觸發(fā)式恒星形成特征的搜尋和1.3 cm波段的譜線巡測

龔 龑

(中國科學(xué)院紫金山天文臺(tái)南京210008)

本論文分為兩個(gè)部分.第1部分基于紫金山天文臺(tái)13.7 m毫米波望遠(yuǎn)鏡的分子譜線巡天項(xiàng)目—“銀河畫卷”計(jì)劃,對(duì)一個(gè)紅外塵泡的樣本和一個(gè)大天區(qū)的12CO/13CO/C18O(J=1?0)成圖觀測,搜尋觸發(fā)式恒星形成的運(yùn)動(dòng)學(xué)特征,并結(jié)合紅外衛(wèi)星和射電連續(xù)譜檔案數(shù)據(jù),研究相應(yīng)的分子團(tuán)塊、年輕恒星分布及恒星形成規(guī)律等.第2部分是利用E ff elsberg-100 m望遠(yuǎn)鏡對(duì)IRC+10216和Orion KL進(jìn)行了1.3 cm波段的譜線巡測.

我們通過對(duì)選定有大質(zhì)量恒星形成跡象的17個(gè)天區(qū)做CO同位素分子譜線的成圖觀測,研究其中30個(gè)紅外塵泡樣本的分子氣體性質(zhì).包裹著紅外塵泡的分子殼層按形態(tài)和可能的演化序列分為:完整型-破損型-不規(guī)則型.與塵泡成協(xié)電離氫區(qū)的動(dòng)力學(xué)年齡分析支持我們提出的演化序列圖像.在其中17個(gè)塵泡周圍分子殼層的速度-位置圖中發(fā)現(xiàn)圓形或弓形的膨脹運(yùn)動(dòng)學(xué)特征,相應(yīng)的膨脹速度為2.5–7.5 km·s?1,這種超聲速的膨脹很可能是由于電離氫區(qū)和周圍分子云相互作用導(dǎo)致的.我們從這17個(gè)天區(qū)內(nèi)證認(rèn)了379個(gè)13CO分子團(tuán)塊和245個(gè)C18O分子團(tuán)塊,并給出了局部熱動(dòng)平衡假設(shè)下的物理參數(shù).分析表明,其中有16個(gè)塵泡周圍都存在成協(xié)的大質(zhì)量恒星形成團(tuán)塊,它們是深入研究“收集和塌縮”模型的理想候選樣本.

我們對(duì)銀道面(216.25?6 l 6218.75?和?0.75?6 b 61.25?)范圍內(nèi)共約3.75deg2天區(qū)進(jìn)行3條CO同位素分子譜線的巡天觀測,該區(qū)域在視線方向上有3個(gè)速度成分:15 km·s?1、27 km·s?1和50 km·s?1,其中27 km·s?1云主要包括一個(gè)處于超臨界狀態(tài)的纖維狀結(jié)構(gòu)S287-main. 巡天工作首次發(fā)表的C18O(J=1?0)成圖數(shù)據(jù)在S287-main中發(fā)現(xiàn)7個(gè)大質(zhì)量C18O團(tuán)塊,分布在分子氣體最大柱密度線上.對(duì)這些團(tuán)塊的質(zhì)量-尺度關(guān)系研究表明:它們均有可能形成大質(zhì)量恒星.對(duì)50 km·s?1成分獲得首次可分辨分子譜線成圖觀測,揭示出其中3個(gè)正在膨脹的較大尺度的分子氣體殼層結(jié)構(gòu).區(qū)域中年輕星分布在電離氫區(qū)S287附近和50 km·s?1成分云中的殼層結(jié)構(gòu)交界處呈現(xiàn)富集,表明這些年輕星的形成很有可能是電離氫區(qū)和殼層結(jié)構(gòu)的膨脹所觸發(fā)的.分子云中無處不在的纖維狀結(jié)構(gòu)的碎裂過程是恒星及星團(tuán)形成的關(guān)鍵環(huán)節(jié).與碎裂理論模型的比較表明,S287-main中大尺度的碎裂是湍動(dòng)主導(dǎo)的,而在形成年輕星的尺度上,引力主導(dǎo)的碎裂起重要作用.對(duì)處于外銀河環(huán)境下分子云的恒星形成關(guān)系的研究發(fā)現(xiàn),它們與在較低質(zhì)量近鄰分子云的研究結(jié)果一致,即:恒星形成率面密度明顯高于從河外星系的Schmidt-Kennicutt關(guān)系所預(yù)期的值.

我們?cè)贗RC+10216和Orion KL的1.3 cm譜線巡測中探測并證認(rèn)出的所有分子譜線均來自這兩個(gè)源中已知的星際分子.在IRC+10216中發(fā)現(xiàn)了78條分子譜線,其中有12條是沒有證認(rèn)出來的“U”線,譜線密度為每GHz約9條.證認(rèn)出的譜線來自18種不同的星際分子.其中23條譜線是第一次在星際介質(zhì)中探測到,還有另外20條譜線是第一次在IRC+10216星周包層中探測到.對(duì)NH3的非局部熱動(dòng)平衡分析表明:大部分的NH3發(fā)射是來源于動(dòng)能溫度至少為(70±20)K的內(nèi)部包層.我們通過研究HC5N的不同13C同位素分子發(fā)現(xiàn)它們的豐度可能不同,并得到同位素豐度比12C/13C=49±9.與暗云TMC-1的對(duì)比分析發(fā)現(xiàn),IRC+10216星周包層中更容易形成含硅的分子.在Orion KL中,我們發(fā)現(xiàn)并證認(rèn)261條譜線,包括164條氫、氦、碳的射電復(fù)合線和13種分子的97條譜線.對(duì)應(yīng)的譜線密度為每GHz約32條,其中有23條分子譜線是第一次在星際介質(zhì)中探測到.15NH3非亞穩(wěn)態(tài)(J>K)系列躍遷在Orion KL中是第一次被探測到.結(jié)合阿塔卡瑪毫米/亞毫米波陣列望遠(yuǎn)鏡(ALMA)對(duì)Orion KL中11種分子的檔案資料,我們討論了這些分子躍遷的發(fā)射區(qū)域.我們還發(fā)現(xiàn)SO2(81,7-72,6)中一個(gè)可能的脈澤發(fā)射成分.我們給出了局部熱動(dòng)平衡假設(shè)下12個(gè)分子的柱密度和它們相對(duì)于H2的豐度.轉(zhuǎn)動(dòng)圖方法分析表明14NH3非亞穩(wěn)態(tài)躍遷中,從J=K+1到J=K+4的躍遷可能示蹤不同的發(fā)射區(qū)域;15NH3亞穩(wěn)態(tài)(J=K)躍遷和非亞穩(wěn)態(tài)躍遷也可能示蹤著不同的區(qū)域.我們還通過譜線對(duì)的強(qiáng)度比估計(jì)了元素豐度比和同位素豐度比:He/H=(8.7±0.7)%、12C/13C=63±17、14N/15N=100±51、D/H=(8.3±4.5)×10?3.

This thesis consists of two parts.The first part,based on the Milky Way Imaging Scroll Painting(MWISP)project,observationally studies a sample of infrared bubbles and a region of 3.75 square degrees in three main CO isotopic lines.It focuses on searching for the signs of triggered star formation,and studying the physical properties of molecular shells and associated clumps,the distribution of young stellar objects,and corresponding star formation laws.The second part is concentrated on a 1.3 cm spectral line survey toward IRC+10216 and Orion KL.

We carried out simultaneous observations of12CO(1-0),13CO(1-0),and C18O(1-0)with the PMO(Purple Mountain Observatory)13.7 m telescope toward 30 infrared bubbles in 17 regions which show massive star formation signposts.On the basis of the morphologies of molecular clouds around these bubbles,we divide resolved targets into three morphological types,“complete”,“broken”,and “irregular”.We propose an evolutionary trend in which the “complete”-“broken” sequence is supported by the dynamic ages of associated H II regions.Toward 17 out of 30 bubbles,we find that their associated molecular shells display circular-or arc-like features in position-velocity diagrams,and indicative of expansion.Their expansion velocities are 2.5–7.5 km·s?1,suggesting that the expansion is supersonic.This is likely due to interactions between H II regions and ambient molecular clouds.The physical properties of the 17 molecular shells are also derived by assuming local thermodynamic equilibrium(LTE).In the associated molecular clouds,we identify 37913CO clumps and 245 C18O clumps,and their physical properties are also derived under the LTE assumption.An analysis suggests that there are massive star-forming molecular clumps around 16 bubbles.A statistical analysis con firms the presence of an over-density of clumps in the periphery of bubbles.We also find that bubble-associated clumps tend to have higher excitation temperatures than unassociated clumps.The clump mass functions(CMFs)of bubbleassociated and unassociated clumps are nearly the same,suggesting that the CMF is littlea ff ected by the feedback of bubbles.

We study the physical properties of molecular clouds and their associated star formation toward the Galactic plane within 216.25?6 l 6218.75?and ?0.75?6 b 61.25?,which covers the molecular cloud complex S287.We reveal three molecular clouds,the 15 km·s?1cloud,the 27 km·s?1cloud,and the 50 km·s?1cloud,in the surveyed region.Investigating their morphology and velocity structures,we find that the 27 km·s?1cloud is likely a ff ected by feedback from the stellar association Mon OB3,and the 50 km·s?1cloud is characterized by three large expanding molecular shells.We discover seven C18O clumps and 15 dust clumps that are likely to form massive stars.Using infrared color-color diagrams,we find 56 Class I and 107 Class II young stellar object(YSO)candidates toward a slightly larger region of 5.0 square degrees.An over-density of YSO candidates is found around the H II region S287 and the intersection of two shells,indicative of triggering.Comparing the observed values of the filament S287-main with fragmentation models,we suggest that turbulence controls the large-scale fragmentation in the filament,while gravitational fragmentation plays an important role in the formation of YSOs on small scales.Using the 1.1 mm dust emission to trace dense gas,we obtain a dense gas fraction of 2.7%–10.4%for the 27 km·s?1cloud.

We carried out a spectral line survey with the Effelsberg-100 m telescope toward IRC+10216 and Orion KL at λ～1.3 cm.In the circumstellar shell of IRC+10216,we find 78 spectral lines,among which 12 remain unidentified.The identified lines are assigned to 18 different molecules and radicals.A total of 23 lines from species known to exist in this envelope are detected for the first time outside the solar system and there are additional 20 lines first detected in IRC+10216.Assuming LTE,we then determine rotational temperatures,column densities,and abundances of 17 detected molecules.A non-LTE analysis of NH3shows that the bulk of its emission arises from the inner envelope with a kinetic temperature of(70±20)K.Potential abundance differences between various13C-bearing isotopologues of HC5N are evaluated.Overall,the isotopic12C/13C ratio is estimated to be 49±9.Finally,a comparison of detected molecules in the λ～1.3 cm range with the dark cloud TMC-1 indicates that silicate-bearing molecules are more predominant in IRC+10216.In Orion KL,we find 261 spectral lines.The identified lines include 164 radio recombination lines(RRLs)and 97 molecular lines.The molecular lines are assigned to 13 different molecular species including rare isotopologues.A total of 23 molecular transitions from species known to exist in Orion KL are detected for the first time in the interstellar medium.Non-metastable(J>K)15NH3transitions are detected in Orion KL for the first time.Based on the velocity information of detected lines and the ALMA images,the spatial origins of molecular emission are constrained and discussed.A narrow feature is found in SO2(81,7-72,6),possibly suggesting the presence of a maser line.Column densities and fractional abundances relative to H2are estimated for 12 molecules with LTE methods.Metastable(J=K)15NH3is found to have a higher excitation temperature than non-metastable15NH3,also indicating that they may trace different regions.Elemental and isotopic abundance ratios are also estimated:He/H=(8.7±0.7)%derived from the ratios between helium RRLs and hydrogen RRLs;12C/13C=63±17 from12CH3OH/13CH3OH;14N/15N=100±51 from14NH3/15NH3;and D/H=(8.3±4.5)×10?3from NH2D/NH3.