Observing planet formation

Planets are thought to form in the circumstellar disks orbiting young stars in formation. According to the core-accretion model, a candidate scenario for Earth-like planets, the interstellar sub-$\mu$m-sized dust particles grow thanks to collisions to mm/cm size and then form km-sized planetesimals via dynamical encounters. Eventually, the rocky planetary cores accrete gas and, depending on the total gas mass attained, a terrestrial planet or a gas giant forms. Modern sub-mm/mm/radio interferometers such as ALMA and VLA detect the thermal emission of dust grains and provide us with an unprecedented sharp view of protoplanetary disks at the spatial scales where planet formation occurs. In recent years, evidence of grain growth in disks has been obtained by extensive sub-mm/mm photometric studies, but so far they only provided disk-averaged estimates of the dust properties. Moreover, the derivation of dust properties from the observed spectral index was done under reasonable - but simplifying - assumptions rather than with a proper modeling of the disk emission. The thesis presents an analysis method that enables - for the first time - the disk structure and the dust properties to be constrained simultaneously by fitting multi-wavelength observations with a self-consistent physical model. The thesis presents also an accelerated version of the computer code that uses modern graphics cards and provides the computational breakthrough needed to exploit the new wealth of information now available. Applying the multi-wavelength analysis to observations of three disks in the Taurus and Ophiuchus star-forming regions, a key result is a radial gradient in the grain-size distribution, with large grains of up to $1\,\mathrm{cm}$ size confined to the inner disk and smaller grains of size $\ll 1\,\mathrm{mm}$ populating the whole disk. Similar results hold for another disk, HD~163296, where in addition the grain size radial profile supports the scenario of enhanced grain growth at the snowline location of the second most abundant volatile in disks, CO. The tool developed in the thesis is also designed to accelerate the analysis of high-resolution observations for demographic studies. By applying the analysis tool to an ALMA disk survey in the Lupus star-forming region, the physical structure of more than 20 disks is obtained, in particular the disks's size and dust mass among other physical parameters. To date, this is the largest sample of disks of the same star-forming region fitted homogeneously with a self-consistent model. Remarkably, the sample is complete in the mass range of 0.7$M_\odot$ to one $M_\odot$. The results are compatible with previous studies based on simpler analyses but also highlight a consistent difference in the disks's luminosity-size correlation between the older ($\sim3\,\mathrm{Myr}$) Lupus and the younger ($\sim1-2\,\mathrm{Myr}$ old) Taurus-Auriga region. The application of the analysis developed in this thesis to multi-wavelength observations of large samples of disks with ALMA will allow us to spatially resolve the early growth of solids in numerous protoplanetary disks, and therefore to provide measurements that will be crucial to inform, test, and refine theoretical models of planet formation.Die Entstehung von Planeten und Sternen ist eng miteinander verknüpft. Der Stern bildet sich im Zentrum einer rotierenden Materiescheibe. Die Planeten entstehen wiederum in der zirkumstellaren Scheibe. Das Kern-Akkretions-Modell beschreibt die allmähliche Entstehung von Planeten in folgender Weise: Interstellare Staubteilchen mit Größen im Submillimeterbereich wachsen durch Kollisionen auf eine Größe von Millimetern bzw. Zentimetern heran. Sie stoßen wieder zusammen und bilden im weiteren kilometergroße Planetesimale. Schließlich akkretieren die felsartigen Planetenkerne Gas und bilden dann, je nach akkretierter Gasmasse, einen erdähnlichen Planeten oder einen Gasriesen. Moderne Interferometer mit Wellenlängen von Submillimeter über Millimeter bis in den Radiobereich wie das Atacama Large Millimetre Array (ALMA) oder das Very Large Array (VLA) detektieren die thermische Emission von Staubkörnern und erlauben eine nie dagewesene Auflösung von protoplanetaren Scheiben bis auf Längenskalen, auf denen sich die Planetenbildung ereignet. In den letzten Jahren haben ausführliche photometrische Studien im Submilli\-meter- und Millimeter-Wellenlängenbereich Hinweise auf Kornwachstum in Scheiben geliefert, allerdings nur gemittelt über die gesamte Scheibe. Zudem wurde die Ableitung der Staubeigenschaften vom beobachteten spektralen Index unter plausiblen, aber stark vereinfachenden, Annahmen durchgeführt. In Rahmen dieser Dissertation wurde eine Analysemethode entwickelt, die es zum ersten Mal erlaubt, gleichzeitig die Struktur der Scheibe und die Eigenschaften des Staubs durch eine Anpassung eines selbstkonsistenten, physikalischen Modells an die Beobachtungen in mehreren Wellenlängenbereichen zu ermitteln. Außerdem wird eine neue Version eines Computercodes präsentiert, die durch die Verwendung moderner Grafikkarten viel schneller ist. Das stellt einen Durchbruch in der Rechenleistung dar, der erforderlich ist, um die riesigen, aktuell verfügbaren Datenmengen zu bewältigen. In der Anwendung der Multiwellenlängen-Analyse auf Beobachtungen dreier Scheiben in Sternentstehungsregionen der Sternbilder Stier (Taurus) und Schlangenträger (Ophiuchus) zeigt sich ein radialer Gradient in der Verteilung der Korngröße. Dabei sind große Körner von bis zu einem Zentimeter Größe auf die innere Scheibe beschränkt. Dagegen sind Körner, die viel kleiner sind als ein Millimeter, in der gesamten Scheibe zu finden. Ähnliche Ergebnisse betreffen eine andere analysierte Scheibe in HD 163296. Dort gilt zusätzlich, dass das Radialprofil der Korngröße ein Szenario unterstützt, in dem verstärktes Kornwachstum genau dort auftritt, wo der zweithäufigste, flüchtige Stoff in Scheiben, nämlich Kohlenmonoxid (CO), gefriert. Das Computerprogramm, das im Rahmen der Dissertation entwickelt wurde, dient auch zur Beschleunigung der zwölf Analysen von hochaufgelösten Beobachtungen in Studien ganzer Populationen von Sternen mit protoplanetaren Scheiben. Konkret wurde das Programm auf mit ALMA beobachtete Scheiben in einer Sternentstehungsregion im Sternbild Wolf (Lupus) angewendet. Daraus wurde die physikalische Struktur von mehr als zwanzig Scheiben abgeleitet. Neben anderen physikalischen Parametern wurden ihre Größen und Staubmassen bestimmt. Bis jetzt ist dies die größte Anzahl von Scheiben aus der gleichen Sternentstehungsregion, die je einheitlich mit einem selbstkonsistenten Modell betrachtet wurde. Es ist bemerkenswert, dass dieser Satz an Scheiben im Massenbereich von 0,7 bis 1 Sonnenmassen und im Strahlungsfluss --- integriert über den Submillimeterbereich --- vollständig ist. Die Ergebnisse sind im Einklang mit vorherigen Arbeiten, die auf einfacheren Analysen beruhten. Allerdings zeigen sie auch einen klaren Unterschied in der Korrelation zwischen der Leuchtkraft und der Größe der Scheiben aus der älteren, ca. drei Millionen Jahre alten Region im Sternbild Wolf und der jüngeren, ca. 1-2 Millionen Jahre alten Population aus dem Grenzgebiet zwischen Stier und Fuhrmann (Auriga). Die Anwendung der Analyse dieser Dissertation auf Multiwellenlängen\--Be\-ob\-achtungen einer großen Zahl von Scheiben, die mit ALMA beobachtet wurden, wird es erlauben, das Wachstum fester Körper im frühen Stadium vieler protoplanetarer Scheiben räumlich aufzulösen. Diese Messungen werden von zentraler Bedeutung sein, um theoretische Modelle der Planetenentstehung aufzustellen, zu testen und sie weiter zu verbessern

An Inner Disk in the Large Gap of the Transition Disk SR 24S

We report new Atacama Large Millimeter/sub-millimeter Array (ALMA) Band 3 observations at 2.75 mm of the TD around SR 24S with an angular resolution of $\sim$0.11''$\times$ 0.09'' and a peak signal-to-noise ratio of $\sim24$. We detect an inner disk and a mostly symmetric ring-like structure that peaks at $\sim$0.32'', that is $\sim$37 au at a distance of $\sim$114.4 pc. The full width at half maximum of this ring is $\sim$28 au. We analyze the observed structures by fitting the dust continuum visibilities using different models for the intensity profile, and compare with previous ALMA observations of the same disk at 0.45 mm and 1.30 mm. We qualitatively compare the results of these fits with theoretical predictions of different scenarios for the formation of a cavity or large gap. The comparison of the dust continuum structure between different ALMA bands indicates that photoevaporation and dead zone can be excluded as leading mechanisms for the cavity formation in SR 24S disk, leaving the planet scenario (single or multiple planets) as the most plausible mechanism. We compared the 2.75 mm emission with published (sub-)centimeter data and find that the inner disk is likely tracing dust thermal emission. This implies that any companion in the system should allow dust to move inwards throughout the gap and replenish the inner disk. In the case of one single planet, this puts strong constraints on the mass of the potential planet inside the cavity and the disk viscosity of about $\lesssim$5 $M_{\rm{Jup}}$ and $\alpha\sim10^{-4}-10^{-3}$, respectively.Comment: Accepted to Ap

Revealing signatures of planets migrating in protoplanetary discs with ALMA multi-wavelength observations

Recent observations show that rings and gaps are ubiquitous in protoplanetary discs. These features are often interpreted as being due to the presence of planets; however, the effect of planetary migration on the observed morphology has not been investigated hitherto. In this work we investigate whether multiwavelength mm/submm observations can detect signatures of planet migration, using 2D dusty hydrodynamic simulations to model the structures generated by migrating planets and synthesising ALMA continuum observations at 0.85 and 3 mm. We identify three possible morphologies for a migrating planet: a slowly migrating planet is associated with a single ring outside the planet's orbit, a rapidly migrating planet is associated with a single ring inside the planet's orbit while a planet migrating at intermediate speed generates one ring on each side of the planet's orbit. We argue that multiwavelength data can distinguish multiple rings produced by a migrating planet from other scenarios for creating multiple rings, such as multiple planets or discs with low viscosity. The signature of migration is that the outer ring has a lower spectral index, due to larger dust grains being trapped there. Of the recent ALMA observations revealing protoplanetary discs with multiple rings and gaps, we suggest that Elias 24 is the best candidate for a planet migrating in the intermediate speed regime.Comment: Accepted for publication in MNRA

Detecting the halo heating from AGN feedback with ALMA

The Sunyaev-Zel'dovich (SZ) effect can potentially be used to investigate the heating of the circumgalactic medium and subsequent suppression of cold gas accretion onto the host galaxy caused by quasar feedback. We use a deep ALMA observation of HE0515-4414 in band 4, the most luminous quasar known at the peak of cosmic star formation (z=1.7), to search for the SZ signal tracing the heating of the galaxy's halo. ALMA's sensitivity to a broad range of spatial scales enables us to disentangle emitting compact sources from the negative, extended SZ signal. We obtain a marginal S-Z detection (~3.3$\sigma$) on scales of about 300 kpc (30-40 arcsec), at the 0.2 mJy level, 0.5 mJy after applying a correction factor for primary beam attenuation and flux that is resolved out by the array. We show that our result is consistent with a simulated ALMA observation of a similar quasar in the FABLE cosmological simulations. We emphasise that detecting an SZ signal is more easily achieved in the visibility plane than in the (inferred) images. We also confirm a marginal detection (3.2$\sigma$) of a potential SZ dip on smaller scales (<100 kpc) already claimed by other authors, possibly highlighting the complex structure of the halo heating. Finally, we use SZ maps from the FABLE cosmological simulations, convolved with ALMA simulations, to illustrate that band 3 observations are much more effective in detecting the SZ signal with higher significance, and discuss the optimal observing strategy.Comment: 13 pages, 16 figures. Accepted for publication by MNRA

The evolution of dust-disk sizes from a homogeneous analysis of 1-10 Myr-old stars

We utilize ALMA archival data to estimate the dust disk size of 152 protoplanetary disks in Lupus (1-3 Myr), Chamaeleon I (2-3 Myr), and Upper-Sco (5-11 Myr). We combine our sample with 47 disks from Tau/Aur and Oph whose dust disk radii were estimated, as here, through fitting radial profile models to visibility data. We use these 199 homogeneously derived disk sizes to identify empirical disk-disk and disk-host property relations as well as to search for evolutionary trends. In agreement with previous studies, we find that dust disk sizes and millimeter luminosities are correlated, but show for the first time that the relationship is not universal between regions. We find that disks in the 2-3 Myr-old Cha I are not smaller than disks in other regions of similar age, and confirm the Barenfeld et al. (2017) finding that the 5-10 Myr USco disks are smaller than disks belonging to younger regions. Finally, we find that the outer edge of the Solar System, as defined by the Kuiper Belt, is consistent with a population of dust disk sizes which have not experienced significant truncation.Comment: ApJ accepted, 38 pages, 16 figures, 68k compatibl

New insights into the nature of transition disks from a complete disk survey of the Lupus star forming region

Transition disks with large dust cavities around young stars are promising targets for studying planet formation. Previous studies have revealed the presence of gas cavities inside the dust cavities hinting at recently formed, giant planets. However, many of these studies are biased towards the brightest disks in the nearby star forming regions, and it is not possible to derive reliable statistics that can be compared with exoplanet populations. We present the analysis of 11 transition disks with large cavities (>20 AU radius) from a complete disk survey of the Lupus star forming region, using ALMA Band 7 observations at 0.3" (22-30 AU radius) resolution of the 345 GHz continuum, 13CO and C18O 3-2 observations and the Spectral Energy Distribution of each source. Gas and dust surface density profiles are derived using the physical-chemical modeling code DALI. This is the first study of transition disks of large cavities within a complete disk survey within a star forming region. The dust cavity sizes range from 20-90 AU radius and in three cases, a gas cavity is resolved as well. The deep drops in gas density and large dust cavity sizes are consistent with clearing by giant planets. The fraction of transition disks with large cavities in Lupus is ~11%, which is inconsistent with exoplanet population studies of giant planets at wide orbits. Furthermore, we present a hypothesis of an evolutionary path for large massive disks evolving into transition disks with large cavities.Comment: 29 pages, 15 figures, Accepted by Ap

The time evolution of Md/M˙M_{\mathrm{d}}/\dot M in protoplanetary discs as a way to disentangle between viscosity and MHD winds

As the classic viscous paradigm for protoplanetary disk accretion is challenged by the observational evidence of low turbulence, the alternative scenario of MHD disk winds is being explored as potentially able to reproduce the same observed features traditionally explained with viscosity. Although the two models lead to different disk properties, none of them has been ruled out by observations - mainly due to instrumental limitations. In this work, we present a viable method to distinguish between the viscous and MHD framework based on the different evolution of the distribution in the disk mass ($M_{\mathrm{d}}$) - accretion rate ($\dot M$) plane of a disk population. With a synergy of analytical calculations and 1D numerical simulations, performed with the population synthesis code \texttt{Diskpop}, we find that both mechanisms predict the spread of the observed ratio $M_{\mathrm{d}}/\dot M$ in a disk population to decrease over time; however, this effect is much less pronounced in MHD-dominated populations as compared to purely viscous populations. Furthermore, we demonstrate that this difference is detectable with the current observational facilities: we show that convolving the intrinsic spread with the observational uncertainties does not affect our result, as the observed spread in the MHD case remains significantly larger than in the viscous scenario. While the most recent data available show a better agreement with the wind model, ongoing and future efforts to obtain direct gas mass measurements with ALMA and ngVLA will cause a reassessment of this comparison in the near future.Comment: 11 pages, 6 figures, 1 table. Published in ApJ

Radio multiwavelength analysis of the compact disk CX Tau: strong free-free variability or anomalous microwave emission?

Protoplanetary disks emit radiation across a broad range of wavelengths, requiring a multiwavelength approach to fully understand their physical mechanisms and how they form planets. Observations at sub-millimeter to centimeter wavelengths can provide insights into the thermal emission from dust, free-free emission from ionized gas, and possible gyro-synchrotron emission from the stellar magnetosphere. This Letter focuses on CX Tau, a ${\sim}0.4\,M_\odot$ star with an extended gas emission and a compact and apparently structureless dust disk, with an average millimeter flux when compared to Class II sources in Taurus. We present Karl G. Jansky Very Large Array (VLA) observations in 4 bands (between 9.0 mm and 6.0 cm) and combine them with archival data from the Atacama Large Millimeter/submillimeter Array (ALMA), the Submillimeter Array (SMA) and the Plateau de Bure Interferometer (PdBI). Such a multiwavelength approach allows to separate the dust continuum from other emissions. After isolating the dust thermal emission, we derived an upper limit of the dust disk extent at 1.3 cm which is consistent with theoretical predictions of a radial drift-dominated disk. Centimeter data show a peculiar behavior: deep observations at 6.0 cm did not detect the source, while at 1.3 cm the flux density is anomalously higher than adjacent bands. Intraband spectral indices suggest a dominant contribution from free-free emission, whereas gyro-synchrotron emission is excluded. To explain these observations, we propose strong variability of the free-free emission with timescales shorter than a month. Another possible interpretation is the presence of anomalous microwave emission from spinning dust grains.Comment: Accepted for publication in A&A Letters; 13 pages, 13 figures, 1 tabl