An observational review is provided of the properties of accretion disks around young stars. It concerns the primordial disks of intermediate- and high-mass young stellar objects in embedded and ...optically revealed phases. The properties were derived from spatially resolved observations and, therefore, predominantly obtained with interferometric means, either in the radio/(sub)millimeter or in the optical/infrared wavelength regions. We make summaries and comparisons of the physical properties, kinematics, and dynamics of these circumstellar structures and delineate trends where possible. Amongst others, we report on a quadratic trend of mass accretion rates with mass from T Tauri stars to the highest mass young stellar objects and on the systematic difference in mass infall and accretion rates.
We present high angular resolution observations (0''.5 x 0''.3) carried out with the Submillimeter Array (SMA) toward the AFGL2591 high-mass star-forming region. Our SMA images reveal a clear ...chemical segregation within the AFGL2591 VLA 3 hot core, where different molecular species (Types I, II, and III) appear distributed in three concentric shells. This is the first time that such a chemical segregation is ever reported at linear scales < or =, slant3000 AU within a hot core. While Type I species (H sub(2)S and super(13)CS) peak at the AFGL2591 VLA 3 protostar, Type II molecules (HC sub(3)N, OCS, SO, and SO sub(2)) show a double-peaked structure circumventing the continuum peak. Type III species, represented by CH sub(3)OH, form a ring-like structure surrounding the continuum emission. The excitation temperatures of SO sub(2), HC sub(3)N, and CH sub(3)OH (185 + or - 11 K, 150 + or - 20 K, and 124 + or - 12 K, respectively) show a temperature gradient within the AFGL2591 VLA 3 envelope, consistent with previous observations and modeling of the source. By combining the H sub(2)S, SO sub(2), and CH sub(3)OH images, representative of the three concentric shells, we find that the global kinematics of the molecular gas follow Keplerian-like rotation around a 40 M sub(middot in circle) star. The chemical segregation observed toward AFGL2591 VLA 3 is explained by the combination of molecular UV photodissociation and a high-temperature (~1000 K) gas-phase chemistry within the low extinction innermost region in the AFGL2591 VLA 3 hot core.
Cardiovascular disease (CVD) is the leading cause of adult mortality in low-income countries but data on the prevalence of cardiovascular risk factors such as hypertension are scarce, especially in ...sub-Saharan Africa (SSA). This study aims to assess the prevalence of hypertension and determinants of blood pressure in four SSA populations in rural Nigeria and Kenya, and urban Namibia and Tanzania.
We performed four cross-sectional household surveys in Kwara State, Nigeria; Nandi district, Kenya; Dar es Salaam, Tanzania and Greater Windhoek, Namibia, between 2009-2011. Representative population-based samples were drawn in Nigeria and Namibia. The Kenya and Tanzania study populations consisted of specific target groups. Within a final sample size of 5,500 households, 9,857 non-pregnant adults were eligible for analysis on hypertension. Of those, 7,568 respondents ≥ 18 years were included. The primary outcome measure was the prevalence of hypertension in each of the populations under study. The age-standardized prevalence of hypertension was 19.3% (95%CI:17.3-21.3) in rural Nigeria, 21.4% (19.8-23.0) in rural Kenya, 23.7% (21.3-26.2) in urban Tanzania, and 38.0% (35.9-40.1) in urban Namibia. In individuals with hypertension, the proportion of grade 2 (≥ 160/100 mmHg) or grade 3 hypertension (≥ 180/110 mmHg) ranged from 29.2% (Namibia) to 43.3% (Nigeria). Control of hypertension ranged from 2.6% in Kenya to 17.8% in Namibia. Obesity prevalence (BMI ≥ 30) ranged from 6.1% (Nigeria) to 17.4% (Tanzania) and together with age and gender, BMI independently predicted blood pressure level in all study populations. Diabetes prevalence ranged from 2.1% (Namibia) to 3.7% (Tanzania).
Hypertension was the most frequently observed risk factor for CVD in both urban and rural communities in SSA and will contribute to the growing burden of CVD in SSA. Low levels of control of hypertension are alarming. Strengthening of health care systems in SSA to contain the emerging epidemic of CVD is urgently needed.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
Aims. We present a mid-IR interferometric survey of the circumstellar environment of a specific class of post-asymptotic giant branch (post-AGB) binaries. For this class the presence of a compact ...dusty disk has been postulated on the basis of various spatially unresolved measurements. The aim is to determine the angular extent of the N-band emission directly and to resolve the compact circumstellar structures. Methods. Our interferometric survey was performed with the MIDI instrument on the VLTI. In total 19 different systems were observed using variable baseline configurations. Combining all the visibilities at a single wavelength at 10.7 μm, we fitted two parametric models to the data: a uniform disk and a ring model mimicking a temperature gradient. We compared our observables of the whole sample, with synthetic data computed from a grid of radiative transfer models of passively irradiated disks in hydrostatic equilibrium. These models are computed with a Monte Carlo code that has been widely applied to describe the structure of protoplanetary disks around young stellar objects (YSO). Results. The spatially resolved observations show that the majority of our targets cluster closely together in the distance-independent size-colour diagram, and have extremely compact N-band emission regions. The typical uniform disk diameter of the N-band emission region is ~40 mas, which corresponds to a typical brightness temperature of 400–600 K. The resolved objects display very similar characteristics in the interferometric observables and in the spectral energy distributions. Therefore, the physical properties of the disks around our targets must be similar. Our results are discussed in the light of recently published sample studies of YSOs to compare quantitatively the secondary discs around post-AGB stars to the ones around YSOs. Conclusions. Our high-angular-resolution survey further confirms the disk nature of the circumstellar structures present around wide post-AGB binaries. The grid of protoplanetary disk models covers very well the observed objects. Much like for young stars, the spatially resolved N-band emission region is determined by the hot inner rim of the disk. Continued comparisons between post-AGB and protoplanetary disks will help to understand grain growth and disk evolution processes, and to constrain planet formation theories. These second-generation disks are an important missing ingredient in binary evolution theory of intermediate-mass stars.
Solar-mass stars form via disk-mediated accretion. Recent findings indicate that this process is probably episodic in the form of accretion bursts1, possibly caused by disk fragmentation2, 3, 4. ...Although it cannot be ruled out that high-mass young stellar objects arise from the coalescence of their low-mass brethren5, the latest results suggest that they more likely form via disks6, 7, 8, 9. It follows that disk-mediated accretion bursts should occur10, 11. Here we report on the discovery of the first disk-mediated accretion burst from a roughly twenty-solar-mass high-mass young stellar object12. Our near-infrared images show the brightening of the central source and its outflow cavities. Near-infrared spectroscopy reveals emission lines typical for accretion bursts in low-mass protostars, but orders of magnitude more luminous. Moreover, the released energy and the inferred mass-accretion rate are also orders of magnitude larger. Our results identify disk-accretion as the common mechanism of star formation across the entire stellar mass spectrum.
Context.
Given the high incidence of binaries among mature field massive stars, it is clear that multiplicity is an inevitable outcome of high-mass star formation. Understanding how massive multiples ...form requires the study of the birth environments of massive stars, covering the innermost to outermost regions.
Aims.
We aim to detect and characterise low-mass companions around massive young stellar objects (MYSOs) during and shortly after their formation phase. By the same means, we also probed the 3.8-μm emission that surrounds these massive protostars, in order to link the multiplicity to their star-forming environment.
Methods.
To investigate large spatial scales, we carried out an
L
′-band high-contrast direct imaging survey seeking low-mass companions (down to
L
bol
≈ 10
L
⊙
or late A-type) around thirteen previously identified MYSOs using the VLT/NACO instrument. From those images, we looked for the presence of companions on a wide orbit, covering scales from 300 to 56 000 au. Detection limits were determined for all targets and we tested the gravitational binding to the central object based on chance projection probabilities.
Results.
We have discovered a total of thirty-nine potential companions around eight MYSOs, the large majority of which have never been reported to date. We derived a multiplicity frequency (MF) of 62 ± 13% and a companion fraction (CF) of 3.0 ± 0.5. The derived stellar multiplicity and companion occurrence are compared to other studies for similar separation ranges. The comparisons are effective for a fixed evolutionary stage spanning a wide range of masses and vice versa. We find an increased MF and CF compared to the previous studies targeting MYSOs, and our results match the multiplicity rates derived among more evolved populations of massive stars. For similar separation ranges, we however confirm a higher multiplicity than that of T Tauri stars (∼30%), showing that the statement in which multiplicity scales with primary mass also extends to younger evolutionary stages. The separations at which the companions are found and their location with relation to the primary star allow us to discuss the implications for the massive star formation theories.
Conclusions.
Our findings do not straightforwardly lift the uncertainty as to the formation process of massive stars as a whole but we rather examine the likely pathways for individual objects. However, the wide distance at which companions are detected rather supports core fragmentation or capture as the main mechanisms to produce wide multiples. We find hints of triggered star formation for one object and discuss the massive star against stellar cluster formation for other crowded fields.
η Car is one of the most luminous and massive stars in our Galaxy and is the brightest mid-IR source in the sky outside our solar system. Since the late 1990s, the central source has dramatically ...brightened at UV and optical wavelengths. This might be explained by a decrease in circumstellar dust extinction. We aim to establish the mid-IR flux evolution and further our understanding of the star’s UV and optical brightening. Mid-IR images from 8−20 μm were obtained in 2018 with VISIR at the Very Large Telescope. Archival data from 2003 and 2005 were retrieved from the ESO Science Archive Facility, and historical records were collected from publications. We present mid-IR images of η Car with the highest angular resolution to date at the corresponding wavelengths (≥0.22″). We reconstruct the mid-IR evolution of the spectral energy distribution of the spatially integrated Homunculus nebula from 1968 to 2018 and find no long-term changes. The bolometric luminosity of η Car has been stable over the past five decades. We do not observe a long-term decrease in the mid-IR flux densities that could be associated with the brightening at UV and optical wavelengths, but circumstellar dust must be declining in our line of sight alone. Short-term flux variations within about 25% of the mean levels could be present.
Summary Background The effect of pretreatment HIV-1 drug resistance on the response to first-line combination antiretroviral therapy (ART) in sub-Saharan Africa has not been assessed. We studied ...pretreatment drug resistance and virological, immunological, and drug-resistance treatment outcomes in a large prospective cohort. Methods HIV-1 infected patients in the PharmAccess African Studies to Evaluate Resistance Monitoring (PASER-M) cohort started non-nucleoside reverse transcriptase inhibitor-based ART at 13 clinical sites in six countries, from 2007 to 2009. We used the International Antiviral Society-USA drug resistance mutation list and the Stanford algorithm to classify participants into three pretreatment drug resistance categories: no pretreatment drug resistance, pretreatment drug resistance with fully active ART prescribed, or pretreatment drug resistance with reduced susceptibility to at least one prescribed drug. We assessed risk factors of virological failure (≥400 copies per mL) and acquired drug resistance after 12 months of ART by use of multilevel logistic regression with multiple imputations for missing data. CD4 cell count increase was estimated with linear mixed models. Findings Pretreatment drug resistance results were available for 2579 (94%) of 2733 participants; 2404 (93%) had no pretreatment drug resistance, 123 (5%) had pretreatment drug resistance to at least one prescribed drug, and 52 (2%) had pretreatment drug resistance and received fully active ART. Compared with participants without pretreatment drug resistance, the odds ratio (OR) for virological failure (OR 2·13, 95% CI 1·44–3·14; p<0·0001) and acquired drug-resistance (2·30, 1·55–3·40; p<0·0001) was increased in participants with pretreatment drug resistance to at least one prescribed drug, but not in those with pretreatment drug resistance and fully active ART. CD4 count increased less in participants with pretreatment drug resistance than in those without (35 cells per μL difference after 12 months; 95% CI 13–58; p=0·002). Interpretation At least three fully active antiretroviral drugs are needed to ensure an optimum response to first-line regimens and to prevent acquisition of drug resistance. Improved access to alternative combinations of antiretroviral drugs in sub-Saharan Africa is warranted. Funding The Netherlands Ministry of Foreign Affairs.
Summary Background There are few data on the epidemiology of primary HIV-1 drug resistance after the roll-out of antiretroviral treatment (ART) in sub-Saharan Africa. We aimed to assess the ...prevalence of primary resistance in six African countries after ART roll-out and if wider use of ART in sub-Saharan Africa is associated with rising prevalence of drug resistance. Methods We did a cross-sectional study in antiretroviral-naive adults infected with HIV-1 who had not started first-line ART, recruited between 2007 and 2009 from 11 regions in Kenya, Nigeria, South Africa, Uganda, Zambia, and Zimbabwe. We did population-based sequencing of the pol gene on plasma specimens with greater than 1000 copies per mL of HIV RNA. We identified drug-resistance mutations with the WHO list for transmitted resistance. The prevalence of sequences containing at least one drug-resistance mutation was calculated accounting for the sampling weights of the sites. We assessed the risk factors of resistance with multilevel logistic regression with random coefficients. Findings 2436 (94.1%) of 2590 participants had a pretreatment genotypic resistance result. 1486 participants (57.4%) were women, 1575 (60.8%) had WHO clinical stage 3 or 4 disease, and the median CD4 count was 133 cells per μL (IQR 62–204). Overall sample-weighted drug-resistance prevalence was 5.6% (139 of 2436; 95% CI 4.6–6.7), ranging from 1.1% (two of 176; 0.0–2.7) in Pretoria, South Africa, to 12.3% (22 of 179; 7.5–17.1) in Kampala, Uganda. The pooled prevalence for all three Ugandan sites was 11.6% (66 of 570; 8.9–14.2), compared with 3.5% (73 of 1866; 2.5–4.5) for all other sites. Drug class-specific resistance prevalence was 2.5% (54 of 2436; 1.8–3.2) for nucleoside reverse-transcriptase inhibitors (NRTIs), 3.3% (83 of 2436; 2.5–4.2) for non-NRTIs (NNRTIs), 1.3% (31 of 2436; 0.8–1.8) for protease inhibitors, and 1.2% (25 of 2436; 0.7–1.7) for dual-class resistance to NRTIs and NNRTIs. The most common drug-resistance mutations were K103N (43 1.8% of 2436), thymidine analogue mutations (33 1.6% of 2436), M184V (25 1.2% of 2436), and Y181C/I (19 0.7% of 2436). The odds ratio for drug resistance associated with each additional year since the start of the ART roll-out in a region was 1.38 (95% CI 1.13–1.68; p=0.001). Interpretation The higher prevalence of primary drug resistance in Uganda than in other African countries is probably related to the earlier start of ART roll-out in Uganda. Resistance surveillance and prevention should be prioritised in settings where ART programmes are scaled up. Funding Ministry of Foreign Affairs of the Netherlands.
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