AKARI Near-Infrared Spectroscopy Towards Young Stellar Objects Aleksandra Ardaseva 1,TakashiOnaka 2 1 University of St Andrews, United Kingdom 2 Department of Astronomy, Graduate School of Science, University of Tokyo, Japan ABSTRACT This project analyses infrared spectra of two Young Stellar Objects (YSO s) in range of 2 5 µm from the data provided by AKARI Infrared Space Telescope. The first star-froming region located towards Sgr B1 showed CO 2 ice, CO ice and XCN absorption features. Column densities of each chemical species were estimated. In the second YSO near GAL 318.05 was identified CO gas emission. After comparison with theoretical models the shift in the wavelength was found. The shiftt might be caused by the expansion of the gas from supernova remnant located nearby. 1. INTRODUCTION Spectroscopy provides astronomers with the method to study chemical composition and physical properties of objects in the space. When a solid absorbs a photon, it will excite an electronic state or a vibration state depending on the energy of the photon. For a photon at the infrared, it will usually excite a vibration sate and an absorption feature appears relating to the lattice structure. De-exitation of the state will emit an infrared photon characteristic for the lattice vibration. Thus the infrared spectrum gives us information on species responsible for the features. Infrared refers to a broad range of wavelengths from 1 to 300 microns and shows absorption features of various gases such as O 2,H 2,N 2. Studying infrared spectrum has several di culties. A major problem is Earth s athmosphere that is opaque at some wavelengths it blocks specific ranges and emits its own radiation. One of such region from 2 to 30 µm is very important for the study of ices, therefore for this purpose space-ground telescopes are used. [Gibb et al., 2004] In this project were used near-infrared (NIR) Ns and Nh slit data from AKARI Infrared Space Telescope. For Ns both high-resolutiong grism (NG) and low-resolution prism (NP) data were used, but for Nh only NG data. Both projects were dedicated to the study of Young Stellar Objects(YSO s). The first project focuses on typical star-formation absorption features, such as CO ice, CO 2 ice and XCN. XCN feature is rarely found in spectra of protostars embedded in dense molecular clouds. It denotes nitrile/isonitrile goup (CN) and unknown structure (X). Its is suggested that the formation of XCN might be caused by ultraviolet radiation from protostars. [Bernstein et al., 2000] The mechanism of both ice and XCN formation is not well understood yet. The second project studies CO gas emission from a star-forming region.
2. PROJECT ONE - 5200149.1 The first project looks at the YSO located towards the Sgr B1 close to the center of the Galaxy. 2.1 Methods In the first project were used Nh high-resolution grism data. After filtering the original image (AKARI manual, 2008) it was possible to extract the image of the spectra. Spectra varies along the slit, therefore plots of flux vs wavelength were produced at various positions with the step of 3 pixels (1 pixel corresponds to 1.46 arcsec). Using software Fityk, continuum was subtracted and Gaussian fit was applied to each spectrum. (Figure 1) Figure 1: Gaussian fit of the spectrum at the shift +6. Green line is the original spectrum. Yellow line shows fitted continuous spectrum. Red lines are Gaussian fit for each feature. From this fit the central wavelengths and the full-width-half-maximum (FWHM) were estimated. values are used to estimate the column density for CO ice, CO 2 ice and XCN from formula (1). These = R e 1 1 0 ( 1 ) 2 d B p a0 FWHM = 2 p ln(2) B where B is the absorption strength, a 0 is the amplitude. The column density gives us an estimate of the abundance of the species in question in the observed object. 2.2 Discussion (1) From the spectrum extracted after filtering were suggested CO ice, CO 2 ice and XCN absorption features. (Figure 2) These features were observed at all positions, but at positions -6 and -9 it was hard to resolve two absorption peaks of XCN and CO, because they merge together. After applying Gaussian fit, the central wavelengths and the FWHM were estimated, which allow the identification of the species responsible for each feature. Theoretical values for CO 2 ice are =4.27 µm, for XCN: =4.6.2 µm and for CO ice: =4.6731 µm. [Gibb et al., 2004] Figure 3 shows the values of central wavelengths for each element at di erent positions. Figure 2: Extracted spectrum at the shift +6
(a) CO 2 ice (b) XCN (c) CO ice Figure 3: Variation of central wavelength along the slit CO ice has red and blue components at 4.681 and 4.665 µm respectively which cannot be resolved, but might shift a peak of the central wavelength. From Figure 3 (a) it is seen that most estimated central wavelengths are larger than the theoretical values that could arise from the redshift. XCN absorption feature is quite rare and is observed in a limited number of embedded YSO s. [Gibb et al., 2004] Before this feature was detected toward Sgr A*, Galactic center region. Our target is also located near the Galactic center region, but at a di erent position - in Sgr B1. XCN feature might be related to the triple bond C-N, most likely OCN- ion, but this needs further investigation. Figure 4: Variation of column densities along the slit. Blue line - XCN, red line - CO 2 ice, green line - CO ice. Using formula (1) were calculated values of column densities at all positions. Figure 4 shows variation of amount of the species at di erent parts of the object. 3. PROJECT TWO - 5201570.1 The second object is also a YSO. The closest object near our YSO is GAL 318.05+00.08. 3.1 Methods For the second project we used Ns data with both NG and NP. Using the same technique as in the first project, we produced a filtered image of spectrum. (Figure 5)
Figure 5: High-resolution grism spectrum of 5201570.1 The spectrum shows di erent features if we move along the slit. At NG shift equal to -11 the spectrum showed absorption features typical for star-forming regions, such as CO 2 ice, CO ice and H 2 O ice. However, at the shift equal to -2 the spectrum changes its shape. (Figure 6) The same features were observed in NP data set. It is important to look at both NP and NG, because high-resolution data (NG) are noisier than low-resolution (NP). (a) NG shift -11 (b) NG shift -2 Figure 6: Two graphs show spectrum graphs at two shifts: -11 and -2. It was suggested that the peak spectrum approximately at 4.2 4.8 µm might be CO gas emission. To prove this we ran a model for CO emission. CO emission spectrum changes its shape with temperature. Therefore, various temperature combinations were simulated to find the best fit adjusting including increasing continuum. To determine the best fit was used equation (2). Dividing this value by the number of datapoints gives the goodness of the fit which tends to 1. NX x 2 (data model) 2 = noise 2 (2) i=1 3.2 Discussion Applying CO gas emission model showed that the model and the observed spectrum have the same shape. However, the observed spectrum is shifted to the left (approximately 1.5 µm) comparing to the model. To solve
this di erence we assume that the object is approaching us (blue-shift). The best fit is obtained when blue-shifted by -4000 km/s and T = 1500 K to the model in both NG and BP data sets. (Figure 7) (a) NG data (b) NP data Figure 7: Boths plots show the observed spectrum and CO gas emission model of 1500 K blue-shifted by -4000 km/s. The blue line with errorbars are the observed spectra and the red line is CO gas model. Such blue-shift might arise from the emission of supernova remnant located nearby. However, such phenomenon is extremely rare, because CO is rapidly destroyed in the environent of supernova remnants. There are several conditions which destroy CO molecules: collisions with He ions, impacts with energetic electrons, energy transfer with Ne +, photodissociation and photoionisation. [Rho et al., 2012] Previously CO gas emission was detected in the young supernova remnant Cassiopeia A (Cas A). Visual comparison of Cas A spectra and 5201570.1 spectra show similarities in shape and wavelength position. Both spectra show double-peaked profiles at 4.65 µm. CO detection in young supernova remnants gives new information about the storage of carbon in interstellar medium. Although it is generally thought that carbon in the ISM is in a form of C+, but these observations suggest that carbon in the ISM can also be in a form of CO, if it is well shielded from the strong interstellar radiation. [Rho et al., 2012] Discovery of CO also impacts understanding of the processes happening in supernova remnants. The detection of CO in 5201570.1 requires further invesitgation and validation. 4. CONCLUSION Both projects showed interesting results that should be investigated further. In 5200149.1 spectrum were identified CO ice, CO 2 ice and XCN absorption features. If confirmed, it would be the first discovery of XCN in the region of Sgr B1. XCN feature is quite rare and not well understood yet. Column densities were estimated for all elements at various slit positions to see variation of amount of the species. In 5201570.1 spectrum was found unusual CO gas emission feature. From the fitted model it was determined, that the temperature of CO gas is 1500 K and it is blue-shifted by - 4000 km/s. Such blue-shift might be associated with supernova remnant emission. It was previously discovered in CasA supernova remnant, and its shape of spectrum line is similar to 5201570.1. More investigation is needed to confirm or not.
ACKNOWLEDGMENTS I would like to thank Prof. Takashi Onaka for the opportunity to do a research at his laboratory at the University of Tokyo. I would also like to thank Tamami Mori-Ito and Dr. Mark Hammonds for their help in my project. I am grateful to the Graduate School of Science and ILO o ce for warm welcome in Japan and for a given chance to work at the University of Tokyo. REFERENCES [1] Bernstein M. P., Sandford S. A., Allamandola L. J., (2000), H, C, N, and O Isotopic Substitution Studies of the 2165 Wavenumber (4.62 micron) XCN Feature Produced By Ultraviolet Photolysis of Mixed Molecular Ices, The Astrophysical Journal, 542 : 894-897 [2] Gibb E. L., Whittet D. C. B., Boogert A. C. A., Tielens A. G. G. M., (2004), Interstellar Ice: The Infrared Space Observatory Legacy, The Astrophysical Journal Supplement Series, 151, 35 [3] Lorente R., Onaka T., Ita Y., Ohyama Y., Tanabe T., Pearson C. et al., (2008), AKARI IRC Data User Manual [4] Rho J., Onaka T., Cami J., Reach W. T., (2012), Spectroscopic Detection of Carbon Monoxide In the Young Supernova Remnant Cassiopeia A, The Astrophysical Journal Letters, 747 : L6 (5pp)