Bright Regions Research Articles

Observational study of hydrocarbons in the bright photodissociation region of Messier 8

Aims. Hydrocarbons are ubiquitous in the interstellar medium, but their formation is still not well understood, depending on the physical environment in which they are found. Messier 8 (M8) is host to one of the brightest H II regions and photodissociation regions (PDRs) in our galaxy. With the observed C2H and c-C3H2 data toward M8, we aim at obtaining their densities and abundances and to shed some light on their formation mechanism. Methods. Using the Atacama Pathfinder Experiment (APEX) 12 m, and the Institut de Radioastronomie Millimétrique (IRAM) 30 m telescopes, we performed a line survey toward Herschel 36 (Her 36), which is the main ionizing stellar system in M8, and an imaging survey within 1.3 × 1.3 pc around Her 36 of various transitions of C2H and c-C3H2. We used both local thermodynamic equilibrium (LTE) and non-LTE methods to determine the physical conditions of the emitting gas along with the column densities and abundances of the observed species, which we compared with (updated) gas-phase photochemical PDR models. In order to examine the role of polycyclic aromatic hydrocarbons (PAHs) in the formation of small hydrocarbons and to investigate their association with the H II region, the PDR and the molecular cloud, we compared archival Galactic Legacy Infrared Mid-Plane Survey Extraordinaire (GLIMPSE) 8 μm and the Spectral and Photometric Imaging Receiver (SPIRE) 250 μm continuum images with the C2H emission maps. Results. We observed a total of three rotational transitions of C2H with their hyperfine structure components and four rotational transitions of c-C3H2 with ortho and para symmetries toward the H II region and the PDR of M8. Fragmentation of PAHs seems less likely to contribute to the formation of small hydrocarbons as the 8 μm emission does not follow the distribution of C2H emission, which is more associated with the molecular cloud toward the north west of Her 36. From the quantitative analysis, we obtained abundances of ~10−8 and 10−9 for C2H and c-C3H2 respectively, and volume densities of the hydrocarbon emitting gas in the range n(H2) ~5 × 104–5 × 106 cm−3. Conclusions. The observed column densities of C2H and c-C3H2 are reproduced reasonably well by our PDR models. This supports the idea that in high-UV flux PDRs, gas-phase chemistry is sufficient to explain hydrocarbon abundances.

Study of a retinal layer model to generate a spike waveform for a color deficient and strabismus individual.

The physiological modeling of retinal layers to provide an insight into how the incoming image is converted into its equivalent spike train that can be decoded by the human brain is a key issue. Most of the retinal layer models concentrate mainly on image compression, edge detection and image reconstruction. A retinal layer model to generate spike waveform corresponding to the visual information is not covered much in the literature. The aim of this study was to develop a mathematical model of retinal layers that has complex neural structures, that can detect the incoming signal and transform the signal into the equivalent spike train. The proposed retinal layer model includes a photoreceptor, an outer plexiform (OPL), an inner plexiform (IPL) and ganglion cell layers exhibiting the properties of compression, luminance and spatial temporal filtering in the processing of visual information. The photoreceptor layer enhances the contrast visibility in the dark region and maintains the same in the bright regions. The OPL is modeled to enhance the contour of the image. The finer detail of the image is extracted by mathematically modeling the IPL. The ganglion cell layer is modeled using the Hodgkin-Huxley model to generate the spike train for the incoming information. The spike train was generated for color deficient individuals namely protanopia, deuteranopia, tritanopia and for individuals suffering from night blindness. Simulation results showed a spike train was generated only for a certain threshold stimulus value. The differences in spike pattern for a normal and visually impaired individual were studied. This may lead to a methodology for earlier diagnosis.

High-Frequency Dynamics of Active Region Moss as Observed by IRIS

The high temporal, spatial and spectral resolution of Interface Region Imaging Spectrograph (IRIS) has provided new insights into the understanding of different small-scale processes occurring at the chromospheric and transition region (TR) heights. We study the dynamics of high-frequency oscillations of an active region (AR 2376) moss as recorded by simultaneous imaging and spectral data of IRIS. Wavelet transformation, power maps generated from slit-jaw images in Si\,IV\,1400\,\AA~passband, and sit-and-stare spectroscopic observations of Si\,IV\,1403\,\AA~spectral line reveal the presence of high-frequency oscillations with $\sim$1--2\,minutes periods in the bright moss regions. The presence of such low periodicities is further confirmed by intrinsic mode functions (IMFs) as obtained by empirical mode decomposition (EMD) technique. We find the evidence of the presence of slow waves and reconnection-like events, and together they cause the high-frequency oscillations in the bright moss regions.

A case study on bright band transition from very light to heavy rain using simultaneous observations of collocated X- and Ka-band radars

The observations of bright band carried out simultaneously with X- and Ka-band radars for the first time over the Indian region have been examined to reveal various contrasting characteristics of bright band at the two wavelengths. The study reports the bright band observations on September 12–13, 2015 at millimeter and centimeter wavelengths and brings out a comparative analysis of the bright band features (e.g., intensity, thickness, height, etc.) under three different rain conditions ranging from very light (<0.1 mm/hr) to light (0.1–3 mm/hr) to heavy (3–5 mm/hr). It is seen that the bright band region at Ka-band is always narrower and situated at a higher altitude than at X-band frequency. Our analysis shows that at Ka-band frequency, the polarimetric fields like LDR can be utilized to detect and determine the bright band features using an appropriate selection of a threshold value of LDR, which is found to be −22 dB in this study and could be associated reasonably with the top and bottom heights of the bright band. This study explores the potential of both radars, particularly the Ka-band radar for probing the bright band effect and estimating its features which would be helpful to improve the quantitative estimates of precipitation.

Haze removal method based on a variation function and colour attenuation prior for UAV remote-sensing images

ABSTRACTAiming at the problem of low visibility of images obtained by UAV in hazy weather, this paper proposes an image dehazing algorithm based on variation function and colour attention prior. A large number of experiments have proved that the sky or other bright regions could affect the estimation of atmospheric light and transmittance. In the experiment, our proposed algorithm divides the images into sky and dark regions and uses the pixels of the dark region to solve the atmospheric light value. According to the region where the pixels are located, the transmittances of the pixels in the sky and non-sky regions are separately estimated and adjusted. The experiment’s results show that the restored image visibility, information entropy and colour saturation are significantly improved, and the algorithm’s computational efficiency is high.

Thermal emission from bow shocks

The Bubble Nebula (or NGC 7635) is a parsec-scale seemingly spherical wind-blown bubble around the relatively unevolved O star BD+60°2522. The young dynamical age of the nebula and significant space velocity of the star suggest that the Bubble Nebula might be a bow shock. We ran 2D hydrodynamic simulations to model the interaction of the wind of the central star with the interstellar medium (ISM). The models cover a range of possible ISM number densities of n = 50−200 cm−3 and stellar velocities of v* = 20−40 km s−1. Synthetic Hα and 24 μm emission maps predict the same apparent spherical bubble shape with quantitative properties similar to observations. The synthetic maps also predict a maximum brightness similar to that from the observations and agree that the maximum brightness is at the apex of the bow shock. The best-matching simulation had v* ≈ 20 km s−1 into an ISM with n ∼ 100 cm−3, at an angle of 60° with respect to the line of sight. Synthetic maps of soft (0.3−2 keV) and hard (2−10 keV) X-ray emission show that the brightest region is in the wake behind the star and not at the bow shock itself. The unabsorbed soft X-rays have a luminosity of ∼1032−1033 erg s−1. The hard X-rays are fainter: ∼1030−1031 erg s−1, and may be too faint for current X-ray instruments to successfully observe. Our results imply that the O star creates a bow shock as it moves through the ISM and in turn creates an asymmetric bubble visible at optical and infrared wavelengths and predicted to be visible in X-rays. The Bubble Nebula does not appear to be unique; it could simply be a favourably oriented, very dense bow shock. The dense ISM surrounding BD+60°2522 and its strong wind suggest that it could be a good candidate for detecting non-thermal emission.

Massive star formation in W51 A triggered by cloud–cloud collisions

Abstract W$\, 51\,$A is one of the most active star-forming regions in the Milky Way, and includes copious amounts of molecular gas with a total mass of ${\sim }6\times 10^{5}\, M_{\odot }$. The molecular gas has multiple velocity components over ∼20 km s−1, and interactions between these components have been discussed as the mechanism that triggered the massive star formation in W$\, 51\,$A. In this paper, we report on an observational study of the molecular gas in W$\, 51\,$A using the new 12CO, 13CO, and C18O (J = 1–0) data covering a 1${^{\circ}_{.}}$4 × 1${^{\circ}_{.}}$0 area of W$\, 51\,$A obtained with the Nobeyama 45 m telescope at 20′ resolution. Our CO data resolved four discrete velocity clouds with sizes and masses of ∼30 pc and 1.0–$1.9\times 10^{5}\, M_{\odot }$ around radial velocities of 50, 56, 60, and 68 km s−1. Toward the central part of the Hii region complex G49.5−0.4 in W$\, 51\,$A, in which the bright stellar clusters IRS 1 and IRS 2 are located, we identified four C18O clumps having sizes of ∼1 pc and column densities of higher than 1023 cm−2, which are each embedded within the four velocity clouds. These four clumps are concentrated within a small area of 5 pc, but show a complementary distribution on the sky. In the position–velocity diagram, these clumps are connected with each other by bridge features having weak intensities. The high intensity ratios of 13CO (J = 3–2)$/$(J = 1–0) also indicate that these four clouds are associated with the Hii regions, including IRS 1 and IRS 2. We also reveal that, in the other bright Hii region complex G49.4−0.3, the 50, 60, and 68 km s−1 clouds show a complementary distribution, with two bridge features connecting between the 50 and 60 km s−1 clouds and the 60 and 68 km s−1 clouds. An isolated compact Hii region G49.57−0.27 located ∼15 pc north of G49.5−0.4 also shows a complementary distribution and a bridge feature. The complementary distribution on the sky and the broad bridge feature in the position–velocity diagram suggest collisional interactions among the four velocity clouds in W$\, 51\,$A. The timescales of the collisions can be estimated to be several 0.1 Myr as crossing times of the collisions, which are consistent with the ages of the Hii regions measured from the sizes of the Hii regions with the 21 cm continuum data. We discuss a scenario of cloud–cloud collisions and massive star formation in W$\, 51\,$A by comparing these with recent observational and theoretical studies of cloud–cloud collision.

Estimation of three-dimensional structure on peripheral fluctuation using fast camera images and magnetic field calculation in Heliotron J

Abstract In Heliotron J, peripheral plasma fluctuation and/or turbulence have been measured by using a fast camera for over 10 years. Many image data are provided as two-dimensional information of time series by a fast camera, and they represent the spatial shape of fluctuation and its complex behavior. From experience in Heliotron J experiment, most of the peripheral fluctuations have elongated spatial structure along magnetic field lines. Because the movement of electrons and ions along the field lines is much faster than their vertical motion, this is natural; the filamentary structure along the field lines should be formed as a fundamental of many types of fluctuations in Heliotron J. In the image of the fast camera, if the fluctuation having a filamentous structure along the magnetic field line is observed as a brighter region than its surrounding plasma, the image of this fluctuation should coincide with the projection image of the magnetic field line passing through the position of this fluctuation. That is, by comparing the projected image of magnetic field lines and the image of fluctuation images, three-dimensional information on peripheral plasma fluctuation can be obtained. Therefore, with the help of geometry of magnetic field lines, to extract three dimensional information about the fluctuation in Heliotron J from the two-dimensional image was tried. As a result, in the analysed range, the filament was along the magnetic field line; it appeared to occur from the O-point in the field of view, and disappear at the X-point. Apparent speed in the poloidal cross-section was approximately 500 m/s. In the future, if this analysis method is applied, more detailed data on fluctuation will be obtained in Heliotron J.

Phase Separation in Liquid Metal Nanoparticles

Nanoparticles produced from gallium-based liquid metal alloys have been explored for developing applications in the fields of electronics, catalysis, and biomedicine. Nonetheless, physical properties, such as phase behavior at micro-/nanosize scale, are still significantly underexplored for such nanoparticles. Here, we conduct an in situ investigation of phase behavior for gallium-based liquid metal nanoparticles and discover the unprecedented coexistence of solid particles in spherical liquid metal shells without the support of a crystalline substrate. The particles can also transform into solid Janus nanoparticles after temperature cycling. In addition, we investigate the optical properties of the nanoparticles before and after phase separation using in situ electron energy-loss spectroscopy. Most importantly, we discover that increasing the content of indium within the nanoparticle can stabilize the solid-core/liquid-shell structure at room temperature. This study provides a foundation to engineer liquid metal nanoparticles for developing new applications in nanoscale optical platforms and shape-configurable transformers.Although various electronic, chemical, and biomedical applications have been demonstrated for nanoparticles made from gallium-based liquid metal alloys, fundamental physical properties such as phase behavior of such nanoparticles are still significantly underexplored. Here, Tang and coworkers present the in situ investigation of phase separation in binary and ternary spherical liquid metal nanoparticles upon cooling, and demonstrate the coexistence of solid core/liquid shell without the support of a crystalline substrate. This study provides insight into engineering such nanoparticles for the development of new applications.Despite gallium-based liquid metal alloys attracting extensive attention for various applications, their phase behavior at the nanoscale is still underexplored. Understanding the impact of phase separation in nanoparticles can be extremely useful for developing new structures and exploring suitable applications. Here, we report on the investigation of phase behavior for spherical nanoparticles made from gallium-based liquid metal alloys upon cooling. We discover the thermally stable coexistence of solid cores in spherical liquid metal shells without the support of a crystalline substrate at room temperature. Given the unique properties of liquid metal, together with the facile process for producing core-shell and Janus nanoparticles, this work will encourage further investigation of the properties of such nanoparticles for developing applications in the fields of electronics, catalysis, nanomedicine, and beyond.

Banded spherulites and twisting lamellae in poly–ε–caprolactone

By crystallization of poly–e–caprolactone (PCL) at temperatures (55 ∘C ≥ Tc ≥ 50 ∘C) close to the melting point (Tm = 61 ∘C), banded spherulites form. These structures show a periodicity of P = 32 µm as determined by polarized optical microscopy (POM). In weakly-to-non-banded spherulites formed under similar conditions, a less sharp distribution of characteristic length scales is observed, that exhibits the same mean value. Within bright and dark regions of the banded spherulites, the two-dimensional molecular order parameter of different crystal directions of the lamellae is deduced from tightly focused (15 × 50 µm2) polarized infrared-spectroscopy measurements. From the oscillation of the order parameters of the crystalline $\underline {a}$ -, $\underline {b}$ -, and $\underline {c}$ -axes, banding in pure PCL is proven to result from lamellae growing in a helicoidal fashion along the spherulites radius. No deviation of lamellar growth and radius is determined within experimental uncertainty (± 4∘). Furthermore, spatially averaged IR-microscopy results in the same characteristic polarization dependence in banded and weakly-to-non-banded spherulites. In conjunction with the mentioned characteristic length scales, we conclude that the mechanism, which results in banding, is also active in non-banded spherulites of PCL.

A universal defect detection approach for various types of fabrics based on the Elo-rating algorithm of the integral image

In order to overcome the shortcoming that a fabric defect detection method can only fit into a certain type of fabric, this paper presents a novel method by integrating the idea of the integral image into the Elo-rating algorithm (IIER), which can detect the defects of various types of fabric speedily. Firstly, the golden sub-blocks are extracted from defect-free images. The whole images are divided into many sub-blocks, and the integral images of these sub-blocks are obtained. Next, the R sub-blocks are randomly selected from these integral sub-blocks, and each block is assigned an initial Elo point. Afterwards, the R sub-blocks are matched against all sub-blocks and the Elo points are updated after each competition. Finally, regions with bright defects accumulate high Elo points and regions with dark defects accumulate low Elo points. Thus, the threshold value image can be obtained by thresholding the final Elo points, in which white, gray and black regions correspond to bright, dark-defect and defect-free regions, respectively. The performance of the proposed method is evaluated on databases of three categories of fabric, namely raw fabric, yarn-dyed fabric and patterned fabric. The experimental results show that the IIER is a universal algorithm, which has high detection rate for different types of fabrics; in particular, the average correct detection rate can reach 100% for dot-patterned fabric. In addition, the detection time can be significantly reduced comparing with the Elo-rating algorithm (ER). Particularly for star-patterned fabric, the average detection time per image is 24.18 seconds less than the ER.

A Comparative Study of the Phase Separation of a Nematic Liquid Crystal in the Self-assembling Drying Protein Drops

ABSTRACTThe drying process, self-assembly of the proteins and the phase separation of a thermotropic liquid crystal (LC) from an initial aqueous solution represent a rich area of study. A focus of this work is to compare the behavior of two different proteins, bovine serum albumin [BSA] and lysozyme [Lys] in the ternary system through optical microscopy. During the drying process, the intensity profile shows three regimes in the presence of LC whereas no intensity variation is observed in its absence in both protein drops. The striking outcome is the presence of an umbilical defect of [+1] strength in every domain near the edge of BSA drop, whereas, each domain has a central dark region surrounded by a bright region in the dried Lys drop. Finally, the crack spacing in the dried Lys drop is reduced in the presence of LC whereas, no significant difference is found in the dried BSA drop.

Localized Microwave and EUV Bright Structures in an Eruptive Prominence

Abstract We study a solar eruptive prominence with flare/coronal mass ejection (CME) event by microwave and extreme ultraviolet (EUV) observations. Its evolution can be divided into three phases: slow rise, fast expansion, and ejection. In the slow-rise phase, the prominence continuously twists for more than one hour with a patch of bright emission appearing around the top. When the north leg interacts with the local small-size loops, the fast expansion is initiated and the flare takes place there. The prominence grows rapidly, and a series of localized brightenings appear in the whole prominence structure. Then the ejection occurs, followed by a CME. In microwave images, the brightness temperature (T b ) at 17 and 34 GHz can be divided into three components. The strongest emission with T b at 25,000–300,000 K is related to the bright flare region near the north foot. The medium T b (10,000–20,000 K) outlines a series of small-scale bright enhancements scattering in the prominence, which are superposed on a weak background with T b at 5000–10,000 K. These localized bright structures, first appearing at the top and then scattering in the entire prominence structure, are cospatial with EUV bright threads, fibers, or spots in both high- and low-temperature passbands. They display significant temporal variations on the scale of 3–5 s in the microwave observations. Thus, the plasma inside the prominence is spatially structured and changes with time in both density and temperature. This behavior could be interpreted in the frame of the small-scale and short-term process of energy releases in the twisted magnetic structure.

HARPS-N Solar RVs Are Dominated by Large, Bright Magnetic Regions

Abstract State-of-the-art radial-velocity (RV) exoplanet searches are currently limited by RV signals arising from stellar magnetic activity. We analyze solar observations acquired over a 3 yr period during the decline of Carrington Cycle 24 to test models of RV variation of Sun-like stars. A purpose-built solar telescope at the High Accuracy Radial-velocity Planet Searcher for the Northern hemisphere (HARPS-N) provides disk-integrated solar spectra, from which we extract RVs and log R HK ′ . The Solar Dynamics Observatory (SDO) provides disk-resolved images of magnetic activity. The Solar Radiation and Climate Experiment (SORCE) provides near-continuous solar photometry, analogous to a Kepler light curve. We verify that the SORCE photometry and HARPS-N log R HK ′ correlate strongly with the SDO-derived magnetic filling factor, while the HARPS-N RV variations do not. To explain this discrepancy, we test existing models of RV variations. We estimate the contributions of the suppression of convective blueshift and the rotational imbalance due to brightness inhomogeneities to the observed HARPS-N RVs. We investigate the time variation of these contributions over several rotation periods, and how these contributions depend on the area of active regions. We find that magnetic active regions smaller than 60 Mm2 do not significantly suppress convective blueshift. Our area-dependent model reduces the amplitude of activity-induced RV variations by a factor of two. The present study highlights the need to identify a proxy that correlates specifically with large, bright magnetic regions on the surfaces of exoplanet-hosting stars.

Transient Dimming of the Bright Fe xiv Emission Region in the Solar Corona of the 21 August 2017 Total Solar Eclipse

During the 21 August 2017 total solar eclipse the corona was observed through a narrowband filter centered at the Fe xiv green emission line at a cadence of 30 frames/s. A bright region on the east limb was observed to dim during the 2 m 35 s totality. Details of the temporal behavior of this region are reported. The likely association of the dimming with a coronal mass ejection (CME) in progress during the eclipse or the small sunspot group with coronal loops near the east limb is discussed.

Phase separation of a nematic liquid crystal in the self-assembly of lysozyme in a drying aqueous solution drop

Abstract

Coordinated UV and X-Ray Spectroscopic Observations of the O-type Giant ξ Per: The Connection between X-Rays and Large-scale Wind Structure* †

Abstract We present new, contemporaneous Hubble Space Telescope STIS and XMM-Newton observations of the O7 III(n)((f)) star ξ Per. We supplement the new data with archival IUE spectra, to analyze the variability of the wind lines and X-ray flux of ξ Per. The variable wind of this star is known to have a 2.086-day periodicity. We use a simple, heuristic spot model that fits the low-velocity (near-surface) IUE wind line variability very well, to demonstrate that the low-velocity absorption in the new STIS spectra of N iv λ1718 and Si iv λ1402 vary with the same 2.086-day period. It is remarkable that the period and amplitude of the STIS data agree with those of the IUE spectra obtained 22 yr earlier. We also show that the time variability of the new XMM-Newton fluxes is also consistent with the 2.086-day period. Thus, our new, multiwavelength coordinated observations demonstrate that the mechanism that causes the UV wind line variability is also responsible for a significant fraction of the X-rays in single O stars. The sequence of events for the multiwavelength light-curve minima is Si iv λ1402, N iv λ1718, and X-ray flux, each separated by a phase of about 0.06 relative to the 2.086-day period. Analysis of the X-ray fluxes shows that they become softer as they weaken. This is contrary to expectations if the variability is caused by periodic excess absorption. Furthermore, the high-resolution X-ray spectra suggest that the individual emission lines at maximum are more strongly blueshifted. If we interpret the low-velocity wind line light curves in terms of our model, it implies that there are two bright regions, i.e., regions with less absorption, separated by 180°, on the surface of the star. We note that the presence and persistence of two spots separated by 180° suggest that a weak dipole magnetic field is responsible for the variability of the UV wind line absorption and X-ray flux in ξ Per.

ALMA CO(2-1) observations in the XUV disk of M83

The extended ultraviolet (XUV) disk galaxies are some of the most interesting objects studied in the last few years. The UV emission, revealed by GALEX, extends well beyond the optical disk after the drop in Hα emission, the usual tracer of star formation. This shows that sporadic star formation can occur in a large fraction of the HI disk at radii up to 3 or 4 times the optical radius. In most galaxies, these regions are poor in stars and are dominated by under-recycled gas; they therefore bear some similarity to the early stages of spiral galaxies and high-redshift galaxies. One remarkable example is M83, a nearby galaxy with an extended UV disk reaching 2 times the optical radius. It offers the opportunity to search for molecular gas and to characterize the star formation in outer disk regions, traced by the UV emission. We obtained CO(2-1) observations with ALMA of a small region in a 1.5′ × 3′ rectangle located at rgal = 7.85′ over a bright UV region of M83. There is no CO detection, in spite of the abundance of HI gas, and the presence of young stars traced by their HII regions. Our spatial resolution (17 pc × 13 pc) was perfectly fitted to detect giant molecular clouds (GMC), but none were detected. The corresponding upper limits occur in a region of the Kennicutt–Schmidt diagram where dense molecular clouds are expected. Stacking our data over HI-rich regions, using the observed HI velocity, we obtain a tentative detection corresponding to a H2-to-HI mass ratio of &lt; 3 × 10−2. A possible explanation is that the expected molecular clouds are CO-dark because of the strong UV radiation field. This field preferentially dissociates CO with respect to H2, due to the small size of the star-forming clumps in the outer regions of galaxies.

Wave Activity in Jupiter's North Equatorial Belt From Near‐Infrared Reflectivity Observations

AbstractHigh spatial resolution images of Jupiter at 1.58–2.28 μm are used to track and characterize a wave pattern observed in 2017 at a planetocentric latitude of 14°N. The wave pattern has a wave number of 18 and spans ∼5° in latitude. One bright crest remains stationary in System III longitude, while the remaining crests move slowly westward. The bright and dark regions of the near‐infrared wave pattern are caused by variations in the vertical location of the upper tropospheric haze layer. A comparison with thermal infrared observations shows a correlation with temperature anomalies in the upper troposphere. The results are consistent with a Rossby wave, generated by flow around a stationary vortex.

Rise and fall of molecular clouds across the M 33 disk

We carried out deep searches for CO line emission in the outer disk of M 33, at R &gt; 7 kpc, and examined the dynamical conditions that can explain variations in the mass distribution of the molecular cloud throughout the disk of M 33. We used the IRAM-30 m telescope to search for CO lines in the outer disk toward 12 faint mid-infrared (MIR) selected sources and in an area of the southern outer disk hosting MA1, a bright HII region. We detect narrow CO lines at the location of two MIR sources at galactocentric distances of about 8 kpc that are associated with low-mass young stellar clusters, and at four locations in the proximity of MA1. The paucity of CO lines at the location of weak MIR-selected sources probably arises because most of them are not star-forming sites in M 33, but background sources. Although very uncertain, the total molecular mass of the detected clouds around MA1 is lower than expected given the stellar mass of the cluster, because dispersal of the molecular gas is taking place as the HII region expands. The mean mass of the giant molecular clouds (GMCs) in M 33 decreases radially by a factor 2 from the center out to 4 kpc, then it stays constant until it drops at R &gt; 7 kpc. We suggest that GMCs become more massive toward the center because of the fast rotation of the disk, which drives mass growth by coalescence of smaller condensations as they cross the arms. The analysis of both HI and CO spectral data gives the consistent result that corotation of the two main arms in this galaxy is at a radius of 4.7 ± 0.3 kpc, and spiral shock waves become subsonic beyond 3.9 kpc. Perturbations are quenched beyond 6.5 kpc, where CO lines have been detected only around sporadic condensations associated with UV and MIR emission.