(for references see the list of publications)
1.2.1 Star formation in cometary globule GC 12
Low and intermediate mass star formation takes mostly place in isolated clusters and in low mass star forming regions. As the stars form in dense gas and dust clouds, which are tightly
Figure 1: Colour coded SOFI image of CG12. The J, H and K bands are coded in blue, green and red, respectively. Square root scaling has been used to better bring out the faint surface brightness structures.
concentrated in the plane, star formation far out of the plane is not a likely event. The well known nearby star forming regions have been studied in great detail from visual to radio wavelengths but less is known of star formation regions off the Galactic plane.
Despite being classified as a cometary globule, together with those in the Gum nebula, the high latitude globule CG 12 is actually a high latitude low and intermediate mass star formation region. With a galactic latitude of 21 degrees and at the distance of∼550 pc CG 12 lies more than 200 pc above the plane. The structure of the globule in molecular line emis-sion has been extensively studied by us at SEST (Haikala and Olberg 2007) and at APEX (Haikala et al. 2006). CG12 is an active site formation site but only the brightest stars in the associated stellar cluster are known. We have conducted NIR JHK imaging of the globule with SOFI at the NTT tele-scope at La Silla. The imaging reveals several new deeply embedded member stars. The visual extinction towards the cluster members ranges from a few magnitudes up to about 40 magnitudes. Besides the two bright NIR reflection nebu-lae surrounding already known member stars, the SOFI imag-ing reveals an hour glass shaped nebulosity with a stellar like source at the “waist”. The hour glass lies at the centre of a highly collimated molecular outflow and is projected on a compact high density molecular core. The NIR imaging shows that active star formation is still going on in this low and intermediate mass star formation region high above the Galactic plane.
1.2.2 Prestellar and protostellar cores in Ori B9
Most stars form in clusters and smaller groups in the densest parts of giant molecular clouds (GMCs). By studying their physical and chemical characteristics, we hope to learn the conditions leading to protostellar collapse and the timescale related to this process.
We have carried out studies of the Ori B9 cloud. The 870 µm dust continuum was observed with the APEX telescope and data on the N2H+(1−0) and N2D+(2−1) lines were obtained with the IRAM telescope (see Figs. 2 and 3). To-gether with our previous H2D+data (Harju et al. 2006), these were used to derive the degree of deuteration and other chem-ical characteristics. Using additional information from far-infrared Spitzer/MIPS maps, prestellar and protostellar cores were identified, and the evolutionary stages of the protostars were estimated. Two of the new submm cores are previously unknown class 0 protostellar candidates. The equal number of prestellar and protostellar cores found implies that the du-ration of the prestellar phase is comparable with the free-fall time. However, this interpretation can be questioned on the basis of chemical data. In some sources, depletion has lead to the disappearance of gas phase N2H+and the most abun-dant ionic species are probably H+and HCO+. The ionization degree was found to be x(e)∼10−7. The estimated ambipo-lar diffusion timescales are∼70−100 times longer than the free-fall time.
The mass of the clump associated with IRAS 05405-0117 was found to be∼14 solar masses. It has an elongated struc-ture and, according to our data, consists of multiple low- to intermediate-mass dense cores. This suggest that it will even-tually form a small stellar group.
1.2.3 Near-infrared radiation as a tracer of cloud mass Near-infrared scattered light was observed towards several interstellar clouds using ESO/NTT and UKIRT telescopes.
In Corona Australis, in accordance with previous theoreti-cal predictions (Juvela et al. 2006), the observed surface brightness could be explained by pure light scattering. The near-infrared surface brightness was found to be an accurate tracer of cloud mass. This means that observations of scat-tered light could become an important new method in studies of dense, quiescent clouds (Juvela et al. 2008; ESO press release 06/08).
Near-infrared observations and the role of dust scatter-ing was investigated also in connection with external galax-ies. We performed a radiative transfer simulation study where the near-infrared reddening signatures of extragalactic dust clouds were examined in a simple plane-parallel model mim-icking a face-on spiral galaxy (Kainulainen et al. 2008).
Based on the simulations, we described the expected wave-length dependency of the reddening signatures for different scale heights of the dust distribution and thereby showed that the observed wavelength dependency can be used as an
indi-Figure 2: The IRAS 05405-0117 clump region. The large plus signs mark the positions of our molecular-line observa-tions. Also shown are the 24 µm peak positions of SMM 4 and IRAS 05405-0117, and the 24µm peak near SMM 5 (small green plus signs). The beam size is shown in the bot-tom left.
Figure 3: N2H+(1−0) and N2D+(2−1) spectra toward IRAS 05405-0117.
cator of the dust scale height. We also investigated the corre-lation between near-infrared reddening and total column den-sity in the models, and concluded that only about 10-20%
of the total mass of dust clouds is recovered by their near-infrared reddening signature.
One question of great current interest in star formation is the possible connection between the mass function of dense cores in molecular clouds and the initial mass function of stars. Recently, dust column density data derived from near-infrared extinction maps were used to derive the core mass function for the nearby Pipe Nebula. Collaborating with the research group responsible for the observational work, we performed a simulation study where we examined the fea-sibility of the method employed in composing the mass func-tion (Kainulainen et al. 2009). Using simulafunc-tions, we scribed the accuracy at which the mass function can be de-rived from dust column density data, and how the essential parameters of the core population affect the accuracy. In par-ticular, we showed that the core mass function can be derived quite reliably for relatively sparse clouds like the Pipe Neb-ula.
1.2.4 MHD phenomena: observations and modelling We have succesfully continued the investigation of the mag-netic field structure, especially the polarity of the field in spots of two active longitudes, in active late-type stars (detected earlier on by the group using surface temperature maps and named as ”active star Hale rule”). Spectroscopic observa-tions, based on which surface temperature maps have been inverted, were started in 1991 with the high resolution spec-trograph SOFIN at the Nordic Optical Telescope, La Palma.
The time series collected since is one of the few most exten-sive and complete existing data sets to study long-term vari-ability (cycles) in active late-type stars. An important devel-opment is the magnetic inversions based on new spectropo-larimetic data with upgraded spectropolarimeter and reduc-tion software, giving the first observareduc-tional proof for the the-oretical prediction of the magnetic field polarity. The work is done in collaboration with astronomers in Uppsala, Sweden, and Potsdam, Germany, most of the collaborators originally having worked in Helsinki Observatory and/or Oulu Univer-sity.
Simultaneously to the observations, local and global MHD models (PENCIL-CODE, MEFISTO; Korpi, K¨apyl¨a, Liljestr¨om, Lindborg, Snellman) have been developed and utilized, to be able to investigate the transformation from solar-like dynamo activity to the activity seen in the active rapid rotators. This includes participation in a solar dynamo benchmark project which is likely to be helpful in the devel-opement and validation dynamo codes (Jouve et al. 2008).
Numerical modeling has yielded new results in a variety of systems: local turbulence models have been utilized to study the turbulent transport of angular momentum (K ¨apyl¨a
& Brandenburg 2008; Liljestr ¨om et al. 2009) and turbu-lent transport coefficients relevant for dynamo action (Bran-denburg et al. 2008; Mitra et al. 2009; K¨apyl¨a & Branden-burg 2009). Numerical studies of convection have, for the first time, revealed a large-scale dynamo (K¨apyl¨a et al. 2008, 2009a,b, in press). The latter project was accepted to the CSC grand challenge programme (DYNAMO08 PId by Ko-rpi), wherefrom 1 660 000 CPU hours of computing time was granted and used during the year 2008. Furthermore, a hydro-dynamic instability discovered earlier in isotropically forced turbulence was studied in detail numerically (K¨apyl¨a et al.
2009c).
1.2.5 Magnetic fields in interstellar clouds
Studies were carried out of the polarized sub-millimeter emis-sion from dust grains in interstellar clouds. In a magnetic field, dust grains remain aligned as long as their rotation speed is significantly larger than their thermal rotation speed.
The grains are believed to be spinned up mainly by radiative torques. The efficiency of radiative torques was investigated using magnetohydrodynamic cloud simulations and detailed radiative transfer modelling (Pelkonen et al. 2009). Results indicate large spatial variations in the polarization efficiency.
In particular, in dense clouds, the dust emission is not likely to probe magnetic fields deeper than a few magnitudes in AV. Through Zeeman effect, magnetic fields cause splitting of some radio lines. Because the split components have oppocite circular polarization, the line-of-sight component of the mag-netic field can be estimated from the Stokes I and V spectra.
With radiative transfer modelling, three-dimensional magne-tohydrodynamic simulations could be compared with existing measurements of the Zeeman effect in cloud cores (Lunttila et al. 2008). Good agreement was found between models of super-Alfv´enic turbulence, combined with self-gravity, and available observations of OH molecule lines. This suggests that the average magentic field of molecular clouds may be low and supports the idea of turbulence as a central factor behind the formation of self-gravitating cloud cores.
1.2.6 The Planck and Herschel satellite projects
We participate in several science projects within the Planck Surveyor satellite consortium. Our emphasis is on studies of dense interstellar clouds. Preparations were made to study Galactic cold and compact cloud cores using Planck data.
The population of cold cores (Tdust < 12 K) is still poorly known and Planck will be the first space borne mission that will be sensitive to their radiation. Methods were developed for the detection of cold cores and for the analysis of their dust emission. We also participate in some other Planck sci-ence projects, including the study of nearby galaxies.
We coordinated a successful open time key program pro-posal which was awarded ∼151 hours of observing time on the Herschel satellite. The aim of this project is to use
Herschel satellite for follow-up observations of a number of cold cloud cores detected in the Planck survey. Herschel will observe wavelengths close to the peak of dust emission, λ∼100−500µm, and, compared to Planck, will have much higher spatial resolution. Therefore, we will be able to study the internal structure of the selected cores, determine their evolutionary stages and their relation to future star formation.
We also participate in the Herschel key programme HiGal that will map a large fraction of the plane of our Galaxy.
1.2.7 Extragalactic background light
Using our understanding of the light scattering in dense in-terstellar clouds of dust, we have been developing a method for the detection of the optical extragalactic background light.
This is the so-called ‘shadow of a dark cloud method”. Based on our previous photometric EBL observing program we have developed a spectroscopic analogy for it. This new tech-nique utilizes the difference between the spectra of the dif-fuse galactic scattered light (absorption line spectrum) and the EBL (pure continuum spectrum with possible discontinu-ities). For the spectroscopic observing program we (Mattila, Lehtinen, Vaisanen) have received earlier 20 hours observing time at ESO VLT telescope and FORS instrument . For the ESO Period 82 18.5 hours were again allocated but only a few hours were realised in 2008/09. The reductions and anal-ysis of these data are currently being performed by Mattila and Lehtinen. The modelling of the FORS spectra requires knowledge of the spectrum of the Local Interstellar Radia-tion Field (ISRF). A new synthetic spectral model is being developed along the lines of an earlier model by Mattila (1980 A&AS 39, 53) based on the high-resolution stellar spectrum library STELIB (Borgne et al. 2003, A&A 4002, 433).
Using data from the ISOPHOT instrument of the ISO satellite, we have completed a study of the extragalactic far-infrared background light (Juvela et al. 2009). The signal represents a significant fraction of the cosmic energy output from stars that has been reprocessed by interstellar dust and is redshifted to far-infrared wavelengths. Our study is the first independent test of the results obtained with the COBE satel-lite some ten years earlier. Our values are in agreement with the published COBE results, confirming the intensity of this extragalactic component at a level of∼1 MJy sr−1 at wave-lengths∼150–180µm.