TiO Bands Research Articles

Highly stable CuO incorporated TiO2 catalyst for photocatalytic hydrogen production from H2O

A CuO incorporated TiO(2) catalyst was found to be an active photo-catalyst for the reduction of H(2)O under sacrificial conditions. The catalytic activity originates from the photogeneration of excited electrons in the conduction bands of both TiO(2) and CuO resulting in a build-up of excess electrons in the conduction band of CuO. Consequently, the accumulation of excess electrons in CuO causes a negative shift in the Fermi level of CuO. The efficient inter-particle charge transfer leads to a higher catalytic activity and the formation of highly reduced states of TiO(2)/CuO, which are stable even under oxygen saturated condition. Negative shift in the Fermi level of CuO of the catalyst TiO(2)/CuO gains the required over-voltage necessary for efficient water reduction reaction. The function of CuO is to help the charge separation and to act as a water reduction site. The amount of CuO and crystalline structure were found to be crucial for the catalytic activity and the optimum CuO loading was ca. approximately 5-10%(w/w).

Photoreduction of silver ions in a colloidal titanium dioxide suspension

The photoreduction of Ag + ions at the surface of colloidal TiO 2 nanoparticles has been investigated. Irradiation of an aqueous solution containing TiO 2 and Ag + ions with near-UV light brings about the growth of a sharp absorption band of colloidal silver in absorption spectrum. The photoreduction of Ag + ions by TiO 2 is optically sensitized by a simple cyanine dye, 3,3′-disulfopropyl-5,5′-dichloro-thiacyanine sodium salt (TC). TC molecules are adsorbed at the surface of TiO 2 nanoparticles in the form of the J-aggregate (JA). Visible-light irradiation of the solution containing TiO 2, Ag + ions, and TC causes the growth of a broad absorption band of the colloidal silver and the decrease of whole absorption bands of TC. Experimental results have been interpreted in terms of electronic energy levels of the colloidal silver and the TC aggregate, as well as energy bands of TiO 2. In the sensitized photoreduction, TiO 2 could play the role of a medium to facilitate efficient electron transfer from the excited TC aggregate to Ag + ions adsorbed at the surface.

A census of AGB stars in Local Group galaxies

Our photometric survey of Local Group galaxies, using a four filter technique based on the method of Wing (1971) allows the identification and subsequent characterization of their late-type stellar content. Two narrow band filters centred on molecular bands of TiO and CN allow us to distinguish between AGB stars of different chemistries (M-type (O-rich) and C-type (C-rich)). The faint dwarf spheroidal galaxy And II, a member of the M 31 subgroup, was surveyed. From photometry in V and i as well as in the narrow band filters TiO and CN we were able to identify 7 new AGB carbon stars, to derive their mean absolute � Miand bolometric magnitude Mbol, their luminosity function, and their spatial distribution. We are unable to establish the true C/M ratio because the few M stars of And II are overwhelmed by foreground Galactic M stars.

Further Results of TiO-Band Observations of Starspots

We present measurements of starspot parameters (temperature and filling factor) on five highly active stars, using absorption bands of TiO, from observations made between 1998 March and 2001 December. We determined starspot parameters by fitting TiO bands using spectra of inactive G and K stars as proxies for the unspotted photospheres of the active stars and spectra of M stars as proxies for the spots. For three evolved RS CVn systems, we find spot filling factors between 0.28 and 0.42 for DM UMa, 0.22 and 0.40 for IN Vir, and 0.31 and 0.35 for XX Tri; these values are similar to those found by other investigators using photometry and Doppler imaging. Among active dwarfs, we measured a lower spot temperature (3350 K) for EQ Vir than found in a previous study of TiO bands, and for EK Dra a lower spot temperature (~3800 K) than found through photometry. For all active stars but XX Tri, we achieved good phase coverage through a stellar rotational period. We also present our final, extensive grid of spot and nonspot proxy stars.

Size effects in the Raman spectra of TiO 2 nanoparticles

To investigate the relationship between the particle size and the Raman bands of TiO 2 nanoparticles, two different size-selected samples of TiO 2 nanoparticles were investigated using transmission electron microscopy (TEM), X-ray absorption near edge structure (XANES), extended X-ray absorption fine structure (EXAFS), and Raman spectroscopy. Analysis of the XANES spectra showed that the nanoparticles in both sample A (particle size ∼30 nm) and sample B (particle size ∼12 nm) have an anatase structure, and that there is no difference between the electronic structure of titanium atoms in TiO 2 nanoparticles and that of titanium atoms in bulk TiO 2. The EXAFS results indicate that a contraction effect occurs in the anatase TiO 2 nanoparticles. In the Raman spectra, both broadening and shifts of the Raman bands with decreasing particle diameter were observed. In this paper, these Raman shifts are attributed to the effects of decreasing particle size on the force constants and vibrational amplitudes of the nearest neighbor bonds.

Characterization of Raman Spectra of Size-Selected TiO2Nanoparticles by Two-Dimensional Correlation Spectroscopy

Nanometer-sized materials have intensively studied due to their unique physical and chemical properties as well as their great potential of technological applications. 1-5 Despite many studies on the structure-property relations of nanoparticles using the various characterization techniques, such an understanding is still to be answered. Therefore, we have recently explored the relationship between the size and structure of size-selected TiO 2 nanoparticles, which are prepared via a hydrolysis method with Ti[OCH(CH3)2]4 as the starting material, by X-ray absorption spectroscopy (XAS). 6 Analysis of the XAS of the samples with an average particle size of ~30 nm (sample A), ~12 nm (sample B), and ~7 nm (sample C) demonstrated that samples A and B have an anatase structure, whereas sample C has a structure very similar to that of the TiO 2 II phase. The TiO2 II phase generally arises only under high-pressure conditions. This difference can be attributed to size-induced radial pressure within the smaller nanoparticles, which plays an important role in the phase of TiO 2 nanoparticles in sample C. In addition, we have studied the relationship between the particle size and change of Raman bands of TiO 2 nanoparticles, sample A and sample B. 7 The obtained Raman spectra showed the broadening and shift of Raman bands with decreasing particle diameter. The origin of Raman shifts can be attributed to the effect of smaller particle size and it affects the force constant and vibrational amplitudes of the nearest neighbor bond. In this study, in order to analyze the vibrational modes of size-selected TiO2 nanoparticles and to the shift of Raman bands of TiO2 nanoparticles with particle size, we have applied two-dimensional (2D) correlation spectroscopy to the particle size-dependent Raman spectra of TiO 2 nanoparticles. Generalized 2D correlation spectroscopy has been applied extensively to the analysis of spectral data sets obtained during the observation of a system under some external perturbation. 8-10 Because of the wide range of applications of this technique, it has become one of the standard analytical techniques for interpreting various types of spectroscopic data. The details of this technique are described elsewhere, 8-10 so no further description is given here.

Identifying Young Brown Dwarfs Using Gravity‐Sensitive Spectral Features

We report the initial results of the Brown Dwarf Spectroscopic Survey Gravity Project to study gravity sensitive features as indicators of youth in brown dwarfs. Low-resolution (R ~ 2000) J-band and optical (R ~ 1000) observations using NIRSPEC and LRIS at the W. M. Keck Observatory reveal transitions of TiO, VO, K I, Na I, Cs I, Rb I, CaH, and FeH. By comparing these features in late-type giants and in old field dwarfs, we show that they are sensitive to the gravity (g = GM/R2) of the object. Using low-gravity spectral signatures as age indicators, we observed and analyzed J-band and optical spectra of two young brown dwarfs, G196-3B (20-300 Myr) and KPNO Tau 4 (1-2 Myr) and two possible low-mass brown dwarfs in the σ Orionis cluster (3-7 Myr). We report the identification of the bands of TiO near 1.24 μm and the A-X band of VO near 1.18 μm together with extremely weak J-band lines of K I in KPNO Tau 4. This is the first detection of TiO and VO in the J band in a substellar-mass object. The optical spectrum of KPNO Tau 4 exhibits weak K I and Na I lines, weak absorption by CaH, and strong VO bands, also signatures of a lower gravity atmosphere. G196-3B shows absorption features in both wavelength regions, like those of KPNO Tau 4, suggesting that its age and mass are at the lower end of published estimates. Whereas σ Ori 51 appears to be consistent with a young substellar object, σ Ori 47 shows signatures of high gravity most closely resembling an old L1.5/L0 and cannot be a member of the σ Orionis cluster.

Probing the Low‐Mass Stellar End of the η Chamaeleontis Cluster

We have identified three faint new members of the η Chamaeleontis cluster. Spectral types of the new members are estimated to be ~M5, based on their TiO band strengths and broadband colors. With an age of 5-8 Myr for the cluster, masses of these new members are estimated to be ~0.08 M☉. All three display strong Li 6708 A absorption and Hα emission features, including one with an Hα emission equivalent width of ~60 A, along with He I 6678 and 7605 A emission features that are characteristics of classical T Tauri stars.

Red supergiants in the Small Magellanic Cloud: The effects of metallicity on narrow-band classification indices

AbstractNarrow-band classification photometry, on a six-color system that measures near-infrared bands of TiO and CN, has been obtained for a set of red supergiants in the Small Magellanic Cloud. To investigate the effects of metallicity on the band-strength indices, comparisons are made to supergiants in the LMC and the Galaxy. Two new variable stars are reported.

The molecular Zeeman effect and diagnostics of solar and stellar magnetic fields

Recent advances in the computation of the Zeeman splitting of molecular lines have paved the way for their use as diagnostics of solar and stellar magnetic fields. A systematic study of their diagnostic capabilities had not been carried out so far, however. Here we investigate how molecular lines can be used to deduce the magnetic and thermal structure of sunspots, starspots and cool stars. First, we briefly describe the Stokes radiative transfer of Zeeman-split molecular lines. Then, we compute Stokes spectra of TiO, OH, CH and FeH lines and investigate their diagnostic capabilities. We also compare the synthetic profiles with observations. Spectra of TiO, OH and FeH are found to be interesting diagnostics of sunspot magnetic fields. This is also true for cool stars, where, however, the OH Stokes V profiles may require very high S/ N data to be reliably employed. Finally we investigate the potential of various molecular bands for high-contrast imaging of the solar surface. The violet CN and CH bands turn out to be most promising for imaging the photosphere, the TiO bands are excellent for imaging sunspot umbrae, while the UV OH band can be used for imaging both the photosphere and sunspots.

The cool circumstellar environment of IRAS 08182-6000: radiative transfer modelling of TiO absorption bands

IRAS 08182−6000 has very red infrared colours, with an excess at K and L, and a silicate absorption feature at 9.7 μm. Optical spectra show strong, narrow absorption bands of TiO and AlO and weaker bands of VO that are indicative of formation in a very cool region, superimposed on a late F-type or early G-type spectrum. It is similar to the unusual star U Equ = IRAS 20547+0247 and both are high-velocity stars. These characteristics, together with spectroscopic indications of low surface gravity, suggest that these stars are of low mass and are possibly at a post-AGB stage of evolution. IRAS 08182−6000 brightened by several magnitudes during 12 yr of photometric observation. Photospheric and some circumstellar atomic absorption lines are evident, as well as atomic lines seen in emission that differ in radial velocity from those in absorption by c. 50 km s−1. Molecular bands of the γ, γ′ and α systems of circumstellar TiO have been modelled using a one-dimensional radiative transfer code that makes use of the most recent laboratory and theoretical spectroscopic data on TiO. It was found necessary to invoke a non-local thermodynamic equilibrium model, which resulted in the determination of different rotational, spin–orbit and vibrational temperatures for the TiO gas, all of which lie well below the temperature at which TiO begins to deplete from the gas phase due to grain formation.

Optical spectra of selected Chamaeleon I young stellar objects

We present optical spectra of eight candidate brown dwarfs and a previously known T Tauri star (Sz 33) of the Chamaeleon I dark cloud. We derived spectral types based on the strength of the TiO or VO absorption bands present in the spectra of these objects as well as on the PC3 index of Martin et al. (1999). Photometric data from the literature are used to estimate the bolometric luminosities for these sources. We apply D'Antona & Mazzitelli(1997) pre-main sequence evolutionary tracks and isochrones to derive masses and ages. Based on the presence of Hα in emission, we confirm that most of the candidates are young objects. Our sample however includes two sources for which we can only provide upper limits for the emission in Hα; whereas these two objects are most likely foreground/background stars, higher resolution spectra are required to confirm their true nature. Among the likely cloud members, we detect one new sub-stellar object and three transition stellar/sub-stellar sources.

LSR 1610-0040: The First Early-Type L Subdwarf

We report the discovery of LSR 1610-0040, a previously unreported r = 17.5 star with a very high proper motion, μ = 146 yr-1. This very cool star (b-i = 6.3) is found to have a peculiar spectrum that does not fit into the standard sequence of late-M dwarfs and L dwarfs. Rather, the spectrum is more typical of an ultracool subdwarf, with weak bands of TiO and no detectable VO. But because LSR 1610-0040 is so much redder than any other sdM known, and because it does not appear to fit well into the sdM sequence, we propose that LSR 1610-0040 be assigned a spectral type sdL, making it the first early-type L subdwarf known. Evidence suggests that LSR 1610-0040 is an old, metal-poor star whose mass is just above the limit of hydrogen burning (M ≈ 0.08 M☉).

Near‐Infrared Line‐Strength Indices and Their Usefulness for Studying the Stellar Populations of Elliptical Galaxies

Until recently, the potential of the near-infrared (NIR) spectral range for interpreting the stellar populations of elliptical galaxies (Es) was still almost unexploited. Previous calibrations of NIR line-strength indices suffer from limitations that translate into severe uncertainties on the predictions of stellar population synthesis models. In order to overcome the above problems, in this thesis we have calibrated new line-strength indices on the basis of a new, extensive NIR stellar library that consists of 706 spectra (ll8348–9020; FWHM p 1.50 A) with a wide range of atmospheric parameters. The new indices have been designed to measure the Ca ii triplet at ll8498, 8542, 8662, the H Paschen series, the Mg i line at l8807, and the TiO bands. Unlike previous definitions, they exhibit reliable pseudocontinua for all spectral types, and the Ca ii index has been corrected for the contamination with Paschen lines in early spectral types. The large sample of spectra, as well as the wide range of atmospheric parameters, allowed us to compute accurate empirical fitting functions for the indices. These, together with the stellar spectra, have been implemented into a recent single stellar population (SSP) synthesis model, concluding that, for SSPs older than 3 Gyr, (1) the age is a secondary parameter, (2) the Ca ii indices of SSPs with intermediate to high metallicity mainly depend on the initial mass function (IMF) slope (m; assuming a Salpeter-like IMF) in the sense that they decrease with increasing m, and (3) the TiO index is an excellent metallicity indicator. The inclusion of M-type stars in our stellar library reveals unprecedented behaviors for metal-rich SSPs, such as the saturation—or even a decreasing trend—of the integrated Ca ii with increasing metallicity. Finally, new spectroscopic data for a sample of 35 Es are interpreted in the light of our model predictions. The most surprising result is the significant anticorrelation between the

Discovery of an Ultracool Subdwarf: LSR 1425+7102, the First Star with Spectral Type [CLC]sd[/CLC]M8.0

We report the discovery of the coolest subdwarf reported to date. The star LSR 1425+7102 was discovered in our survey for faint high proper motion stars in the northern sky. Follow-up spectroscopy revealed the star to be a very red object with the characteristic signature of M subdwarfs: strong CaH bands but relatively weaker TiO bands. The CaH molecular band at ≈6900 A is particularly strong. By extrapolating the empirical relationship between the strength of the CaH molecular band and that of the subdwarf subtype, we conclude that LSR 1425+7102 is the first star to be discovered with spectral type sdM8.0.

Sunspot and starspot interiors as seen from molecular lines

Molecular lines represent powerful diagnostics of stellar and solar magnetic fields. They are currently successfully used for studying the thermal and magnetic structure of sunspots. The present calculations show how molecular lines can been used for testing stellar surface images and measuring magnetic fields inside spatially unresolved starspots. Particularly, observational capabilities for the registration of the Stokes V signal from starspots in the TiO band at 7055 Å are estimated.

Periodic photometric variability of the brown dwarf Kelu-1

We have detected a strong periodicity of 1.80±0.05 h in photometric observations of the brown dwarf Kelu-1. The peak-to-peak amplitude of the variation is ∼1.1 per cent (11.9±0.8 mmag) in a 41-nm wide filter centred on 857 nm and including the dust/temperature-sensitive TiO and CrH bands. We have identified two plausible causes of variability: surface features rotating in to and out of view and so modulating the light curve at the rotation period; or ellipsoidal variability caused by an orbiting companion. In the first scenario, we combine the observed v sin i of Kelu-1 and standard model radius to determine that the axis of rotation is inclined at 65°±12° to the line of sight.

The identification of SS73 71 as a new symbiotic star

We report spectroscopic observations of a new symbiotic star. This star is located in our Galaxy in the direction of the bulge and was discovered during a southern spectroscopic survey of Hα emission line objects. The star, SS73 71, after Sanduleak & Stephenson (1973), shows characteristics of a symbiotic star: the presence of TiO bands and emission lines of ions of higher ionization such as He ii4686 Å and [O iii]5007 Å. Here, we present some of the main spectroscopic features, line intensities and, whenever possible, some physical parameters.

Photometric and TiO modelling of the starspots on AG Dor and HU Vir

In this work, spot parameters are determined for two stars, namely, AG Dor and HU Vir, from their photometric variability and λ7055 Å TiO band depth. Applying the photometric modelling code spotpic, we have found two modulating spots together with a non-modulating component on the photosphere of AG Dor in December 96/January 97. The spot temperatures are constrained to be between 4300 K and 4600 K for the modulating component, and cooler, viz., 4000 K, for the non-modulating one. Therefore, the non-modulating component of the spot distribution on AG Dor at this epoch is held to be totally responsible for the presence of the TiO band in the spectra of this K1 dwarf with photospheric temperature of 5000 K. AG Dor's total coverage of spots is determined to be between 4% and 7% for the modulating component and 26.5% for the non-modulating one. For HU Vir, the light and colour curves were modelled with two spots with temperatures of 4450 and 4050 K. In this case we did not include the non-modulating component in the model.

Variability and spectral classification of LMC giants: Results from DENIS and EROS

We present the first cross-identifications of sources in the near-infrared DENIS survey and in the micro-lensing EROS survey in a field of about 0.5 square degrees around the optical center (OC) of the Large Magellanic Cloud. We analyze the photometric history of these stars in the EROS data base and obtain light-curves for about 800 variables. Most of the stars are long period variables (Miras and Semi-Regulars); a few Cepheids are also present. We also present new spectroscopic data on 126 asymptotic giant branch stars in the OC field, 30 previously known and 96 newly discovered by the DENIS survey. The visible spectra are used to assign a carbon-(C-) or oxygen-rich (O-rich) nature to the observed stars on the basis of the presence of molecular bands of TiO, VO, CN, C2. For the remaining of the stars we used the () color to determine whether they are O-rich or C-rich. Plotting versus KS we find three very distinct period-luminosity relations, mainly populated by Semi-Regular of type a (SRa), b (SRb) and Mira variables. Carbon-rich stars occupy mostly the upper part of these relations. We find that of the asymptotic giant branch population are long period variables (LPVs).