Dynamic walking stability of the TUlip robot by means of the extrapolated center of mass

The TUlip robot was created to participate in the teensize league of Robocup. The TUlip robot is a bipedal robot intended for dynamic walking. It has six degrees of freedom for each leg: three for the hip, one for the knee and two for the ankle. This paper elaborates on the algorithm for the sideways control during gait. The algorithm uses the extrapolated center of mass (XcoM) to achieve limit cycle stability. The algorithm is tested in simulation using a linear inverted pendulum and, then, experimentally applied to the TUlip robot. The result is an adaptive behavior of the TUlip robot, promising for future application to legged robot stability

The absence of the 10 um silicate feature in the isolated Herbig Ae star HD 100453

We analyse the optical and IR spectra, as well as the spectral energy distribution (UV to mm) of the candidate Herbig Ae star HD100453. This star is particular, as it shows an energy distribution similar to that of other isolated Herbig Ae/Be stars (HAEBEs), but unlike most of them, it does not have a silicate emission feature at 10 um, as is shown in Meeus (2001). We confirm the HAEBE nature of HD100453 through an analysis of its optical spectrum and derived location in the H-R diagram. The IR spectrum of HD100453 is modelled by an optically thin radiative transfer code, from which we derive constraints on the composition, grain-size and temperature distribution of the circumstellar dust. We show that it is both possible to explain the lack of the silicate feature as (1) a grain-size effect - lack of small silicate grains, and (2) a temperature effect - lack of small, hot silicates, as proposed by Dullemond (2001), and discuss both possibilities.Comment: 9 pages, 7 figures; accepted by A&

Can neutral and ionized PAHs be carriers of the UV extinction bump and the diffuse interstellar bands?

Up to now, no laboratory-based study has investigated polycyclic aromatic hydrocarbon (PAH) species as potential carriers of both the diffuse interstellar bands (DIBs) and the 2175 A UV bump. We examined the proposed correlation between these two features by applying experimental and theoretical techniques on two specific medium-sized/large PAHs (dibenzorubicene C30H14 and hexabenzocoronene C42H18) in their neutral and cationic states. It was already shown that mixtures of sufficiently large, neutral PAHs can partly or even completely account for the UV bump. We investigated how the absorption bands are altered upon ionization of these molecules by interstellar UV photons. The experimental studies presented here were realized by performing matrix isolation spectroscopy with subsequent far-UV irradiation. The main effects were found to be a broadening of the absorption bands in the UV combined with slight red shifts. The position of the complete pi - pi* absorption structure around 217.5 nm, however, remains more or less unchanged which could explain the observed position invariance of the interstellar bump for different lines of sight. This favors the assignment of this feature to the interstellar PAH population. As far as the DIBs are concerned, neither our investigations nor the laboratory studies carried out by other research groups support a possible connection with this class of molecules. Instead, there are reasonable arguments that neutral and singly ionized cationic PAHs cannot be made responsible for the DIBs.Comment: 11 pages, 7 figures, 1 tabl

Mobile travel services: A three-country study into the impact of local circumstances

In this paper we explore the difference in acceptance patterns of mobile services that are related to travelling in three countries: Finland, The Netherlands and New Zealand. The objective of this paper is to understand differences in the use of Mobile Travel Services in three countries that differ with regard to national travel patterns. This paper also contributes to the discussion of the relevance of the Technology Acceptance Model for mobile applications by focusing on the importance of context characteristics, such as the degree of mobility of the user, the social situation people are in, and their need for social interaction. Based on surveys in the three countries as executed in 2009, we use structural equation modelling to find differences in patterns. The paper concludes that context factors have an impact on the relation between the core concepts as used in TAM and DOI approach, and that t here is a clear need for closer research in the moderating effect of physical (e.g. mobile and fixed context) and social context, as well as the need for social interaction. Moreover it is clear that country specific characteristics play a role in the acceptance of mobile travel services. As we pointed out in many of our research projects before the acceptance and use of mobile services requires deep understanding from individual, context and technology related characteristics and their mutual interactions

Mobile travel services: the effect of moderating context factors

This article has two objectives: (1) to draw an international comparison regarding the acceptance of mobile travel services in three countries with different profiles when it comes to travelling and mobile telecommunications, and (2) to extend relevant literature on mobile applications, more specifically travel services, by including context-related concepts, taking moderating factors like location, mobility of users, physical, and social context into account. Based on surveys that were carried out in 2009, structural equation modelling is used to identify differences in patterns in the use of mobile travel services and in the role of context-related variables. the conclusion of this article is that context-related factors, that is, mobility and (physical and social) context, have an impact on the relationship between the core concepts of technology Acceptance model (TAM) and Diffusion of Innovation (DoI) research. many studies on the acceptance and use of mobile services indicate that a deep understanding is needed of individual, context-related, and technological characteristics and the way they interact. this is also highly relevant to the travel industry, which wants to utilize the opportunities provided by mobile technology

The characteristics of the IR emission features in the spectra of Herbig Ae stars: Evidence for chemical evolution

Herbig Ae/Be stars are a class of young pre-main sequence stellar objects of intermediate mass and are known to have varying amounts of natal cloud material still present in their direct vicinity. We characterise the IR emission bands, due to fluorescence by PAH molecules, in the spectra of Herbig Ae/Be stars and link observed variations to spatial aspects of the mid-IR emission. We analysed two PAH dominated spectra from a sample of 15 Herbig Ae/Be stars observed with Spitzer and derive profiles of the major PAH bands. The shape and the measured band characteristics show pronounced variations between the two Spitzer spectra. Those variations parallel those found between three ISO spectra of other, well-studied, Herbig Ae/Be stars. The derived profiles are compared to those from a broad sample of sources. The Spitzer and ISO spectra exhibit characteristics commonly interpreted respectively as interstellar matter-like (ISM), non-ISM-like, or a combination of the two. We argue that the PAH emission detected from the sources exhibiting a combination of ISM-like and non-ISM-like characteristics indicates the presence of two dissimilar, spatially separated, PAH families. As the shape of the individual PAH band profiles reflects the composition of the PAH molecules involved, this demonstrates that PAHs in subsequent, evolutionary linked stages of star formation are different from those in the general ISM, implying active chemistry. None of the detected PAH emission can be associated with the (unresolved) disk and is thus associated with the circumstellar cloud. This implies that chemical changes may already occur in the natal cloud and not necessarily in the disk

ISO spectroscopy of circumstellar dust in 14 Herbig Ae/Be systems: towards an understanding of dust processing

We present Infrared Space Observatory (ISO) spectra of fourteen isolated Herbig Ae/Be (HAEBE) stars, to study the characteristics of their circumstellar dust. These spectra show large star-to-star differences, in the emission features of both carbon-rich and oxygen-rich dust grains. The IR spectra were combined with photometric data ranging from the UV through the optical into the sub-mm region. We defined two key groups, based upon the spectral shape of the infrared region. The derived results can be summarized as follows: (1) the continuum of the IR to sub-mm region of all stars can be reconstructed by the sum of a power-law and a cool component, which can be represented by a black body. Possible locations for these components are an optically thick, geometrically thin disc (power-law component) and an optically thin flared region (black body); (2) all stars have a substantial amount of cold dust around them, independent of the amount of mid-IR excess they show; (3) also the near-IR excess is unrelated to the mid-IR excess, indicating different composition/location of the emitting material; (4) remarkably, some sources lack the silicate bands; (5) apart from amorphous silicates, we find evidence for crystalline silicates in several stars, some of which are new detections; (6) PAH bands are present in at least 50% of our sample, and their appearance is slightly different from PAHs in the ISM; (7) PAH bands are, with one exception, not present in sources which only show a power-law continuum in the IR; their presence is unrelated to the presence of the silicate bands; (8) the dust in HAEBE stars shows strong evidence for coagulation; this dust processing is unrelated to any of the central star properties (such as age, spectral type and activity).Comment: 15 pages, accepted by A&

ISO spectroscopy of circumstellar dust in the Herbig Ae systems AB Aur and HD 163296

Using both the Short- and Long-wavelength Spectrometers on board the Infrared Space Observatory (ISO), we have obtained infrared spectra of the Herbig Ae systems AB Aur and HD 163296. In addition, we obtained ground-based N band images of HD 163296. Our results can be summarized as follows: (1) The main dust components in AB Aur are amorphous silicates, iron oxide and PAHs; (2) The circumstellar dust in HD 163296 consists of amorphous silicates, iron oxide, water ice and a small fraction of crystalline silicates; (3) The infrared fluxes of HD 163296 are dominated by solid state features; (4) The colour temperature of the underlying continuum is much cooler in HD 163296 than in AB Aur, pointing to the existence of a population of very large (mm sized) dust grains in HD 163296; (5) The composition and degree of crystallization of circumstellar dust are poorly correlated with the age of the central star. The processes of crystallization and grain growth are also not necessarily coupled. This means that either the evolution of circumstellar dust in protoplanetary disks happens very rapidly (within a few Myr), or that this evolution is governed by factors other than stellar mass and age.Comment: 6 pages, 2 figures, accepted for publication in Astronomy & Astrophysic

On the interplay between flaring and shadowing in disks around Herbig Ae/Be stars

Based on the SED, Herbig stars have been categorized into two observational groups, reflecting their overall disk structure: group I members have disks with a higher degree of flaring than their group II counterparts. We investigate the 5-35 um Spitzer IRS spectra of a sample of 13 group I sources and 20 group II sources. We focus on the continuum emission to study the underlying disk geometry. We have determined the [30/13.5] and [13.5/7] continuum flux ratios. The 7-um flux excess with respect to the stellar photosphere is measured, as a marker for the strength of the near-IR emission produced by the inner disk. We have compared our data to self-consistent passive-disk model spectra, for which the same quantities were derived. We confirm the literature result that the difference in continuum emission between group I and II sources can largely be explained by a different amount of small dust grains. However, we report a strong correlation between the [30/13.5] and [13.5/7] flux ratios for Meeus group II sources. Moreover, the [30/13.5] flux ratio decreases with increasing 7-um excess for all targets in the sample. To explain these correlations with the models, we need to introduce an artificial scaling factor for the inner disk height. In roughly 50% of the Herbig Ae/Be stars in our sample, the inner disk must be inflated by a factor 2 to 3 beyond what hydrostatic calculations predict. The total disk mass in small dust grains determines the degree of flaring. We conclude, however, that for any given disk mass in small dust grains, the shadowing of the outer (tens of AU) disk is determined by the scale height of the inner disk (1 AU). The inner disk partially obscures the outer disk, reducing the disk surface temperature. Here, for the first time, we prove these effects observationally.Comment: 4 pages, 3 figures, accepted by A&