Gabriel A. P. Franco

Optical polarimetry toward the Pipe nebula: Revealing the importance of the magnetic field ⋆

Context. Magnetic fields are proposed to play an important role in the formation and support of self-gravitating clouds and the formation and evolution of protostars in such clouds. Aims. We attempt to understand more precisely how the Pipe nebula is affected by the magnetic field. Methods. We use R-band linear polarimetry collected for about 12 000 stars in 46 fields with lines of sight toward the Pipe nebula to investigate the properties of the polarization across this dark cloud complex. Results. Mean polarization vectors show that the magnetic field is locally perpendicular to the large filamentary structure of the Pipe nebula (the “stem”), indicating that the global collapse may have been driven by ambipolar diffusion. The polarization properties clearly change along the Pipe nebula. The northwestern end of the nebula (B59 region) is found to have a low degree of polarization and high dispersion in polarization position angle, while at the other extreme of the cloud (the “bowl”) we found mean degrees of polarization as high as ≈15% and a low dispersion in polarization position angle. The plane of the sky magnetic field strength was estimated to vary from about 17 μG in the B59 region to about 65 μG in the bowl. Conclusions. We propose that three distinct regions exist, which may be related to different evolutionary stages of the cloud; this idea is supported by both the polarization properties across the Pipe and the estimated mass-to-flux ratio that varies between approximately super-critical toward the B59 region and sub-critical inside the bowl. The three regions that we identify are: the B59 region, which is currently forming stars; the stem, which appears to be at an earlier stage of star formation where material has been through a collapsing phase but not yet given birth to stars; and the bowl, which represents the earliest stage of the cloud in which the collapsing phase and cloud fragmentation has already started.

DETAILED INTERSTELLAR POLARIMETRIC PROPERTIES OF THE PIPE NEBULA AT CORE SCALES

We use R-band CCD linear polarimetry collected for about 12,000 background field stars in 46 fields of view toward the Pipe nebula to investigate the properties of the polarization across this dark cloud. Based on archival Two Micron All Sky Survey data, we estimate that the surveyed areas present total visual extinctions in the range 0.6 mag ? AV ? 4.6 mag. While the observed polarizations show a well-ordered large-scale pattern, with polarization vectors almost perpendicularly aligned to the clouds long axis, at core scales one sees details that are characteristics of each core. Although many observed stars present degrees of polarization that are unusual for the common interstellar medium (ISM), our analysis suggests that the dust grains constituting the diffuse parts of the Pipe nebula seem to have the same properties as the normal Galactic ISM. Estimates of the second-order structure function of the polarization angles suggest that most of the Pipe nebula is magnetically dominated and that turbulence is sub-Alv?nic. The Pipe nebula is certainly an interesting region to investigate the processes that prevailed during the initial phases of low-mass stellar formation.

On the radiation driven alignment of dust grains: Detection of the polarization hole in a starless core

Aims. We aim to investigate the polarization properties of a starl ess core in a very early evolutionary stage. Linear polariza tion data reveal the properties of the dust grains in the distinct phas es of the interstellar medium. Our goal is to investigate how the polarization degree and angle correlate with the cloud and core gas. Methods. We use optical, near infrared and submillimeter polarization observations toward the starless object Pipe-109 in the Pipe nebula. Our data cover a physical scale range of 0.08 to 0.4 pc, comprising the dense gas, envelope and the surrounding cloud. Results. The cloud polarization is well traced by the optical data. The near infrared polarization is produced by a mixed population of grains from the core border and the cloud gas. The optical and near infrared polarization toward the cloud reach the maximum possible value and saturate with respect to the visual extinction. Th e core polarization is predominantly traced by the submillimeter data and have a steep decrease with respect to the visual extinction. Modeling of the submillimeter polarization indicates a magnetic field main direction projected onto the plane-of-sky and loss of grain alignment for densities higher than 6× 10 4 cm −3 (or AV> 30 mag). Conclusions. Pipe-109 is immersed in a magnetized medium, with a very ordered magnetic field. The absence of internal source of radiation significantly a ffects the polarization effi ciencies in the core, creating a polarization hole at the cen ter of the starless core. This result supports the theory of dust grain alignment via r adiative torques

YOUNG STARLESS CORES EMBEDDED IN THE MAGNETICALLY DOMINATED PIPE NEBULA

The Pipe Nebula is a massive, nearby dark molecular cloud with a low star formation efficiency which makes it a good laboratory in which to study the very early stages of the star formation process. The Pipe Nebula is largely filamentary and appears to be threaded by a uniform magnetic field at scales of a few parsecs, perpendicular to its main axis. The field is only locally perturbed in a few regions, such as the only active cluster-forming core B59. The aim of this study is to investigate primordial conditions in low-mass pre-stellar cores and how they relate to the local magnetic field in the cloud. We used the IRAM 30 m telescope to carry out a continuum and molecular survey at 3 and 1 mm of early- and late-time molecules toward four selected starless cores inside the Pipe Nebula. We found that the dust continuum emission maps trace the densest regions better than previous Two Micron All Sky Survey (2MASS) extinction maps, while 2MASS extinction maps trace the diffuse gas better. The properties of the cores derived from dust emission show average radii of ~0.09 pc, densities of ~1.3×105 cm–3, and core masses of ~2.5 M ☉. Our results confirm that the Pipe Nebula starless cores studied are in a very early evolutionary stage and present a very young chemistry with different properties that allow us to propose an evolutionary sequence. All of the cores present early-time molecular emission with CS detections in the whole sample. Two of them, cores 40 and 109, present strong late-time molecular emission. There seems to be a correlation between the chemical evolutionary stage of the cores and the local magnetic properties that suggests that the evolution of the cores is ruled by a local competition between the magnetic energy and other mechanisms, such as turbulence.

Spectroscopic behaviour of the unusual Ae star HD 190073

The results of high-resolution spectroscopy of the peculiar Ae star HD 190073 obtained within the framework of a cooperative observing programme in 1994-2002 are presented. The temporal behaviour of the Hα ,H β ,H γ ,H δ ,H e λ 5876 A, Na  D and other circumstellar line profiles are investigated in detail. Special attention has been paid to the analysis of the deep multicomponent blueshifted Ca  H and K absorption lines. It has been found that the fine structure of their profiles is variable on timescales from months to decades. The analysis of the circumstellar spectrum of HD 190073, rich in shell-like and emission lines with narrow absorption cores, allows us to conclude that all absorption lines and cores are likely to be of photospheric origin. The emission lines are variable in time and demonstrate signs of a stellar wind as well as a dense equatorial circumstellar disk. As a preliminary hypothesis, we propose that a global magnetic field of a specific topology can be responsible for the formation of stable latitudinal stratification of the outflowing gas resulting in appearance of the complex structure of the Ca  H and K line profiles. We emphasise that a measurement of the stellar magnetic field and an investigation of its detailed configuration would be an important step in understanding the nature of HD 190073.

Local interstellar medium kinematics towards the Southern Coalsack and Chamaeleon–Musca dark clouds

The results of a spectroscopic programme aiming to investigate the kinematics of the local interstellar medium components towards the Southern Coalsack and Chamaeleon-Musca dark clouds are presented. The analysis is based upon high-resolution (R � 60 000) spectra of the insterstellar NaI D absorption lines towards 63 B-type stars (d 6 500 pc) selected to cover these clouds and the connecting area defined by the Gala ctic coordinates: 308 ◦ > l > 294 ◦ and 22 ◦ 6 b 6 5 ◦ . The radial velocities, column densities, velocity disper sions, colour excess and photometric distances to the stars are used to understand the kinematics and distribution of the interstellar cloud components. The ana lysis indicates that the interstellar gas is distributed in two extended sheet-like structures perme ating the whole area, one at d 6 60 pc and another around 120-150 pc from the Sun. The nearby feature is approaching to the local standard of rest with average radial velocity of 7 km s −1 , has low average column density log NNa I � 11.2 cm −2 and velocity dispersion b � 5 km s −1 . The more distant feature has column densities between 12.3 6 log NNa I 6 13.2, average velocity dispersion b � 2.5 km s −1 and seems associated to the dust sheet observed towards the Coalsack, Musca and Chamaeleon direction. Its velocity is centered around 0 km s −1 , but there is a trend for increasing from 3 km s −1 near b = 1 ◦ to 3 km s −1 near b = 18 ◦ . The nearby low column density feature indicates a general outflow from the Sco-Cen association, in aggreement with several independent lines of data in the general searched direction. The dust and gas feature around 120 ‐ 150 pc seem to be part of an extended large scale feature of similar kinematic properties, supposedly identified with the interaction zon e of the Local and Loop I bubbles. Assuming that the interface and the ring-like volume of dense neutral matter that would have been formed during the collision of the two bubbles have similar properties, our results rather suggest that the interaction zone between the bubbles is twisted and folded.

INFRARED AND OPTICAL POLARIMETRY AROUND THE LOW-MASS STAR-FORMING REGION NGC 1333 IRAS 4A

We performed J- and R-band linear polarimetry with the 4.2 m William Herschel Telescope at the Observatorio del Roque de los Muchachos and with the 1.6 m telescope at the Observatorio do Pico dos Dias, respectively, to derive the magnetic field geometry of the diffuse molecular cloud surrounding the embedded protostellar system NGC 1333 IRAS 4A. We obtained interstellar polarization data for about three dozen stars. The distribution of polarization position angles has low dispersion and suggests the existence of an ordered magnetic field component at physical scales larger than the protostar. Some of the observed stars present intrinsic polarization and evidence of being young stellar objects. The estimated mean orientation of the interstellar magnetic field as derived from these data is almost perpendicular to the main direction of the magnetic field associated with the dense molecular envelope around IRAS 4A. Since the distribution of the CO emission in NGC 1333 indicates that the diffuse molecular gas has a multi-layered structure, we suggest that the observed polarization position angles are caused by the superposed projection of different magnetic field components along the line of sight.

TRACING THE MAGNETIC FIELD MORPHOLOGY OF THE LUPUS I MOLECULAR CLOUD

Deep R-band CCD linear polarimetry collected for fields with lines of sight toward the Lupus I molecular cloud is used to investigate the properties of the magnetic field within this molecular cloud. The observed sample contains about 7000 stars, almost 2000 of them with a polarization signal-to-noise ratio larger than 5. These data cover almost the entire main molecular cloud and also sample two diffuse infrared patches in the neighborhood of Lupus I. The large-scale pattern of the plane-of-sky projection of the magnetic field is perpendicular to the main axis of Lupus I, but parallel to the two diffuse infrared patches. A detailed analysis of our polarization data combined with the Herschel/SPIRE 350 ?m dust emission map shows that the principal filament of Lupus I is constituted by three main clumps that are acted on by magnetic fields that have different large-scale structural?properties. These differences may be the reason for the observed distribution of pre- and protostellar objects along the molecular cloud and the cloud?s?apparent evolutionary stage. On the other hand, assuming that the magnetic field is composed of?large-scale and turbulent components, we find that the latter is rather similar in all three clumps. The estimated plane-of-sky component of the large-scale magnetic field ranges from about 70 to 200 ?G in these clumps. The intensity increases toward the Galactic plane. The mass-to-magnetic flux ratio is much smaller than unity, implying that Lupus I is magnetically supported on large scales.

MAGNETICALLY DOMINATED PARALLEL INTERSTELLAR FILAMENTS IN THE INFRARED DARK CLOUD G14.225-0.506*

The G14.225-0.506 infrared dark cloud (IRDC G14.2) displays a remarkable complex of parallel dense molecular filaments projected on the plane of the sky. Previous dust emission and molecular-line studies have speculated whether magnetic fields could have played an important role in the formation of such long-shaped structures, which are hosts to numerous young stellar sources. In this work we have conducted a vast polarimetric survey at optical and near-infrared wavelengths in order to study the morphology of magnetic field lines in IRDC G14.2 through the observation of background stars. The orientation of interstellar polarization, which traces magnetic field lines, is perpendicular to most of the filamentary features within the cloud. Additionally, the larger-scale molecular cloud as a whole exhibits an elongated shape also perpendicular to magnetic fields. Estimates of magnetic field strengths indicate values in the range

Molecular outflow launched beyond the disk edge

320 - 550\,\mu