While the Sun is a quiet and well-balanced star now, during its first few million years it possessed a strong magnetic field and actively accreted material from its circumstellar environment. Theoretical models predict that under certain circumstances the interaction of a strongly magnetic star and its circumstellar disk may lead to short bursts of increased accretion onto the star (D’Angelo & Spruit 2012). Examples for this phenomenon may be the members of a group of young eruptive stars called EXors. Their prototype, EX Lup, had its historically largest outburst in 2008. Spectroscopic evidence suggests that the mass accretion proceeds through the same magnetospheric accretion channels both in quiescence and in outburst but with different mass flux (Sicilia-Aguilar et al. 2012). To characterize for the first time EX Lup’s magnetic field, we obtained spectropolarimetric monitoring for it with the CFHT/ESPaDOnS. We detected strong, poloidal magnetic field with a prominent cool polar cap and an accretion spot above it. We compared our results with numerical simulations, in order to check the applicability of the d’Angelo & Spruit model as an explanation of EX Lup’s accretion outbursts. If EX Lup is a good proxy for the proto-Sun, similar magnetic field-disk interactions and outbursts might have happened during the early evolution of the Solar System as well.

Recent advanced simulations of protoplanetary disks allow us to search for observational constraints to identify the magnetic field activity in protoplanetary disks. With our 3D radiation non-ideal magneto-hydrodynamical (MHD) models including irradiation from an Herbig type star we are able to model the thermal and dynamical evolution in a so far never reached detail (Flock et al. 2017). The activity of the magneto-rotational instability in the inner hot ionized regions comes along with a magnetic dynamo. The oscillations in the mean toroidal magnetic field with a timescale of 10 local orbits can slightly bend the inner dust rim and so the irradiation surface. This causes a clear variability pattern in the near infrared (NIR) emission at the dust inner rim surface. Another way to identify the presence of magnetic fields are to search for polarization signatures. Using 3D non-ideal MHD simulations of the outer disk regions (Flock et al. 2015) we calculate synthetic images of the intrinsically polarized continuum emitted by aspherical grain aligned with the dominantly toroidal magnetic field (Bertrang et al. 2017). Our results show a clear radial polarization pattern for face-on observed disk, similar to recent observations by Ohashi et al. (2018). Additionally, we are even able to see the change of the polarization pattern inside the vortex as the poloidal magnetic field dominates therein.

The Atacama Large Millimeter/submillimeter Array (ALMA) is providing important advances in studies of star formation. In particular, polarimetry can reveal the disk magnetic configuration, a crucial ingredient in many processes, as, for example, the transport of angular momentum. We analized ALMA Band 7 (870 μm) polarimetric data at 0.”2 resolution for the young rotating disk/jet systems DG Tau and CW Tau, to find magnetic signatures. From the Stokes I, U, Q maps, we derive the linear polarization intensity, $P = \sqrt {{Q^2} + {U^2}} $ , the linear polarization fraction, and the polarization angle. The alignment of the latter with the disk minor axis (Fig. 1) shows that self-scattering of dust thermal emission rather than magnetic alignment dominates the polarization in both targets (Bacciotti et al. 2018). However, several dust properties can be diagnosed comparing the polarization data with the models of self-scattering (e.g. Kataoke et al. 2017, Yang et al. 2017). The maximum grain size turns out to be in the range 50 - 70 μm for DG Tau and 100 - 150 μm for CW Tau. The asymmetry of the polarized intensity in DG Tau, observed for the first time around a T Tauri star, indicates that the disk is flared. Moreover, the observed belt-like feature may betray the presence of a disk substructure. In contrast, the polarization maps of CW Tau indicate that here the grains have settled to the disk midplane. Polarimetry is thus very important in studies of the dust evolution.

Numerous numerical studies suggest that magnetic fields influence the transport of dust and gas, the disk chemistry, the migration of planetesimals within the disk, and above all the accretion of matter onto the star. In short: Magnetic fields are crucial for the evolution of planet-forming disks. First indirect comparisons of theory and observations support this picture (Flock et al. 2017); however, profound observational constraints are still pending. Recent studies show that the intrinsically polarized continuum emission, the classical tracer of magnetic fields, might trace other physics as well (radiation field or dust grain size). The nearly face-on protoplanetary disk HD 142527 shows predominantly radial polarization vectors consistent with aspherical grains aligned by a toroidal magnetic field (Fig. 1; Bertrang et al. 2017a,b; Ohashi et al. 2018). However, the number of cutting-edge polarization observations presenting inconclusive data, for which these three different origins of polarization are not clearly distinguishable, increases continuously. We present a solution to this polarized ambiguity: observations and simulations of the most direct tracer of magnetic fields, polarized gas emission, in combination with multi-wavelength continuum polarization observations will disentangle the sources of continuum polarimetry with ALMA (Bertrang et al. 2017a,b; Bertrang & Cortés in prep.).

One idea for the origin of magnetic fields in massive stars suggests that the magnetic field is the fossil remnant of the Galactic ISM magnetic field, amplified during the collapse of the magnetised gas cloud. A search for the presence of magnetic fields in massive stars located in active sites of star formation led to the detection of rather strong magnetic fields in a few young stars. Future spectropolarimetric observations are urgently needed to obtain insights into the mechanisms that drive the generation of kG magnetic fields during high-mass star formation.

AK Sco is an SB2 system formed by two nearly identical Herbig Ae stars, with Teff = 6500K and log g = 4.5, surrounded by a circumbinary disk. This actively accreting system is of special interest among the pre-main-sequence binaries because of its prominent ultraviolet excess and the high eccentricity of its orbit. Moreover, recent spectropolarimetric observations using HARPSpol indicate the presence of a weak magnetic field in the secondary component (Järvinen et al. 2018). An abundance analysis of both components has shown that all elements have a solar abundance in the two stars, except for Li and Ba. These elements are enhanced by 2.2 and 0.5 dex, respectively, in the A component and by 2.4 and 0.5 dex, respectively, in the B component.

Herbig Ae/Be-type stars are analogs of T Tauri stars at higher masses. Since the confirmation of magnetospheric accretion using Balmer and sodium line profiles in the Herbig Ae star UX Ori, a number of magnetic studies have been attempted, indicating that about 20 Herbig Ae/Be stars likely have globally organized magnetic fields. The low detection rate of magnetic fields in Herbig Ae stars can be explained by the weakness of these fields and rather large measurement uncertainties. The obtained density distribution of the root mean square longitudinal magnetic field values revealed that only a few stars have magnetic fields stronger than 200 G, and half of the sample possesses magnetic fields of about 100 G or less. We report on the results of our analysis of a sample of presumably single Herbig Ae/Be stars based on recent observations obtained with HARPSpol attached to ESO’s 3.6m telescope. Knowledge of the magnetic field structure combined with the determination of the chemical composition are indispensable to constrain theories on star formation and magnetospheric accretion in intermediate-mass stars. As of today, magnetic phase curves have been obtained only for two Herbig Ae/Be stars, HD 101412 and V380 Ori.

We present Doppler images of the active dwarf star V1358 Ori using high-resolution spectra from the NARVAL spectropolarimeter mounted on the Bernard Lyot Telescope. The spectra were taken between 09-20 Dec, 2013 with a resolution of R=80000. Doppler imaging was carried out with our new generation multi-line Dopper imaging code iMap (Carroll et al. 2012). 40 individual photospheric lines were selected by line depth, temperature sensitivity and blends. Two data subsets were formed to get two consecutive Doppler images. Prominent cool spots at lower latitudes are found on both maps. At 0.5 phase there is a prominent equatorial feature on both maps. Weaker polar features can be seen on the first map, which somewhat diminishes for the second map. On the first image there is a cool surface feature at 30 degrees latitude which seems to fade greatly on the second map. Around 0.75 phase, a new spot seems to form. These changes suggest a rapid surface evolution. Spot displacements may also indicate surface differential rotation, which was derived by cross-correlating the two subsequent Doppler images (see e.g. Kővári et al. 2012). We fit the latitudinal correlation peaks with a sine-squared law. The fit suggests solar-type surface differential rotation with a shear parameter of α=0.02±0.02. The shear parameter fits the ${P_{{\rm{rot}}}} - |\alpha | $ diagram in Kővári et al. (2017) quite well.

The origin of polarized emission from protoplanetary disks is uncertain. Three mechanisms have been proposed for such polarized emission so far, (1) grain alignment with magnetic fields, (2) grain alignment with radiation gradients, and (3) self-scattering of thermal dust emission. Aiming to observationally identify the polarization mechanisms, we present ALMA polarization observations of the 0.87 mm dust continuum emission toward the circumstellar disk around HD 142527 with a spatial resolution of ∼0.2 arcsec as shown in Ohashi et al. (2018). We confirm that the polarization vectors in the northern region are consistent with self-scattering because of a flip of the polarization vectors. Furthermore, we show that the polarization vectors in the southern region are consistent with grain alignment by magnetic fields, although self-scattering cannot be ruled out. To understand these differences between the polarization mechanisms, we propose a simple grain size segregation model: small dust grains ( $\mathbin{\lower.3ex\hbox{$\buildrel<\over {\smash{\scriptstyle\sim}\vphantom{_x}}$}} $ 100 microns) are dominant and aligned with magnetic fields in the southern region, and middle-sized (∼100 microns) grains in the upper layer emit self-scattered polarized emission in the northern region. The grain size near the middle plane in the northern region cannot be measured because the emission at 0.87 mm is optically thick. However, it can be speculated that larger dust grains ( $\mathbin{\lower.3ex\hbox{$\buildrel>\over {\smash{\scriptstyle\sim}\vphantom{_x}}$}} $ cm) may accumulate near this plane. These results are consistent with those of a previous analysis of the disk, in which large grain accumulation and optically thick emission from the northern region were found. This model is also consistent with theories where smaller dust grains are aligned with magnetic fields. We find that the magnetic fields are toroidal, at least in the southern region.

After successfully retrieving the known rotation period P = 42.076 d in the Herbig Ae star HD 101412 using spectroscopic signatures of accretion tracers (Schöller et al. 2016), we have studied magnetospheric accretion in the Herbig Ae SB2 system HD 104237 using spectroscopic parameters of the He i 10830, Paγ, and He i 5876 lines, formed in the accretion region. Employing 21 spectra obtained with ISAAC and X-shooter, we found that the temporal behavior of these parameters can be explained by a variable amount of matter being accreted in the region between the star and the observer. Using a periodogram analysis, we examined the possible origin of the accretion flow in HD 104237 and considered the following four scenarios: matter flows from the circumbinary envelope, mass exchange between the system’s components, magnetospheric accretion (MA) from the disk onto the star, and fast high-latitude accretion from a disk wind onto a weakly magnetized star. Based on a correlation analysis, we were able to show that the primary component is responsible for the observed emission line spectrum of the system. Since we do not find any correlation of the spectroscopic parameters with the phase of the orbital period (P ≍ 20 d), we can reject the first two scenarios. We found a variation period of about 5 d, which likely represents the stellar rotation period of the primary and favors the MA scenario.

We show the results of global 3D magnetohydrodynamics simulations of an accretion disk with a rotating, weakly magnetized central star (Takasao et al. 2018). The disk is threaded by a weak large-scale poloidal magnetic field. The central star has no strong stellar magnetosphere initially and is only weakly magnetized. We investigate the structure of the accretion flows from a turbulent accretion disk onto the star. Our simulations reveal that fast accretion onto the star at high latitudes is established even without a stellar magnetosphere. We find that the failed disk wind becomes the fast, high-latitude accretion as a result of angular momentum exchange mediated by magnetic fields. The rapid angular momentum exchange occurs well above the disk, where the Lorentz force that decelerates the rotational motion of gas can be comparable to the centrifugal force. Unlike the classical magnetospheric accretion model, fast accretion streams are not guided by magnetic fields of the stellar magnetosphere. Nevertheless, the accretion velocity reaches the free-fall velocity at the stellar surface owing to the efficient angular momentum loss at a distant place from the star. Our model can be applied to Herbig Ae/Be stars whose magnetic fields are generally not strong enough to form magnetospheres, and also provides a possible explanation why Herbig Ae/Be stars show indications of fast accretion.

Magnetic fields are a key component in star formation theories. Nevertheless, their exact role in the formation of stars is still a matter of debate. The process of angular momentum transportation by the disturbance caused during magnetic field reconnection still needs theoretical formulation in terms of the collapsing cloud’s parameters. The purposes of this study are: to model the critical mass of a magnetized, gravitating and turbulent star forming molecular cloud (MC) and to formulate the momentum carried out by a magnetic field through magnetic field reconnection in terms of the MC’s parameters. By applying theoretical modeling, we show how angular momentum transported via an Alfvén wave can be described in terms of mass, radius and dispersion velocity of a collapsing cloud core and a model equation of the critical mass for a gravitating, turbulent, and magnetized molecular cloud core. The outflow of angular momentum by magnetic fields facilitates the inflow of mass. On the other side, magnetic pressure prevents collapse. Therefore, magnetic fields have a dual purpose in the process of star formation. This momentum outflow triggers the inflow of mass to conserve angular momentum. The results show that Alfvén waves are like a machine that extracts angular momentum from a magnetized collapsing cloud core. Thus the total angular momentum transported by magnetic field at a distance R from the core’s center depends on the size, mass and turbulent velocity dispersion of the collapsing cloud core.

We report the detection of the Zeeman effect in the 44 GHz Class I methanol maser line toward the star forming region DR21W. The 44 GHz methanol masers in this source occur in a ∼3” linear structure that runs from northwest to southeast, with the two dominant components at each end, and several weaker maser components in between. Toward a 93 Jy maser in the dominant northwestern component, we find a significant Zeeman detection of −23.4 ± 3.2 Hz. If we use the recently published result of Lankhaar et al. (2018) that the F=5-4 hyperfine transition is responsible for the 44 GHz methanol maser line, then their value of z = −0.92 Hz mG−1 yields a line-of-sight magnetic field of Blos =25.4 ± 3.5 mG. If Class I methanol masers are pumped in high density regions with n∼107–8 cm−3, then magnetic fields in these maser regions should be a few to several tens of mG. Therefore, our result in DR21W is certainly consistent with the expected values.

Using the above noted splitting factor in past Zeeman effect detections in Class I methanol masers reported by Sarma & Momjian (2011) and Momjian & Sarma (2017) in the star forming regions OMC-2 and DR21(OH) result in Blos values of 20.0 ± 1.2 mG and 58.2 ± 2.9 mG, respectively. These are also consistent with the expected values.

Massive stars ( ${\rm{M}} > \,8{M_ \odot }$ ) 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. We carried out a polarimetric survey with the Submillimeter Array of 14 massive star forming clumps in continuum emission at a wavelength of 0.89 mm. This unprecedentedly large sample of massive star forming regions observed by a submillimeter interferometer before the advent of ALMA revealed compelling evidence of strong magnetic influence on the gas dynamics from 1 pc to 0.1 pc scales. We found that the magnetic fields in dense cores tend to be either parallel or perpendicular to the mean magnetic fields in their parental molecular clumps. Furthermore, the main axis of protostellar outflows does not appear to be aligned with the mean magnetic fields in the dense core where outflows are launched. These findings suggest that from 1 pc to 0.1 pc scales, magnetic fields are dynamically important in the collapse of clumps and the formation of dense cores. From the dense core scale to the accretion disk scale of ∼102 au, however, gravity and angular momentum appear to be more dominant relative to the magnetic field.

We give an overview of Focus Meeting 5 on the new field of Terra-Astronomy.

Astronomy has an enviable wealth of historical observations. Some verge on the archaeological, and display rare events such as novæ and supernovæ; others range up to 100 or more years in age, and bear unique information about events that will never repeat in detail. Yet most astronomers today know little of those resources and the scientific potential which they harbour, so rather infrequent use is made today of those historical data. The problem is that historical data were perforce obtained in analogue formats, and because of those formats the data too tend to be regarded as hailing from a culture whose scientific significance is passé. But the medium is not the message! Astronomy’s archives of photographic observations constitute an irreplaceable resource. The change in technology from analogue to electronic recording in the late 20th Century was abrupt, and it left most of today’s astronomers unable to handle and use photographic data, and led to a general skepticism of the value of photographic observations for present-day studies of variability in the cosmos. But that is precisely what older data can do; in particular, the older the data the more reliable the base-line against which one can measure new trends, refine orbital parameters, discern period modulations, etc.

Several problems contribute to difficulties in interpreting transient celestial phenomena as described in Chinese records. Frameworks are an overarching problem. Tianwen, the modern Chinese term for astronomy, in pre-modern times included meteorological phenonemena and was concerned with omenology. Manuscripts that include star charts and comets but also meteorological phenomena and omen reading texts were routinely reframed in modern scholarship to appear as if they included only astronomical content. The scope of pre-modern tianwen, however, was broader than its modern sense. Pre-modern celestial phenomena had political and religious significance. Apparent ambiguity arises from the presence of both meteorological and astronomical phenomena in a single category and from features of the classical Chinese language. Accounting for these problems is essential for research into transient phenomena using historical archives.

Due to its unique 400-year duration, the sunspot number is a central reference for understanding the long-term evolution of solar activity and its influence on the Earth environment and climate. Here, we outline current data recovery work. For the sunspot number, we find historical evidence of a disruption in the source observers occurring in 1947–48. For the sunpot group number, recent data confirm the clear southern predominance of sunspots during the Maunder Minimum, while the umbra-penumbra ratio is similar to other epochs. For the Dalton minimum, newly recovered historical observations confirm a higher activity level than in a true Grand Minimum.

Analysis of historical records of eclipses of the Sun and Moon between 720 BC and AD 1600 gives a measure of the time difference, TT − UT = ΔT. The first derivative in time along a smooth curve fitted to the values of Δ T measures the changes in the length of the day (lod). The average rate of change of the lod is found to be significantly less than that expected on the basis of tidal friction. Fluctuations on a time-scale of centuries to millennia are mainly attributed to the effects of post-glacial uplift and core-mantle coupling.

The first visibility of the lunar crescent signals the start of a new month in the Islamic calendar. The eminent astronomer Ḥabash al-Ḥāsib sib developed a method of uncompromising complexity for predicting the visibility of the lunar crescent. He derived his threshold value from a moonwatch carried out at different places in Iraq on November 17th, 860 CE. We will allude to a few modern visibility criteria as well and highlight the uncertainties of today’s calculations when converting historical Arabic into Julian or Gregorian dates. Tables of first visibility of the lunar crescent for different locations are provided for the purpose of date conversions. Since the Islamic calendar was based on the observation of the lunar crescent, historical dates imply information on positive and negative sightings of the lunar crescent. From such information estimations of cloudiness in different regions of the Islamic world can be extracted.