar specifications, and then to apply this value in any stellar model, regardless of mass or composition. However, there has been growing evidence that the use of a solar-calibrated mixing length is not always appropriate.
In this study, we investigate the scope of validity of a solar-calibrated mixing length over a range of evolutionary phases and input physics for very metal-poor stars, and find that the solar-calibrated mixing length is widely ineffective at reproducing the observed properties of such stars.

Microlensing
2018-01-09 11:45 - 12:45 | Aula Gratton
I will review the microlensing and pixel lensing techniques and show how they play a
fundamental role in the searches for galactic dark matter, in the study of the galactic structure and in
detecting planets around faint stars whose cahacteristics are in general not accessible to other methods.
We show that dedicated and well planned experiments would even allow to detect more exotic objects
as primordial massive black holes (if any) in nearby galaxies and/or to put strong constraints on
their existence.

Dust Production from Evolved Stars in the Local Group
2017-12-19 11:45 - 12:45 | Aula Gratton
Infrared observations of nearby galaxies and the Milky Way show that there are two main sources of ISM dust: the winds of evolved stars and supernovae ejecta. However, the total dust contribution from evolved stars relative to supernovae, and how it changes with metallicity, is less certain. Infrared photometric and spectroscopic Spitzer Surveys of the Large and Small Magellanic Clouds (LMC, SMC): Surveying the Agents of Galaxy Evolution (SAGE) resulted in the discovery of thousands of evolved stars. Here, I will describe how the composition and quantity of dust produced by these stars depends on metallicity. I will also discuss how the mid-IR stellar populations of the Magellanic Clouds can be used as a template for potential observations with JWST, and how we have applied this to our observing programs of Local Group galaxies and SN1987A with JWST

High contrast imaging and the search for young exo-planets
2017-12-12 11:45 - 12:45 | Aula Gratton
High contrast imaging has enormously progressed in the last twenty years, thanks to the development of extreme AO, efficient coronagraph, and various differential imaging techniques. The two frontier instruments are GPI at Gemini South and SPHERE at VLT. We are now routinely achieving contrast of better than 10^5 with a record of 4×10^6 at separations below 0.5 arcsec, This has allowed the discovery and characterization of about a dozen young planets, of a couple of dozens of brown dwarfs, and direct detection of several tens disks in scattered light. These data open new windows on the very early phases of planet formation, when disks are still present. Both theory and data indicate that while clearly planets form in the late phase of the protostellar disks, the relation between disk and planets is very complex. Also, the spectra we are obtaining for the young planets indicate systematic differences between young and old objects with similar temperature. I will briefly present the technological and scientific aspects of this very new and exciting science

Mass loss of AGB stars and RSG in the Magellanic Clouds
2017-12-05 11:45 - 12:45 | Aula Gratton
I will discuss recent work that I am involved in regarding the determination of
the mass-loss rates (MLRs) of a significant sample of Asymptotic Giant Branch stars and
Red Supergiants in the Magellanic Clouds. This mostly involves modelling the
spectral energy distributions to obtain dust MLRs, but we also used ALMA to detect CO emission
in a few LMC objects. I will also discuss possible ways forward in this field

Oumuamua, the first asteroid coming from another planetary system
2017-11-30 11:45 - 12:45 | Aula Gratton
The discovery of 1I/’Oumuamua, the first interstellar object found
transiting in our Solar System, provided astronomers with an extremely
unusual new target, and with a few surprises. We will briefly present
the discovery story and the unique properties of this new temporary
visitor of our Solar System.

Oumuamua, the first asteroid coming from another planetary system
2017-11-30 11:45 - 12:45 | OAR Monte Porzio Catone – aula Gratton, Via Frascati 33, Roma, RM, 00136, Italia

The other 95%: insights from strong gravitational lensing
2017-11-28 11:45 - 12:45 | Aula Gratton
In the standard cosmological model ninety-five percent of the energy content of the universe consists of dark energy and dark matter. Even though their abundance seems well determined, very little is known about their fundamental nature. I will describe how we can learn about the physics of the dark sector by studying in detail its gravitational effect on the trajectories of photons as they travel across the universe, a phenomenon known as strong gravitational lensing. In the first part of the talk, I will use strong gravitational lenses with a time variable background source to measure the expansion rate of the universe (Hubble constant) to 3.8% precision. This result is completely independent of the local distance ladder and the cosmic microwave background, and thus provides a new opportunity to understand whether the tension between the two arises from systematic uncertainties or may be indicative of new physics. In the second part of the talk I will describe how strong lensing can be used to detect the presence of dark subhalos independent of their stellar content. This measurement tests a fundamental prediction of the cold dark matter model, i.e. that galaxies should be surrounded by large numbers of dark satellite subhalos. Proof that such satellites do not exist would force a revision of the model in favor of more exotic alternatives like warm dark matter. I will conclude by discussing the exciting future of strong lensing as a tool for cosmology, in light of the huge numbers of strong lenses that are being discovered in the current generation of wide field astronomical surveys.

Fibrillar Structures in the Solar Chromosphere
2017-11-21 11:45 - 12:45 | Aula Gratton
I present observations of dynamic, slender bright fibrils seen in high-quality narrow-band Ca II H images from the SuFI instrument onboard the SUNRISE balloon-borne solar observatory. We have shown that these slender Ca II H fibrils (SCFs) map magnetic fields in the low solar chromosphere derived from magnetostatic extrapolation of the photospheric field obtained with SUNRISE/IMaX and SDO/HMI. Our analysis reveals the prevalence of both kink and sausage waves in the SCFs, propagating at high-frequencies (up to 30 mHz), with speeds on the order of 9-15 km/s. The estimated energy flux (~ 15 kW/m^2) carried by the observed waves is marginally enough to heat the chromosphere (and perhaps the corona). Characteristics of these waves differ from those reported for other fibrillar structures, which, however, were observed mainly in the upper solar chromosphere.
Furthermore, I present observations of fibrillar structures through the mid-to-high solar chromosphere from coordinated observations of an active region with the Interface Region Imaging Spectrograph (IRIS) explorer and the Atacama Large Millimeter/submillimeter Array (ALMA). This provides us with temperature distributions of the entire field of view at multiple chromospheric heights, and in particular, of the fibrillar structures. A detail comparison between fibrillar structures simultaneously observed at several millimetre and ultraviolet bands of, e.g., ALMA 1.3 mm as well as IRIS Mg II h & k, Si IV, C II, and O I has provided us with new insights into the nature of these thread-like structures.
