Focus Meeting 4 - Abstracts
Magnetized Protoplanetary Disks
Lizano, Susana
Protoplanetary disks are expected to form as a result of the gravitational collapse of magnetized rotating dense cores. I will discuss the structure and emission of models of magnetized accretion disks irradiated by the central star expected to form in this process. The mass-to-flux ratio is a critical parameter for their structure. This ratio will be determined observationally in the near future with radio interferometers like ALMA and VLA. I will also discuss the modeling of millimeter observations of the disk around the young star HL Tau.
Review on Observations of magnetic fields in star-forming clouds
Soler, Juan Diego
This review examines observations of magnetic fields in molecular clouds, that is, at spatial scales ranging from tens to tenths of parsecs and densities up to hundreds of particles per cubic centimetre. I will briefly summarize the techniques for observing and mapping magnetic fields in molecular clouds. I will review important examples of observational studies using each technique and its implications for our understanding of the role of the magnetic field in the molecular cloud formation and evolution. Finally, I will briefly discuss the prospects for advances in our observational capabilities with telescopes and instruments now beginning operation or under construction.
Roles of magnetic fields during the main accretion phase : theoretical landscape
Hennebelle, Patrick
Magnetic field is believed to play a fundamental role during the main accretion phase of stars. First of all the magnetic braking is likely regulating the amount of angular momentum that is available to form the planet-forming disks second of all magnetic field is though to reduce fragmentation. Last but not least, jets and outflows are likely a consequence of the magneto-centrifugal mechanism. These three aspects, depend on how the field is getting transported and how it is coupled to the gas, which itself is a consequence of the micro-physical processes. The talk will address these issues and stress the large uncertainties that remain to be clarified.
Magnetic fields of T Tauri stars and their inner accretion discs
Donati, Jean-Francois
Magnetic fields are known to play a key role in the early life of stars and their planets, as they form from collapsing dense pre-stellar cores that progressively flatten into large-scale accretion discs and eventually settle as young suns orbited by planetary systems. Pre-main-sequence (PMS) phases, in which central protostars feed from surrounding planet-forming accretion discs, are especially crucial for understanding how worlds like our Solar System are born. In my talk I will review the latest results on magnetic fields of PMS stars and their inner accretion discs from optical spectroscopic and spectropolarimetric surveys, focussing on low-mass T Tauri stars (TTSs) either still surrounded by their accretion discs (classical TTSs) or in the process of exhausting it (transition and weak-line TTSs). I will also outline future prospects in this field using forthcoming near infrared spectropolarimetric facilities like SPIRou at CFHT.
Observations of magnetic fields in Herbig Ae/Be stars
Schoeller, Markus
Models of magnetically driven accretion reproduce many observational properties of T Tauri stars. For the more massive Herbig Ae/Be stars, the corresponding picture has been questioned lately, in part driven by the fact that their magnetic fields are typically one order of magnitude weaker. Indeed, the search for magnetic fields in Herbig Ae/Be stars has been quite time consuming, with a detection rate of about 10%, also limited by the current potential to detect weak magnetic fields. Over the last two decades, magnetic fields were found in about twenty objects and for only two Herbig Ae/Be stars was the magnetic field geometry constrained. Also, several studies were undertaken to investigate the time dependence of spectroscopic tracers of magnetospheric accretion (MA). Overall, it seems that while there is proof that MA is present in some Herbig Ae stars, there is less evidence for the Herbig Be stars. In this review talk I will provide an overview about the current state of the MA research in Herbig Ae/Be stars and will give an outlook on what we might expect in the future.
The role of magnetic field in the formation and evolution of filamentary molecular clouds
Inutsuka, Shu-ichiro
Recent observations have emphasized the importance of the formation and evolution of magnetized filamentary molecular clouds in the process of star formation. Theoretical and observational investigations have provided convincing evidence for the formation of molecular cloud cores by the gravitational fragmentation of filamentary molecular clouds. The size and total angular momentum of a protoplanetary disk are supposed to be related directly to the rotational property of the parental molecular cloud core where the central protostar and surrounding disk are born. In this review we summarize our current understanding of various processes that are required in describing the filamentary molecular clouds and try to understand the origin of angular momenta of molecular cloud cores and its link to the mass function of cores and the stellar initial mass function.
Observations of magnetic fields during the main accretion phase
Lai, Shih-Ping
It is widely believed that magnetic fields play an important role in the early stages of star formation (McKee \& Ostriker 2007, Crutcher 2012). Particularly, magnetic braking can remove angular momentum efficiently and has been widely considered in theoretical models of disk formation (Li et al. 2014, PPIV, and the reference therein). Early models assume that the magnetic field is aligned parallel to the rotation axis at the beginning of the disk formation simulations (e.g., Allen et al. 2003, ApJ, 599, 363). However, recent CARMA observations have shown that magnetic fields around protostars tend to be misaligned with the rotation axis, as traced by the outflow axis (Hull et al. 2013, 2014), although other (single-dish) observations found contrary results, especially on larger scales (Davidson et al. 2011, Chapman et al. 2013). In this talk, I will review recent observations of magnetic fields toward Class 0 sources that have shed some light on this important question.
The role of magnetic fields in the early stages of star formation - confronting observations with theoretical expectations
Galametz, Maud
Magnetic fields are believed to redistribute angular momentum efficiently: they could explain the order-of-magnitude difference observed between the large angular momentum of the protostellar envelope and that of a main sequence star. The Class 0 phase is the main accretion phase. If the role of the magnetic fields during that stage is still unclear, they might have a key impact on the evolution of the young star and its surrounding protoplanetary disk. In order to study the structure of B from 50 to 2000 AU scales, we have acquired polarization observations of low-mass protostars with the SMA and for one of them, B335, with ALMA. Polarization is detected in all objects. We will present results on the orientation of B with respect to the outflow direction and show that a relation might exist between the misalignment and the rotational energy at the envelope scale. On the small scales traced with ALMA, we detect a large-scale poloidal magnetic field in the outflow direction and a strongly pinched B in the equatorial direction. Our results suggest that the magnetized collapse shows a high level of organization from the 2000 AU down to 50 AU scales. MHD simulations can help interpreting the results since we can test different assumptions of the dust alignment mechanisms, projection and instrumental effects. We will present simulations of a gravitational collapse of a magnetized core generated using the RAMSES code. The POLARIS code is used to produce maps of the Stokes parameters for different assumptions on the dust alignment. We will present synthetic observations including instrumental effects directly comparable to our observations. Our approach, confronting the most state-of-the-art observations of B to the theoretical expectations of a magnetized star formation scenario ultimately allows us to directly address the phenomenon of magnetic braking during protostar formation and will give key clues on the pristine properties of B fields in star-forming material.
Unveiling the role of magnetic field on the formation of solar type stars
Valdivia, Valeska
Star formation takes place in the densest regions of molecular clouds, where the interplay between gravity, magnetic fields and turbulence shapes the structure and set the main properties of star formation. As the gravitational collapse proceeds, the gas must evacuate more than 99.999% of its initial angular momentum during the short main accretion phase. Using cutting edge numerical simulations including the effect of ambipolar diffusion, we study the evolution of the magnetic field during the gravitational collapse. Using the state-of-the-art code POLARIS we compute the Stokes parameters and produce synthetic observations of mm/submm polarised dust emission directly comparable to available interferometric observations.
Helical Magnetic Fields in Molecular Clouds? A New Method to Determine the Line-of-Sight Magnetic Field Structure in Molecular Clouds
Tahani, Mehrnoosh
We present a new method to find the line-of-sight strength and morphology of magnetic fields in star forming regions using Faraday rotation measurements. In this method, we use rotation measure data from the literature and adopt a simple approach, based on relative measurements, to estimate the amount of rotation measure induced by the molecular clouds versus that from the rest of the Galaxy. We then use a chemical evolution code, along with extinction maps of each cloud, to find the electron column density of the molecular cloud at the position of each rotation measure data point. Combining the rotation measures produced by the molecular clouds and the electron column density, we calculate the line-of-sight magnetic field strength and direction.We applied this method to four relatively nearby regions of Orion A, Orion B, Perseus, and California. In the California cloud and Orion A, we found clear evidence that the magnetic fields at one side of these filamentary structures were pointing towards us and were pointing away from us at the other side. This behaviour is consistent with a helical magnetic field morphology. In the vicinity of available Zeeman measurements in Orion A, Orion B, and Perseus, we found magnetic field values of -23 ± 38 µG, -129 ± 28 µG, and 32 ± 101 µG, respectively, which are in agreement with the Zeeman Measurements.
Revealing magnetic fields towards massive protostars: a multi-scale approach using masers and dust
Dall'Olio, Daria
Magnetic fields play a significant role during star formation processes, hindering the fragmentation and the collapse of the parental cloud, and affecting the accretion mechanisms and feedback phenomena. However, several questions still need to be addressed to clarify the importance of magnetic fields at the onset of high-mass star formation, such as at what evolutionary stage their action becomes relevant, how strong they are, and at what spatial scales they act. Furthermore, the magnetic field parameters are still poorly constrained especially at small scales, i.e. few astronomical units from the central object, where the accretion disc and the base of the outflow are located. Thus we need to probe magnetic fields at different scales, at different evolutionary steps and possibly with different tracers. I will show that the magnetic field morphology around high-mass protostars can be successfully traced at different scales by observing maser and dust polarised emission. A confirmation that they are effective tools is indeed provided by our recent results from 6.7 GHz MERLIN observations of the massive protostar IRAS 18089-1732, where we found that the small-scale magnetic field probed by methanol masers is consistent with the large-scale magnetic field probed by dust. Moreover I will present results obtained from our ALMA Band 7 polarisation observations of G9.62+0.20, which is a massive star-forming region with a sequence of cores at different evolutionary stages. We resolve several protostellar cores embedded in a bright and dusty filamentary structure. I will then discuss the magnetic field morphology and strength in different cores and how it can be interpreted within the evolutionary sequence of the star formation process which is occurring across the filament.
Magnetic field structures in star-forming regions revealed by imaging polarimetry at multi-wavelengths
Kwon, Jungmi
Magnetic fields are ubiquitous in various scales of astronomical objects, and it is considered as playing significant roles from star to galaxy formations. However, the role of the magnetic fields in star forming regions is less well understood because conventional optical polarimetry is hampered by heavy extinction by dust. We have been conducting extensive near-infrared polarization survey of various star-forming regions from low- and intermediate-mass to high-mass star-forming regions, using IRSF/SIRPOL in South Africa. Not only linear but also circular polarizations have been measured for more than a dozen of regions. Both linear and circular polarimetric observations at near-infrared wavelengths are useful tools to study the magnetic fields in star forming regions, although infrared circular polarimetry has been less explored so far. In this presentation, we summarize our results of the near-infrared polarization survey of star forming regions and its comparison with recent submillimeter polarimetry results. Such multi-wavelength approaches can be extended to the polarimetry using ALMA, SPICA in future, and others. We also present our recent results of the first near-infrared imaging polarimetry of young stellar objects in the Circinus molecular cloud, which has been less studied but a very intriguing cluster containing numerous signs of active low-mass star formation.
Magnetic field and accretion in the young eruptive star EX Lupi
Kospal, Agnes
While the Sun is a quiet and well-balanced star nowadays, during its first few million years it possessed a strong magnetic field and accreted actively. Theoretical models predict that under certain circumstances the interaction of a strongly magnetic star and its circumstellar disk may lead to short busts of increased accretion onto the star (d'Angelo & Spruit 2012). The observable examples of this phenomenon may be a group of young stars called EXors, named after the prototype EX Lupi, which show irregular brightenings due to elevated accretion. EX Lupi had its historically largest outburst in 2008. Spectroscopic evidence from the quiescent and outburst periods suggests that the mass accretion proceeds through the same magnetospheric accretion channels in both periods but with different mass flux (Sicilia-Aguilar et al. 2012). However, no information on the magnetic field of EX Lup can be found in the literature. Here, we explore the magnetic field structure of EX Lup using spectropolarimetric monitoring with the CFHT/ESPaDOnS. We detected strong and largely poloidal topology with a prominent cool polar cap and an accretion spot above it. We compare our results with numerical simulations, in order to check the applicability of the d'Angelo & Spruit model as an explanation of EX Lupi's accretion outbursts. If EX Lup is a good proxy for the proto-Sun, similar magnetic field-disk interactions and the resulting outbursts might have happened during the early evolution of the Solar System as well.
The impact of non-ideal effects on the circumstellar disk evolution and their observational signitures
Tsukamoto, Yusuke
In this talk, we discuss the impact of non-ideal effects on the ciucumstellar disk formation, and the observational signatures created by them.It has been recognized that non-ideal MHD effects (Ohmic diffusion, Hall effect, ambipolar diffusion) play crucial roles for the circumstellar disk formation and evolution. the Hall effect notably changes the magnetic torques in the envelope around the disk, and strengthens or weakens the magnetic braking depending on the relative orientation of magnetic field and angular momentum (Tsukamoto+15b, Tsukamoto+17). This suggests that the bimodal evolution of the disk size occurs in the early disk evolutionary phase, which is suggested by the recent disk observation of Class 0 YSOs (e.g., Yen+17). Ohmic and ambipolar diffusion decouple the gas and the magnetic field, and significantly reduces the magnetic torque in the disk, which enables the formation of the circumstellar disk. They set an upper limit to the magnetic field strength of ~ 0.1 G around the disk.Hall effect and ambipolar diffusion imprint the observable characteristic velocity structures in the envelope of Class 0/I YSOs. Hall effect forms a counter-rotating envelope around the disk. Our simulations show that counter rotating envelope has the size of 100-1000 AU and several recent ALMA observations actually infers such structures. Ambipolar diffusion causes the significant ion-neutral drift in the envelopes. Our simulations show that the drift velocity of ion could become 100-1000 m/s and it would be observable by ALMA.
Magnetic field detections in Herbig Ae SB2 systems
Jaervinen, Silva
Studies of the presence of magnetic fields of Herbig Ae/Be stars are extremely important because they enable us to improve our insight into how the magnetic fields of these stars are generated and how they interact with their environment, including their impact on the planet formation processes and the planet-disk interaction. We report new detections of weak mean longitudinal magnetic fields in the close Herbig Ae double-lined spectroscopic binary AKSco and in the presumed spectroscopic Herbig Ae binary HD 95881 based on observations obtained with HARPSpol attached to ESO's 3.6 m telescope. Such studies are important because only very few close spectroscopic binaries with orbital periods below 20d are known among Herbig Ae stars. Our detections favour the conclusion that the previously suggested low incidence (5-10%) of magnetic Herbig Ae stars can be explained by the weakness of these fields and the limited accuracy of the published measurements. The search for magnetic fields and the determination of their geometries in close binary systems will play an important role for understanding the mechanisms that can be responsible for the magnetic field generation.
Magnetic fields and massive star formation
Zhang, Qizhou
Massive stars (M > 8Msun) often form in parsec-scale molecular clumps that collapse and fragment, leading to the birth of a cluster of stellar objects. The role of magnetic fields during the formation of massive dense cores is still not clear. The steady improvement in sensitivity of (sub)millimeter interferometers over the past decade enabled observations of dust polarization of large samples of massive star formation regions. In this talk, I will present a legacy survey carried out with the Submillimeter Array of massive star forming clumps in polarized continuum emission at a wavelength of 0.89mm. This unprecedentedly large sample of massive star forming regions observed by a submm interferometer before the ALMA era revealed compelling evidence of strong magnetic influence on the gas dynamics from 1pc to 0.1pc scales. I will present the results from the SMA legacy survey as well as our followup studies of this sample at higher angular resolutions with ALMA that probe the role of magnetic fields at scales < 0.01pc.
Trying to Make Sense of Polarization Patterns in Circumstellar Disks
Stephens, Ian
In the era of ALMA, we can now resolve polarization within circumstellar disks at (sub)millimeter wavelengths. While we initially hoped that these observations would give us insight on magnetic fields, the observed polarization patterns indicate other possible polarization mechanisms. These mechanisms include polarization from scattering and emission from grains aligned with the radiation anisotropy. Can we make sense of all these disk polarization observations? In this presentation, I will show many polarization observations toward disks, and I will discuss the theoretical expectations for different polarization mechanisms. Hopefully, the answer to the question will be an astounding "Yes!"
Magnetized disk models of young circumstellar disks
Flock, Mario
In this talk I will present our recent magnetized models of young protoplanetary disks (based on our recent work Flock et al. 2017a and Flock et al. 2015). The simulation results allow us to search for observational constraints which can be directly compared with our recent simulations. For this talk I will focus on two aspects: first we post-process our models to search for the polarization signature by magnetically aligned dust grains. Our results show a clear radial polarization pattern for face-on observed disk systems due to the dominant toroidal magnetic field, shown in the mm continuum emission by the dust.The second one is caused by the activity of the magnetic dynamo caused by the magneto rotational instability. This causes a clear variability pattern emitted at the dust inner rim surface which could be observed with near infrared light curve observations.
Interstellar Dust Grain Alignment
Andersson, B-G
Of the available probes of interstellar magnetic fields, dust induced dichroic extinction and emission, are the observationally most straightforward, both in terms of facilities and calibration. Its interpretation has, however, long been handicapped by the lack of a quantitative, observationally supported, understanding of the required grain alignment. Over the last decade this deficiency has been remedied, with the establishment, and observational confirmation, of Radiative Alignment Torque (RAT) theory. With our growing confidence of RAT alignment, measurements of dust induced polarization can now not only be used for more reliable characterization of the magnetic field, but also to constrain and probe other characteristics of the environment and the dust. I will review the basics of RAT alignment, some of the empirical support of the theory and some of the new tools for interstellar astronomy made possible by these developments.
The Formation of Dense Gas within Magnetized Molecular Clouds: A BLASTPol Study of the Vela C Giant Molecular Cloud
Fissel, Laura
I will show comparisons between the magnetic field morphology of the young giant molecular cloud Vela C, as traced by the BLASTPol balloon-borne sub-mm polarimeter, and the orientation of elongated molecular gas structures, as traced by molecular line maps from the Mopra telescope. We find that low-density tracers 12CO and 13CO are statistically more likely to align parallel to the magnetic field, while intermediate or high density tracers show either no preferential alignment or a tendency for alignment perpendicular to the magnetic field. The transition from parallel to perpendicular orientation occurs at a molecular hydrogen number density of approximately 103 cm-3, though there are indications that this transition density may be much lower for the “Centre-Ridge” cloud sub-region, which harbours the highest column density filaments in Vela C and has already formed several high mass stars. Our results suggest that the magnetic field in Vela C is strong enough to have influenced the formation of cloud sub-structures, and further imply that the orientation of the magnetic field with respect to the convergent flows that created Vela C may have affected the efficiency with which dense gravitationally unstable molecular gas was formed in the cloud sub-regions.