mid-IR Source Research Articles

Frequency-comb-referenced mid-IR sources for next-generation environmental sensors

We report on recent development of IR spectrometers based on non-linear optical generation and quantum-cascade lasers. Frequency stabilization and referencing to optical frequency-comb synthesizers is described. Their characteristics for spectroscopic as well as metrological applications are pointed out. The potential of the combination of mid-IR sources with enhancement cavities for high-resolution and sensitive measurements of gas spectra is illustrated. Finally, we describe wave-front engineering of long wavelength beams for spatial control and imaging applications.

Subpicosecond fiber-based soliton-tuned mid-infrared source in the 97–149 μm wavelength region

We present a fiber-based mid-IR source of ultrafast laser pulses tunable in the 9.7-14.9 microm spectral region based on difference-frequency mixing between an Er:fiber laser and a first-order frequency-shifted soliton pulse. We have measured subpicosecond pulses at a repetition rate of 37 MHz, with a maximum power of 1.5 microW.

VARIABLE RADIO SOURCES IN THE GALACTIC PLANE

Using three epochs of VLA observations of the Galactic Plane in the first quadrant taken ~15 years apart, we have conducted a search for a population of variable Galactic radio emitters in the flux density range 1-100 mJy at 6 cm. We find 39 variable sources in a total survey area of 23.2 sq deg. Correcting for various selection effects and for the extragalactic variable population of active galactic nuclei, we conclude there are ~1.6 Galactic sources per sq deg which vary by more than 50% on a time scale of years (or shorter). We show that these sources are much more highly variable than extragalactic objects; more than 50% show variability by a factor >2 compared to <10% for extragalactic objects in the same flux density range. We also show that the fraction of variable sources increases toward the Galactic center (another indication that this is a Galactic population), and that the spectral indices of many of these sources are flat or inverted. A small number of the variables are coincident with mid-IR sources and two are coincident with X-ray emitters, but most have no known counterparts at other wavelengths. Intriguingly, one lies at the center of a supernova remnant, while another appears to be a very compact planetary nebula; several are likely to represent activity associated with star formation regions. We discuss the possible source classes which could contribute to the variable cohort and followup observations which could clarify the nature of these sources.

Characteristics of infrared point sources associated with OH masers

We collect 3249 OH maser sources from the literature published up to April 2007, and compile a new catalog of OH masers. We look for the exciting sources of these masers and their infrared properties from IRAS and MSX data, and make a statistical study. MSX sources associated with stellar 1612 MHz OH masers are located mainly above the blackbody line; this is caused by the dust absorption of stellar envelopes, especially in the MSX_A band. The mid-IR sources associated with stellar OH masers are concentrated in a small region in an [A]–[D] vs. [A]–[E] diagram with a small fraction of contamination; this gives us a new criterion to search for new stellar OH masers and distinguish stellar masers from unknown types of OH masers. IR sources associated with 1612 MHz stellar OH masers show an expected result: the average flux of sources with F60 > F25 increases with increasing wavelength, while those with F60 < F25 vary little with wavelength, because the sources with F60 < F25 are much hotter than those with F60 > F25.

Lasers on overtone transitions of carbon monoxide molecule

The experimental and theoretical study of lasers working on the first vibrational overtone of the CO molecule is reviewed. A theoretical model of the active medium of the CO laser and its verification are presented. The CO laser operation in a wide spectral range (2.5–4.2 μm) that spans the atmospheric transparency window is demonstrated in the review of experimental works. It is shown that the overtone CO laser can serve as an effective mid-IR source for atmospheric spectroscopy, long-distance transportation of laser radiation, remote laser sensing, etc.

MULTIPLICITY OF THE PROTOSTAR SERPENS SMM 1 REVEALED BY MILLIMETER IMAGING

The Serpens SMM 1 region was observed in the 6.9 mm continuum with an angular resolution of about 0.6 arcsec. Two sources were found to have steep positive spectra suggesting emission from dust. The stronger one, SMM 1a, is the driving source of the bipolar jet known previously, and the mass of the dense molecular gas traced by the millimeter continuum is about 8 solar mass. The newly found source, SMM 1b, positionally coincides with the brightest mid-IR source in this region, which implies that SMM 1b is yet another young stellar object. SMM 1b seems to be less deeply embedded than SMM 1a. SMM 1 is probably a protobinary system with a projected separation of 500 AU.

A proto brown dwarf candidate in Taurus

Aims. We search for brown dwarfs at the Class 0/I evolutionary stage, or proto brown dwarfs. Methods. We present a multi wavelength study, ranging from optical at 0.8 μm to radio wavelengths at 6 cm, of a cool, very faint, and red multiple object, SSTB213 J041757, detected by Spitzer toward the Barnard 213 dark cloud, in Taurus. Results. The SED of SSTB213 J041757 displays a clear excess at long wavelengths resembling that of a Class I object. The mid-IR source has two possible counterparts, A and B, in the near-IR and optical images, and the 350 μm observations detect clear extended emission, presumably from an envelope around the two sources. The position of A & B in the (Ic− J) versus (J − [3.6]) colour-colour diagram is consistent with them being Galactic sources and not extragalactic contaminants. A proper-motion study confirms this result for A, while it is inconclusive for B. The temperature and mass of the two possible central objects, according to COND evolutionary models, range between 1550−1750 K and 3−4 M_(Jupiter), and 950−1300 K and 1−2 M_(Jupiter), for A and B, respectively. The integrated SED provides bolometric temperatures and luminosities of 280 K and 0.0034 L_⊙, assuming that the emission at wavelengths > 5 μm is associated with component A, and 150 K and 0.0033 L_⊙, assuming that the emission at wavelengths > 5 μm is associated with component B, which would imply the SSTB213 J041757 object has a luminosity well below the luminosity of other very low luminosity objects discovered up to date. Conclusions. With these characteristics, SSTB213 J041757 seems to be a promising, and perhaps double, proto brown dwarf candidate.

Ultra-stable, widely tunable and absolutely linked mid-IR coherent source

We report on a new coherent source that, using a phase-lock scheme to an optical frequency-comb synthesizer, achieves a 10-Hz intrinsic linewidth, is tunable from 4 to 4.5 microm with a presettable absolute frequency and, when coupled to a high-finesse cavity, can provide a short-term absorption sensitivity of 1.3 x 10(-11) cm(-1)Hz,(-1/2). These unique spectral features make this source a precise tool for molecular physics.

RADIO CONTINUUM EMISSION AND WATER MASERS TOWARD CB 54

We present high angular resolution observations of water masers at 1.3 cm and radio continuum emission at 1.3, 3.6 and 6 cm towards the Bok globule CB 54 using the Very Large Array. At 1.3 cm, with subarsecond angular resolution, we detect a radio continuum compact source located to the south-west of the globule and spatially coincident with a mid-infrared embedded object (MIR-b). The spectral index derived between 6 and 1.3 cm (alpha=0.3+/-0.4) is flat, consistent with optically thin free-free emission from ionized gas. We propose the shock-ionization scenario as a viable mechanism to produce the radio continuum emission observed at cm frequencies. Water masers are detected at two different positions separated by 2.3'', and coincide spatially with two mid-infrared sources: MIR-b and MIR-c. The association of these mid-IR sources with water masers confirms that they are likely protostars undergoing mass-loss, and they are the best candidate as driving sources of the molecular outflows in the region.

Mid-IR short-pulse OPCPA with micro-Joule energy at 100kHz

We present a novel mid-IR source based on optical parametric chirped pulse amplification (OPCPA) generating 96 fs pulses (9.0 cycles) at 3.2 mm with an energy of 1.2 microJ, at a repetition rate of 100 kHz. The amplified spectrum supports a minimum Fourier transform limited pulse duration of 45 fs, or 4.2 cycles. Our use of OPCPA allows the direct amplification of few-cycle pulses at this mid-IR wavelength, and is inherently scalable to higher energies. The seed source for the system is based on difference frequency generation (DFG) between two outputs of the same fibre laser: this source is expected to be intrinsically CEP stable.

PROPERTIES OF PROTOSTARS IN THE ELEPHANT TRUNK IN THE GLOBULE IC 1396A

Extremely red objects, identified in the early Spitzer Space Telescope observations of the bright-rimmed globule IC 1396A and photometrically classified as Class I protostars and Class II T Tauri stars based on their mid-infrared (mid-IR) colors, were spectroscopically observed at 5.5-38 μm (Spitzer Infrared Spectrograph), at the 22 GHz water maser frequency (National Radio Astronomy Observatory Green Bank Telescope), and in the optical (Palomar Hale 5 m) to confirm their nature and further elucidate their properties. The sources photometrically identified as Class I, including IC 1396A:α, γ, δ, e, and ζ, are confirmed as objects dominated by accretion luminosity from dense envelopes, with accretion rates 1-10 × 10^–6 M☉ yr^–1 and present stellar masses 0.1-2 M☉. The Class I sources have extremely red continua, still rising at 38 μm, with a deep silicate absorption at 9-11 μm, weaker silicate absorption around 18 μm, and weak ice features including CO2 at 15.2 μm and H2O at 6 μm. The ice/silicate absorption ratio in the envelope is exceptionally low for the IC 1396A protostars, compared to those in nearby star-forming regions, suggesting that the envelope chemistry is altered by the radiation field or globule pressure. Only one 22 GHz water maser was detected in IC 1396A; it is coincident with a faint mid-IR source, offset from near the luminous Class I protostar IC 1396A:γ. The maser source, IC 1396A:γb, has luminosity less than 0.1 L☉, the first H2O maser from such a low-luminosity object. Two near-infrared (NIR) H2 knots on opposite sides of IC 1396A:γ reveal a jet, with an axis clearly distinct from the H2O maser of IC 1396A:γb. The objects photometrically classified as Class II, including IC 1396A:β, θ, Two Micron All Sky Survey (2MASS)J 21364964+5722270, 2MASSJ 21362507+5727502, LkHα 349c, Tr 37 11-2146, and Tr 37 11-2037, are confirmed as stars with warm, luminous disks, with a silicate emission feature at 9-11 μm, and bright Hα emission; therefore, they are young, disk-bearing, classical T Tauri stars. The disk properties change significantly with source luminosity: low-mass (G-K) stars have prominent 9-11 emission features due to amorphous silicates while higher-mass (A-F) stars have weaker features requiring abundant crystalline silicates. A mineralogical model that fits the wide- and low-amplitude silicate feature of IC 1396A:θ requires small grains of crystalline olivine (11.3 μm peak) and another material to to explain its 9.1 μm peak; reasonable fits are obtained with a phyllosilicate, quartz, or relatively large (greater than 10 μm) amorphous olivine grains. The distribution of Class I sources is concentrated within the molecular globule, while the Class II sources are more widely scattered. Combined with the spectral results, this suggests two phases of star formation, the first (4 Myr ago) leading to the widespread Class II sources and the central O star of IC 1396 and the second (less than 1 Myr ago) occurring within the globule. The recent phase was likely triggered by the wind and radiation of the central O star of the IC 1396 H II region.

THE LARGE MAGELLANIC CLOUD'S TOP 250: CLASSIFICATION OF THE MOST LUMINOUS COMPACT 8 μm SOURCES IN THE LARGE MAGELLANIC CLOUD

To ascertain the nature of the brightest compact mid-infrared (mid-IR) sources in the Large Magellanic Cloud (LMC), we have applied an updated version of Buchanan et al.'s Two Micron All Sky Survey (2MASS)–Midcourse Space Experiment (MSX) color classification system, which is based on the results of Spitzer Space Telescope spectroscopy, to a mid-IR (8 μm) flux-limited sample of 250 LMC objects for which 2MASS and MSX photometry is available. The resulting 2MASS–MSX (“JHK8”) color-based classifications of these sources, which constitute the most mid-IR-luminous objects in the LMC, were augmented, cross-checked, and corrected where necessary via a variety of independent means, such that only 46 sources retain tentative classifications and only 10 sources cannot be classified at all. The sample is found to consist primarily of carbon-rich asymptotic giant branch (AGB) stars (35%), red supergiants (RSGs) (18%), and compact H ii regions (32%), with additional, small populations of oxygen-rich AGB stars (∼5%), dusty, early-type emission-line stars (∼3%), and foreground, O-rich AGB stars in the Milky Way (∼3%). The very large ratio of C-rich to O-rich objects among the luminous and heavily dust-enshrouded AGB stars in our LMC IR source sample is consistent with the hypothesis that carbon stars form easily in lower metallicity environments. We demonstrate that very luminous C-rich and O-rich AGB stars and RSGs, identified here primarily on the basis of their JHK8 colors, also appear as distinct clusters in Spitzer Infrared Array Camera (IRAC)/Multiband Imaging Photometer for Spitzer (MIPS) color–color diagrams. Thus, in principle, the infrared spectrograph (IRS)-based IR photometric classification techniques applied here to the LMC can be applied to any external galaxy whose most luminous IR point sources are detectable and resolvable by 2MASS and Spitzer.

Broadly tunable single-frequency cw mid-infrared source with milliwatt-level output based on difference-frequency generation in orientation-patterned GaAs

A narrow-linewidth mid-IR source based on difference-frequency generation of an amplified 1.5 microm diode laser and a cw Tm-doped fiber laser in orientation-patterned (OP) GaAs has been developed and evaluated for spectroscopic applications. The source can be tuned to any frequency in the 7.6-8.2 microm range with an output power of 0.5 mW. The measured characteristics of the OP-GaAs sample demonstrate a high quality of the material.

Discovery of the Dust-Enshrouded Progenitor of SN 2008S with Spitzer

We report the discovery of the progenitor of the recent type IIn SN 2008S in the nearby galaxy NGC 6946. Surprisingly, it was not found in deep, pre-explosion optical images of its host galaxy taken with the Large Binocular Telescope, but only through examination of archival Spitzer mid-IR data. A source coincident with the SN 2008S position is clearly detected in the 4.5, 5.8, and 8.0 micron IRAC bands, showing no evident variability in the three years prior to the explosion, yet is undetected at 3.6 and 24 micron. The distinct presence of ~440 K dust, along with stringent LBT limits on the optical fluxes, suggests that the progenitor of SN 2008S was engulfed in a shroud of its own dust. The inferred luminosity of 3.5x10^4 Lsun implies a modest mass of ~10 Msun. We conclude that objects like SN 2008S are not exclusively associated with the deaths or outbursts of very massive eta Carinae-like objects. This conclusion holds based solely on the optical flux limits even if our identification of the progenitor with the mid-IR source is incorrect.

The Mid‐Infrared Properties of X‐Ray Sources

We combine the results of the Spitzer IRAC Shallow Survey and the Chandra XBootes Survey of the 8.5 deg2 Bootes field of the NOAO Deep Wide-Field Survey to produce the largest comparison of mid-IR and X-ray sources to date. The comparison is limited to sources with X-ray fluxes >8 × 10−15 ergs cm−2 s−1 in the 0.5-7.0 keV range and mid-IR sources with 3.6 μm fluxes brighter than 18.4 mag (12.3 μJy). In this most sensitive IRAC band, 85% of the 3086 X-ray sources have mid-IR counterparts at an 80% confidence level based on a Bayesian matching technique. Only 2.5% of the sample have no IRAC counterpart at all based on visual inspection. Even for a smaller but a significantly deeper Chandra survey in the same field, the IRAC Shallow Survey recovers most of the X-ray sources. A majority (65%) of the Chandra sources detected in all four IRAC bands occupy a well-defined region of IRAC [3.6] − [4.5] versus [5.8] − [8.0] color-color space. These X-ray sources are likely infrared-luminous, unobscured type I AGNs with little mid-infrared flux contributed by the AGN host galaxy. Of the remaining Chandra sources, most are lower luminosity type I and type II AGNs whose mid-IR emission is dominated by the host galaxy, while approximately 5% are either Galactic stars or very local galaxies.

Compact mid-IR sources east of Galactic Center source IRS5

Aims. Mid-infrared observations of the Galactic Center show among the extended mini-spiral a number of compact sources. Their nature is of interest because they represent an interaction of luminous stars with the mini-spiral material or mass losing sources that are enshrouded in dust and gas shells. Characterizing their nature is necessary to obtain a complete picture of the different stellar populations and the star formation history of the cental stellar cluster in general. Prominent compact MIR sources in the Galactic Center are either clearly offset from the mini-spiral (e.g. the M2 super-giant IRS 7 and the bright dust enshrouded IRS 3) or have been identified earlier with bright bow shock sources (e.g. IRS 21, 1W, 10W and IRS 5). There are, however, four less prominent compact sources east of IRS 5, the natures of which were unclear until now. Methods. We present near-infrared K-band long slit spectroscopy of the four sources east of IRS 5 obtained with the ISAAC spectrograph at the ESO VLT in July 2005. We interpret the data in combination with high angular resolution NIR and MIR images obtained with ISAAC and NACO at the ESO VLT. Results. The K � -band images and proper motions show that the sources are multiple. For all but one source we find dominant contributions from late type stars with best overall fits to template stars with temperatures below 5000 K. Conclusions. The brightest sources contained in IRS 5NE, 5E and 5S may be asymptotic giant branch stars and a part of the MIR excess may be due to dust shells produced by the individual sources. However, in all cases an interaction with the mini-spiral cannot be excluded and their broad band infrared SEDs indicate that they could be lower luminosity counterparts of the identified bow shock sources. In fact, IRS 5SE is associated with a faint bow shock and its spectrum shows contributions from a hotter early type star which supports such a classification.

Chandra and Spitzer observations of CDFS X-ray obscured QSOs

We present Chandra and Spitzer data of the 186, extragalactic, hard 2-10 keV X-ray selected sources, which lie in the central part of the Chandra Deep Field South (CDFS). For the vast majority of sources (99.5%) there is a spectroscopic or photometric redshift available. We classify 17 sources as X-ray obscured QSOs, according to strictly X-ray criteria, i.e. defined as having large hydrogen column densities (N_H>10^22 cgs) and luminosities (Lx>10^44 cgs). The surface density of X-ray obscured QSOs is ~210 sq. deg. We find 18 candidate Compton thick N_H>10^24 cgs sources, of which three have QSO luminosities (L_x>10^44 cgs). The X-ray obscured QSO comprise a mixed bag of objects, covering the redshift range z=1.3-4.3. Eight of these show narrow line optical spectra, two show no obscuration in their optical spectra presenting Broad Lines, while for the other seven there is only a photometric redshift available. About half of the X-ray obscured QSOs show high X-ray to optical flux ratios, X/O>1, and red colours, I-3.6>4. Combination of the X-ray with the mid-IR 8 or 24 micron flux can be used as an additional diagnostic to sift out the heavily obscured AGN. All X-ray selected QSOs present red mid-IR colours and can be easily separated among mid-IR sources, demonstrating that mid-IR selection provides a powerful tool for the detection of obscured QSOs.

High-power, tunable difference frequency generation source for absorption spectroscopy based on a ridge waveguide periodically poled lithium niobate crystal

A novel waveguide for difference frequency generation in the mid-IR spectral region at 3.52 mum is characterized. High mid-IR power of 15 mW and an external conversion efficiency of up to 19 %W( -1) have been obtained. An optical beam propagation factor M(2) =1.18 was determined using the second moment method. A simple 2-f absorption spectra demonstrates the potential of this mid-IR source for high precision trace gas sensing applications.

Wavelength-agile mid-infrared (5-10 μm) generation using a galvano-controlled KTiOPO_4 optical parametric oscillator

A wavelength-agile mid-infrared (IR) ZnGeP2 (ZGP) optical parametric oscillator (OPO) using a galvano-controlled double-crystal KTiOPO4 (KTP) OPO was demonstrated. The mid-IR wavelength was tuned by varying the KTP OPO pump wavelength while the ZGP crystal angle remained fixed. Rapid tuning of the KTP OPO was achieved by changing the crystal angle by using the galvano scanner. Our mid-IR source can jump to a different wavelength without scanning through the intermediate wavelengths while also permitting continuous-wavelength scanning. The mid-IR source can be tuned from approximately 5to10 microm at a phase-matching angle of 51 degrees , while the pump wavelength is controlled in the 1.95-2.2 microm range.

Watt-level, high-repetition-rate, mid-infrared pulses generated by wavelength conversion of an eye-safe fiber source

We report on a mid-infrared (mid-IR) source consisting of an approximately 10 W average-power, linearly polarized 1.54 microm wavelength pulsed fiber source pumping an optical parametric oscillator. From this source, we obtained average power in excess of 1 W in the 3.8-4.0 microm wavelength range at a pulse repetition frequency of 100 kHz. With a slightly different setup, we also achieved an average power of 0.25 W at 4.5 microm wavelength. To our knowledge, these values represent the highest mid-IR power obtained through wavelength conversion of an eye-safe fiber source.