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Register a new userYSO search toward the boundary of the Central Molecular Zone with near-infrared polarimetry
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Added by
Tatsuhito Yoshikawa
Publication date
2014
fields
Physics
and research's language is
English
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We have carried out near-infrared polarimetry toward the boundary of the Central Molecular Zone, in the field of (-1.4 deg $lesssim l lesssim$ -0.3 deg and 1.0 deg $lesssim l lesssim$ 2.9 deg, $|b|lesssim$ 0.1 deg), using the near-infrared polarimetric camera SIRPOL on the 1.4 m Infrared Survey Facility telescope. We have selected 112 intrinsically polarized sources on the basis of the estimate of interstellar polarization on Stokes $Q/I-U/I$ planes. The selected sources are brighter than $K_S=14.5$ mag and have polarimetric uncertainty $delta P<1,%$. Ten of these distinctive polarized sources are fit well with spectral energy distributions of young stellar objects when using the photometry in the archive of the Spitzer Space Telescope mid-infrared data. However, many sources have spectral energy distributions of normal stars suffering heavy interstellar extinction; these might be stars behind dark clouds. Due to the small number of distinctive polarized sources and candidates of young stellar object, we cannot judge if there is a decline of them outside the Central Molecular Zone. Many of massive candidates of young stellar object in the literature have only small intrinsic polarization. This might suggest that their masses are 4-15 M$_{{rm sun}}$, whose intrinsic polarization has been expected to be small.
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We present a NIR polarimetric map of the 1deg by 1deg region toward the Galactic center. Comparing Stokes parameters between highly reddened stars and less reddened ones, we have obtained a polarization originating from magnetically aligned dust grains at the central region of our Galaxy. The distribution of position angles shows a peak at the parallel direction to the Galactic plane, suggesting a toroidal magnetic field configuration. However, at high Galactic latitudes, the peak of the position angles departs from the direction of the Galactic plane. This may be a transition of a large-scale magnetic field configuration from toroidal to poloidal.
We conducted near-infrared (JHKs) imaging polarimetry toward the infrared dark cloud (IRDC) M17 SWex, including almost all of the IRDC filaments as well as its outskirts, with the polarimeter SIRPOL on the IRSF 1.4 m telescope. We revealed the magnetic fields of M17 SWex with our polarization-detected sources that were selected by some criteria based on their near-IR colors and the column densities toward them, which were derived from the Herschel data. The selected sources indicate not only that the ordered magnetic field is perpendicular to the cloud elongation as a whole, but also that at both ends of the elongated cloud the magnetic field appears to bent toward its central part, i.e., large-scale hourglass-shaped magnetic field perpendicular to the cloud elongation. In addition to this general trend, the elongations of the filamentary subregions within the dense parts of the cloud appear to be mostly perpendicular to their local magnetic fields, while the magnetic fields of the outskirts appear to follow the thin filaments that protrude from the dense parts. The magnetic strengths were estimated to be ~70-300 microG in the subregions, of which lengths and average number densities are ~3-9 pc and ~2-7x10^3 cm^{-3}, respectively, by the Davis-Chandrasekhar-Fermi method with the angular dispersion of our polarization data and the velocity dispersion derived from the C^{18}O (J=1-0) data obtained by the Nobeyama 45 m telescope. These field configurations and our magnetic stability analysis of the subregions imply that the magnetic field have controlled the formation/evolution of the M17 SWex cloud.
We present a spectroscopic follow-up of photometrically-selected young stellar object (YSO) candidates in the Central Molecular Zone of the Galactic center. Our goal is to quantify the contamination of this YSO sample by reddened giant stars with circumstellar envelopes and to determine the star formation rate in the CMZ. We obtained KMOS low-resolution near-infrared spectra (R ~4000) between 2.0 and 2.5 um of sources, many of them previously identified, by mid-infrared photometric criteria, as massive YSOs in the Galactic center. Our final sample consists of 91 stars with good signal-to-noise ratio. We separate YSOs from cool late-type stars based on spectral features of CO and Br_gamma at 2.3 um and 2.16 um respectively. We make use of SED model fits to the observed photometric data points from 1.25 to 24 um in order to estimate approximate masses for the YSOs. Using the spectroscopically identified YSOs in our sample, we confirm that existing colour-colour diagrams and colour-magnitude diagrams are unable to efficiently separate YSOs and cool late-type stars. In addition, we define a new colour-colour criterion that separates YSOs from cool late-type stars in the H-Ks vs H-[8.0] diagram. We use this new criterion to identify YSO candidates in the |l| < 1.5, |b|<0.5 degree region and use model SED fits to estimate their approximate masses. By assuming an appropriate initial mass function (IMF) and extrapolating the stellar IMF down to lower masses, we determine a star formation rate (SFR) of ~0.046 +/- 0.026 Msun/yr assuming an average age of 0.75 +/- 0.25 Myr for the YSOs. This value is lower than estimates found using the YSO counting method in the literature. Our SFR estimate in the CMZ agrees with the previous estimates from different methods and reaffirms that star formation in the CMZ is proceeding at a lower rate than predicted by various star forming models.
Near-infrared polarimetry of point sources reveals the presence of a toroidal magnetic field in the central 20 x 20 region of our Galaxy. Comparing the Stokes parameters between high extinction stars and relatively low extinction ones, we have obtained a polarization originating from magnetically aligned dust grains at the central region of our Galaxy of at most 1-2 kpc. The derived direction of the magnetic field is in good agreement with that obtained from far-infrared/submillimeter observations, which detect polarized thermal emission from dust in the molecular clouds at the Galactic center. Our results show that by subtracting foreground components, near-infrared polarimetry allows investigation of the magnetic field structure at the Galactic center. The distribution of the position angles shows a peak at around 20deg, nearly parallel to the direction of the Galactic plane, suggesting a toroidal magnetic configuration.
Previous radio observations revealed widespread gas-phase methanol (CH$_3$OH) in the Central Molecular Zone (CMZ) at the Galactic center (GC), but its origin remains unclear. Here, we report the discovery of CH$_3$OH ice toward a star in the CMZ, based on a Subaru $3.4$-$4.0 mu$m spectrum, aided by NASA/IRTF $L$ imaging and $2$-$4 mu$m spectra. The star lies $sim8000$ au away in projection from a massive young stellar object (MYSO). Its observed high CH$_3$OH ice abundance ($17%pm3%$ relative to H$_2$O ice) suggests that the $3.535 mu$m CH$_3$OH ice absorption likely arises in the MYSOs extended envelope. However, it is also possible that CH$_3$OH ice forms with a higher abundance in dense clouds within the CMZ, compared to within the disk. Either way, our result implies that gas-phase CH$_3$OH in the CMZ can be largely produced by desorption from icy grains. The high solid CH$_3$OH abundance confirms the prominent $15.4 mu$m shoulder absorption observed toward GC MYSOs arises from CO$_2$ ice mixed with CH$_3$OH.
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