NEW INFRARED DIFFUSE INTERSTELLAR BANDS IN THE GALACTIC CENTER Tom Geballe (Gemini) Paco Najarro and Diego de la Fuente (Departamento de Astrofsica en el Centro de Astrobiologia, Madrid Don Figer, Center for Detectors, Rochester Institute of Technology Barret Schlegelmilch, UCLA Gemini Science Meeting, July 17, 2012
Central 1,000 ly (~300 pc) diam. Central Molecular Zone (CMZ) massive black hole 4 x 10 6 M sun densest & most massive concentration of stars 3 clusters of young and hot stars (only a few million years old) interstellar gas and dust (6 GMCs) Spitzer + VLA image
optical infrared CMZ Spiral arm Spiral arm Spiral arm PROBLEM: gas and dust along the line of sight (esp. in the CMZ and in the intervening spiral arms. Dust produces 30 mag (factor of 10 6 ) attenuation at visible wavelengths. Must observe GC in other wavebands.
Why study hot stars in the GC? understand massive star formation and its recent history in the GC understand unusual stars (Pistol, IRS8) determine ISM elemental abundances IRS8 Sgr A* and Central Cluster
Push studies of GC hot stars to shorter wavelengths. Almost all previous spectra of hot stars in GC are K-band spectra. (for good reason extinction), but 1. Many more diagnostic lines in H and J bands. 2. Some hot stars are surrounded by warm dust shells,. which emit strongly in the IR. Contamination of the stellar spectrum by c.s. dust emission less at shorter wavelengths (eg, Quintuplet Stars). unratioed He I GCS3-2 Gemini North (NIFS)
He I He I 3P-3D H I 10-4 Hydrogen Brackett series (n-4) H I 11-4 11-4 10-4 Normalized Normalized sig Sig gnal gnal N I 16-4 H I H I 14-4 15-4 15-4 14-4 H I 13-4 13-4 N I H I 12-4?????? P Cygni profile?? Gemini North (NIFS)
????? Gemini North (NIFS) Present in stars of all spectral types Invariant in wavelength from star to star Almost invariant in strength from star to star interstellar Not reported previously in stars or ISM Do not match wavelengths of simple molecules Diffuse Interstellar Bands??
What are DIBs? First two bands discovered: λλ5780, 5797 in ζ Per, ρ Leo (Mary Lea Heger, Lick, 1919) some broad ( diffuse ), many narrow Stationary (interstellar) Strengths scale roughly with extinction, but different DIBs not tightly correlated from sightline to sightline family of carriers. diffuse narrow
Only seen in diffuse ISM (n~100 cm -3, T~30 K) not in dark clouds Mostly in the visible None known in UV Few previously known in the IR (> 1µm) Plot courtesy of Ben McCall
Jenniskens & Desert 1994 Hobbs et al. 2008 Tuairisg et al. 2000 Hobbs et al. 2009 (A growing problem) None has been identified. Greatest unsolved problem in astronomical spectroscopy -! (90 years) General consensus: large carbon-bearing molecules, but no specific identification has survived scrutiny. Heger 1919 Merrill & Wilson 1938 Merrill & Wilson 1960 Herbig 1966 Herbig 1975 Herbig 1988
Cygnus OB2 stars If the H-band absorption features are DIBs, they should be detectable toward other diffuse clouds with DIBs. Najarro (unpublished data) Telescopio Nazionale Galileo (2002)
CONFIRMATION for the four strongest DIBs: relative strengths ~consistent with ratio of extinctions A V (Cyg OB2) ~ 6 mag A V (GC-diff.) ~ 20 mag Geballe et al. (2011)
~13 H-band DIBs (some need more confirmation) variety of widths and depths, and groupings longest wavelength DIBs currently known No K-band DIBS detected (τ < 0.02) Geballe et al. (2011)
Where are the DIBs (diffuse ISM) located on the 8 kpc GC sightline? CMZ Oka et al. 2005) probably mostly in the GC
WHAT ARE THEY GOOD FOR? Demonstrate that DIBs can survive in a harsher environment than Galactic plane diffuse clouds. Probes of ISM in inaccessible regions (eg, OB associations in distant parts of the Galaxy) Probes of (diffuse) gas in obscured AGN/ULIRGS. DIBs are sufficiently narrow to provide velocity info. (ULIRG) hydrocarbon abs. signpost of diffuse gas
WHAT ARE THEY GOOD FOR? (cont.) IDENTIFICATION? Correlation studies with other DIBs to determine if they and H-band DIBs arise from the same molecule or family of molecules. caution has been tried before with few useful results Better chance of identification via lab spectroscopy(?) (longest wavelength DIBS; H-band less congested than the optical) But caution - ~10 7 organic molecules known on earth ~10 200 stable molecules of weight < 750 containing only C, H, O, N, and S - Ben McCall
There is no better way to lose a scientific reputation than to speculate on the carrier for the diffuse bands. - William Klemperer